Tag Archives: roofing

TPO on Top: Why the Roofing Material Continues to Shine

by Mike Mendoza

Thermoplastic polyolefin (TPO) is a relatively new material, but makes up one of the fastest-growing sectors of the commercial roofing industry.  All images courtesy Firestone Building Products

Thermoplastic polyolefin (TPO) is a relatively new material, but makes up one of the fastest-growing sectors of the commercial roofing industry. All images courtesy Firestone Building Products

As building codes and environmental trends continue to evolve, so too does the diversity of roofing material options. These factors, along with others, make construction specifiers’ roofing installation and selection prowess of utmost importance.

Among the available choices is thermoplastic polyolefin (TPO). Despite the fact this technology is fairly new in the scheme of roof materials, it makes up one of the fastest-growing sectors of the commercial roofing industry. That growth brings with it constant improvements to the chemical composition of the membrane to achieve greater durability and longevity.

Nevertheless, representatives for the Single Ply Roofing Industry (SPRI) still point to early 1990s TPO installations that continue to perform thanks to proper installation and maintenance. SPRI data indicates about 371.6 million m2 (4 billion sf) of TPO was installed in North America between 2005 and 2010, with nearly the entirety still performing without issue.1

“We at SPRI are confident TPO roofing systems will continue to provide quality and value for many years to come,” said the association’s technical director, Mike Ennis.

Indeed, a 2013 study that used ASTM D6878, Standard Specification for Thermoplastic Polyolefin-based Sheet Roofing (revised to address concerns of prolonged exposure in extreme heat climates) shows the material’s durability.2

“The heat aging exposure at (116 C [240 F]) was extended from 670 hours to 5400 hours (32 weeks),” the study states. “To meet these new requirements, it is critical for TPO roofing formulations to contain high-quality resins combined with tailored stabilization, flame retardants, and membrane design.”

Results showed TPO roofing membranes produced using the right polymer formation and stabilization can perform in some of the most extreme climate conditions: “Depending on the climate zone, 1.5-mm (60-mil) membranes may last up to 25 years or more.”

Beyond durability, TPO single-ply roofing membranes offer several other performance and installation advantages. These assemblies can provide resistance to ultraviolet (UV) rays, ozone, and chemical exposure. Further, the reflective surface meets the U.S. Environmental Protection Agency’s (EPA’s) Energy Star requirements, and is both recyclable and composed of recycled content.

Knowing the options
Settling on the ideal TPO roofing system relies on identifying and analyzing the characteristics surrounding climate and condition of a commercial building’s existing roof surface. Factors include:
● potential for tears and abrasions;
● area wind speeds;
● UV exposure; and
● various surfaces (e.g. vertical parapet walls) and slopes.

When it comes to color choice, white is among the most popular option for reasons discussed later in this article. However, it is not the sole TPO variety to consider, particularly if another hue better suits a rooftop. In addition to white, TPO membranes often come in gray and tan.

Since TPO membranes are installed fully-adhered, ballasted, or mechanically fastened, they work with a range of building envelope designs, offering architects flexibility. Architects most commonly select TPO roofing for flat or low-slope installations because of its cost-effectiveness, easy installation, and heat-welded seams that prevent moisture penetration.

Keeping cool
A specific driving factor for choosing TPO is within the realm of cool roofing. The Cool Roof Rating Council (CRRC) was created in 1998 to “develop accurate and credible methods for evaluating and labeling the solar reflectance and thermal emittance of roofing products,” according to the nonprofit organizatioRoof Surface Properties (CRRC)n’s history and bylaws. In the 15 years or so since, a lot of roofing products now come in what can be considered ‘cool’ varieties.3 Field-applied coatings, single-plies, tiles, and others fit this bill, which the council defines as a surface that reflects and emits the sun’s heat back into the sky rather than transferring it to a building.

A roof’s solar reflectivity and thermal emittance (the ability to release absorbed heat), are both measurable factors, according to the U.S. Green Building Council (USGBC). The Green Building Alliance, a chapter serving the Greater Pittsburgh, Laurel Highlands, and Northwest Pennsylvania branches, says as many as 90 percent of roofs in the United States are, “poorly designed and built with dark, non-reflective heat-absorbing materials,” causing rooftop temperatures to hover as many as 50 C (90 F) above that of the air.

The more reflective a TPO surface, the more likely it will comply with increasingly stringent building codes. Therefore, white TPO membranes—the most reflective—can be a suitable choice for those striving for maximum energy savings and environmental benefits.

The U.S. Department of Energy (DOE) touches on this concept in its 2010 study entitled, “Guidelines for Selecting Cool Roofs,” stating a conventional dark-colored surface reflects about 20 percent of incoming sunlight while a “cool” light-colored one (white being the lightest) reflects as much as 80 percent.4

EPA goes one step further in its “Reducing Urban Heat Islands: Compendium of Strategies” focusing on cool roofs. The report uses a diagram from the Lawrence Berkeley National Laboratory (LBNL) to compare the solar reflectance of black, metal, and white roofs.5

Roof Reflectance, EmittanceOn a hot, sunny summer day, a black roof that reflects five percent of the sun’s energy and emits more than 90 percent of the heat it absorbs can reach 82 C (180 F). A metal roof will reflect most of the sun’s energy while releasing about a fourth of the heat it absorbs; it can warm to 70 C (160 F). A cool roof will reflect and emit the majority of the sun’s energy and reach a peak temperature of 48 C (120 F).

CRRC lists white, tan, and gray TPO roofs among its ‘environmentally friendly’ options, and white and tan are compliant with California’s Title 24 Energy Efficiency Building Standards. Cool roof requirements have been adopted in several U.S. building energy codes, and an increasing percentage of electric utilities have begun offering rebates for cool roofing materials, including TPO, that help conserve energy and reduce buildings’ environmental impacts.

According to USGBC, TPO and other cool roof options yield the following benefits:
● utility rebate opportunities;
● lower indoor temperatures;
● reduced maintenance costs (partially due to the material’s longer lifespans);
● improved air quality resulting from a reduction of emissions such as mono-nitrogen oxides and carbon dioxides in the atmosphere;
● mitigated heat island indexes (i.e. less heat creation in dense, urban areas); and
● reduced energy bills because less air-conditioning is needed during the summer.

Conclusion
Not unlike other roofing product manufacturers, those offering TPO will recommend consulting a design professional to ensure proper roofing system selection, conformance to building codes, and insurance requirements. Such customizations further emphasize evaluating a roof’s current condition is paramount when determining how best to repair or replace it. Further with ever-changing code and material options available, it is important for construction professionals to remain educated about the changing commercial building products landscape.

Of course, amidst the increasingly popular cool roofing and TPO possibilities, it is crucial to be cognizant of the fact roofing materials are not the sole remedy to increase overall building performance. It takes a holistic approach to have a building perform at peak efficiency.

This fact emphasizes the importance of building products manufacturers working closely with architects and construction specifiers—as well as consultants, contractors, facility operators, and building owners—who share similar goals of increasing overall building performance.

Together, these professionals should consider the entire building envelope to continue revolutionizing the commercial building industry, yielding an impact that reaches far beyond their individual buildings, cities, and immediate populations.

Notes
1 Visit www.spri.org/pdf/spri_responds_to_pro_roofing_article_on_tpo.pdf. (back to article)
2 Visit msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_08e0/0901b803808e057f.pdf. (back to article)
3 See the council’s “Reducing Urban Heat Islands: Compendium of Strategies” by visiting www.epa.gov/hiri/resources/pdf/CoolRoofsCompendium.pdf. (back to article)
4 The DOE study can be found at www1.eere.energy.gov/femp/pdfs/coolroofguide.pdf. (back to article)
5 See S.L. Konopacki et al’s 1998 report, “Demonstration of Energy Savings of Cool Roofs,” also known as LBNL-40673. (back to article)

Mike MendozaMike Mendoza is the thermoplastic polyolefin (TPO) product manager for Firestone Building Products. He is responsible for researching market trends and directing the planning and development of thermoplastic products. Mendoza previously spent 13 years at Owens Corning where he served in various roles including as a southwest region sales manager for roofing, strategic product planning manager for asphalt, and director of global sourcing for insulation. He can be reached at mendozamichael@fsdp.com.

To Test or Not to Test…? A guide to field quality control

Photo © BigStockPhoto/Vichaya Kiatying-Angsulee

Photo © BigStockPhoto/Vichaya Kiatying-Angsulee

by Sean M. O’Brien, PE, LEED AP, and David Artigas, PE

When properly implemented, field and laboratory testing of buildings and their systems and components can yield a wealth of useful information about construction quality, watertightness, durability, longevity, and other critical performance criteria.

Test results can help designers better evaluate ‘as-built conditions,’ understand any problems with the installation, and develop solutions appropriate to the specific problems that prompted the testing. When improperly implemented, however, testing can yield misleading results, lead designers to incorrect conclusions, and cause unnecessary expenses related to remedial work that may not really be warranted by the in-situ conditions.

In some cases, specifying inappropriate standards or performance criteria can create confusion or incite debate between the design and construction teams, especially in the event of a perceived failure. This article reviews some of the common test methods and procedures used in contemporary construction, with a focus on how the purpose of, and results from, these tests are often misunderstood.

This site-built interior chamber creates differential pressure across a curtain wall system for a test under American Architectural Manufacturers Association (AAMA) 503, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls, and Sloped Glazing Systems.

This site-built interior chamber creates differential pressure across a curtain wall system for a test under American Architectural Manufacturers Association (AAMA) 503, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls, and Sloped Glazing Systems. Images courtesy Simpson Gumpertz & Heger

The spray rack above has been positioned on the window exterior for AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products.

The spray rack above has been positioned on the window exterior for AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products.

 

 

 

 

 

 

 

 

 

 

 

Most important (but least asked) question
With dozens of industry organizations publishing thousands of test standards for buildings and building systems, there is almost always a quick answer to the questions: “What do I test and how do I test it?” However, the more important question, and one about which designers and contractors are often less sure, is “Why do I test?”

The answer to this question will almost always dictate the best method to use, the timing of the test, the pass/fail criteria, and sometimes whether it should be performed at all. In the case of poorly specified tests, the wrong tests are often performed ‘because it was in the specifications’ or ‘because the contractor owes us testing.’ Especially with fast-track construction projects, debates over testing are often brushed aside in favor of doing whatever the specification demands, regardless of the value of that testing.

In the authors’ experience, requiring designers to explain the reasoning behind their specified test methods or procedures can be an extremely useful exercise, either during the design process or as part of pre-construction activities. In the case of a designer having correctly specified test methods, the discussion can provide valuable information to the rest of the project team, giving everyone involved a better understanding of the reasons behind the testing. For improperly specified tests, the conversation can help avoid unnecessary testing and the resulting time/expense, as well as identify the correct test methods to determine the desired information.

Flood testing of a membrane waterproofing system on a rooftop parking deck occurs the finished paving is installed.

Flood testing of a membrane waterproofing system on a rooftop parking deck occurs the finished paving is installed.

Dye is used to color water during a roof flood test; it can help link interior leaks to specific areas of the roof above.

Dye is used to color water during a roof flood test; it can help link interior leaks to specific areas of the roof above.

Windows, doors, and curtain walls
Some of the most commonly tested building components are fenestration products—windows, doors, and curtain walls. For new construction, testing is most often specified as a quality control measure to ensure the installed system(s) meet the specified performance requirements for air and water penetration resistance.

The most common requirements are for testing in accordance with various American Architectural Manufacturers Association (AAMA) standards, depending on the system being evaluated. This requires the designer or specifier to know what type of system is specified as well as the relevant performance requirements to establish the appropriate test method.

There are different standards for different components, and some contain multiple test methods or options. For example, AAMA 501.1, Standard Test Method for Water Penetration of Windows, Curtain Walls, and Doors Using Dynamic Pressure, includes a method to test curtain fenestration for water penetration under dynamic wind pressure that requires a large fan—essentially, an airplane engine/propeller—and calibrated spray racks. There is also AAMA 501.2, Quality Assurance and Diagnostic Water Leakage Field Check of Installed Storefronts, Curtain Walls, and Sloped Glazing Systems, which is much easier to perform as it uses a simple handheld nozzle to spray along gaskets and joints. In this case, specifying testing per AAMA 501 is insufficient—the specific test method from that standard needs to be called out, as the two options vary greatly in scope and complexity.

In previous versions of the AAMA 501 standard, a third option (501.3) was available to perform water leakage testing under static pressure differential (Figure 1). Despite this test being pulled from the standard and replaced by AAMA 503-02, Voluntary Specification for Field Testing of Newly Installed Storefronts, Curtain Walls, and Sloped Glazing Systems, references to the 501.3 method can still be found in specifications written today. This often results in confusion when the time comes to perform the tests.

AAMA 501.2 is intended as a field check for water leakage—a simple, economical method to verify the general quality of the curtain wall installation. For compliance with a specified level of air- and water-penetration resistance, AAMA 503 must be used. It is important to note 501.2 is solely intended for fixed glazing systems; the high nozzle pressure used would likely cause moderate to severe leakage on operable vents or window products due to the inherent limitations of seals and gaskets used in operable fenestration.

For testing window assemblies (both fixed and operable) for compliance with a specified air- and water-penetration, AAMA 502, Voluntary Specification for Field Testing of Newly Installed Fenestration Products, is typically specified. This involves building a chamber on the interior of the product to allow for negative interior air pressure and using a spray rack to wet the exterior of the window (Figure 2).

It is important to understand this test method has a specific definition of what constitutes a leak. For example, water on the sill members that does not pass the innermost projection of the window is not considered a leak, since it does not reach a point where it can damage interior finishes. This can come as a surprise to designers witnessing the test and seeing water on the sill, only to find out that by the standard’s strict definition, the window is considered to not have leaked. For this reason, some specifiers add their own language regarding the definition of leakage, but may have difficulty holding a manufacturer to this definition in the event of a dispute.

It is also important for designers to clearly specify the pass/fail criteria for windows. This information can be derived from the performance class (e.g. R, CW, or AW) and grade for the product being tested, but is often misinterpreted, leading to confusion during the test or attempts to hold installers/manufacturers to unrealistic or non-industry-standard performance criteria.

Another caveat of AAMA 502 (since the 2008 revision) as well as AAMA 503 is they only apply to newly installed fenestration products. The standards define ‘new’ as products installed before issuance of the certificate of occupancy for the building or products that have been installed for less than six months.

This recent development is often a source of debate, as this means a window installed for six months and one day, even in an unoccupied/incomplete building, is no longer subject to AAMA 502 and therefore cannot be tested for compliance with the manufacturer’s stated performance criteria. In simpler terms, the manufacturer of the window is only held to its stated performance criteria for six months. For older products, AAMA 511, Voluntary Guideline for Forensic Water Penetration Testing of Fenestration Products, contains diagnostic procedures for identifying known leaks, but is not specifically intended to evaluate in-situ performance of non-leaking windows.

When specifying testing, it is important to make the distinction between test specifications, standard test methods, and testing guides. Each of these types of documents is used for a different, but often similar, purpose. Standard test methods, such as ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls, by Uniform or Cyclic Static Air Pressure Difference, contains specific information on how to physically test these various components, what equipment to use, and related information.

Test specifications, such as AAMA 502, provide procedural information on the testing, the relevance/applicability of the testing, and related administrative information, and typically reference standard test methods (e.g. ASTM E1105) for the physical test procedures.

Finally, testing guides, such as ASTM E2128, Standard Guide for Evaluating Water Leakage of Building Walls, are usually more general in nature and cover a wide range of components and procedures rather than focus on one specific area of the building enclosure. Similar to test specifications, these guides include procedural/administrative requirements and reference standard test methods for the actual testing procedures. Due to their non-specific nature, including ‘compliance’ with a testing guide such as ASTM E2128 as a specification requirement is likely to result in confusion, as it can be interpreted in many ways for many different components. A testing specification and pass/fail criteria must be clearly identified in the contract documents.

This depicts laboratory testing of brick masonry for compressive strength.

This depicts laboratory testing of brick masonry for compressive strength.

Water leakage through brick veneer/cavity wall can be seen from the interior through openings in the backup wall. The veneer does not provide actual waterproofing for this assembly due to presence of drainage plane and weather-resistant barriers.

Water leakage through brick veneer/cavity wall can be seen from the interior through openings in the backup wall. The veneer does not provide actual waterproofing for this assembly due to presence of drainage plane and weather-resistant barriers.

Roofing assemblies
Leakage from roofing systems, especially in the case of low-slope assemblies, can result in significant interior damage when left unchecked. There are many different methods for testing roofs, but not all are compatible with all assembly types. Understanding which methods can be used for which systems is key to specifying the appropriate test, whether as part of a specification for quality control in new installations or as part of remedial/troubleshooting efforts.

In this article, the authors focus on large-scale testing of roof areas, as opposed to smaller-scale testing of specific detail conditions (which is most often done using spray racks/nozzles or localized flood testing). As discussed, the roof’s configuration, as well as the specific membrane type, must be considered when specifying a test method.

The most obvious method of testing a roof—flooding it with water—can be effective in some cases, but extremely damaging in others. Flood-testing is best-suited to inverted roof membrane assemblies (IRMAs) where the membrane is installed directly over the structural deck, with insulation and ballast or other overburden above. In those cases, the testing is performed once the membrane and flashings are complete but prior to the installation of any overburden (Figure 3).

For this test, which is standardized in ASTM D5957, Standard Guide for Flood Testing Horizontal Waterproofing Installations, water is ponded over the system for a period of 24 to 72 hours, during which time the interior is reviewed for leaks. The depth of water must be reviewed to ensure the structural capacity of the roof is not exceeded, as every inch of water adds approximately 0.24 kPa (5 psf) of load. This can be challenging on large or complex roofs, where the deck slope may require compartmentalizing the test into smaller areas. Any leaks resulting from this test are likely to produce only localized damage which gets repaired along with the leaking component(s).

This test procedure is not appropriate for traditional ‘membrane-over-insulation’ roof systems, since leakage through the membrane may wet (and necessitate the replacement of) large areas of roof insulation. Especially in the case of a concrete roof deck, leakage through the membrane could go unnoticed or travel far from the original location as the deck retains the water, allowing large areas of insulation to become damaged and making it difficult to determine the leak’s source. These risks can be reduced by flooding only small areas at a time (limiting the amount of water that could enter the roof), in which case the water can be dyed to provide confirmation of leak sources if multiple areas are flooded in sequence (Figure 4).

There are several test methods available for traditional insulated roofing systems that do not carry the same risk of large-scale damage. These methods typically rely on specialized equipment to detect wet insulation below the membrane. Infrared (IR) thermography uses an infrared camera that visualizes temperature differences on surfaces by measuring and processing emitted radiation.

For an insulated roof, wet insulation will tend to retain more heat and cool slower than dry insulation. Since moisture from roof leaks is often trapped in the system for an extended period, scanning of a roof with suspected or known leaks shortly after the sun has set can help identify areas of wet insulation.

The IR camera measures the surface temperature of the membrane, so this method cannot be used on ballasted roofs since the ballast (e.g. gravel) will cool off uniformly and mask any small temperature differences on the membrane below. Similarly, testing on a windy day may yield misleading results as airflow over the membrane surface may even out temperature differences or cause the wet areas to cool off to the same temperature as the surroundings before the scan is made.

IR scanning of a roof is relatively efficient since large roof areas can be surveyed relatively quickly (some companies even offer aerial surveys, which can be economical for very large, open roof areas). A second method, often referred to as electrical capacitance/impedance (EC) testing, uses handheld or rolling (push-cart) equipment that sends electrical pulses into the roof system and measures the insulation’s ability to retain electrical charge. Wet areas will tend to hold a charge for less time than dry, allowing for relative comparison between areas. Similar to infrared, this method requires an exposed roof membrane since the scanner needs to be in close proximity to the insulation to be effective. For this method to be effective, the roof membrane needs to be non-conductive, making it ineffective on most ethylene propylene diene monomer (EPDM) assemblies or on membranes with metallized reflective coatings. For both of these methods, secondary verification (i.e. roof probes) of suspected wet materials should always be specified to confirm the efficacy of the test for the specific application.

A more recently developed test method uses specialized equipment to pinpoint specific defects in the membrane. In this method, a potential difference is created between the wetted roof surface and the grounded roof deck. Any breaches in the membrane create, in effect, a short circuit in the system which can be detected using specialized equipment.

This method can be used on both traditional and IRMA systems, but—similar to EC testing—the roof membrane must be nonconductive for the method to work. For new construction, especially on traditional roof systems, a grounding screen can be added below the membrane or cover board to provide more positive leak detection and become part of a permanently installed leak detection system. This type of system can be especially beneficial for vegetated roofing assemblies where the often significant amount of overburden can make locating leakage sites extremely difficult.

This infrared image shows air leakage around a window perimeter during a whole-building test.

This infrared image shows air leakage around a window perimeter during a whole-building test.

Brick masonry and exterior walls
Brick masonry has been a common building material in the United States since the colonial period, and mass masonry walls continued to be built through the first half of the 20th century. While the basic process of brick manufacturing has not changed much, modern technology allows the creation of brick typically much stronger and has greater uniformity of properties than historic brick. Historic lime mortars typically are softer and more permeable than modern cement mortars, which allows them to absorb greater stress within the wall from expansion and contraction or enables the wall greater capacity to ‘breathe.’

Concerns with historic masonry fall under two, often related, headings: the masonry’s structural capacity and durability. While it certainly is true modern masonry manufactured and constructed to meet modern standards should result in durable construction, it is not always necessary to hold historic masonry to these same modern standards, as the historic materials often have more than the necessary capacity to provide a long service life with good performance. Also, certain properties being lesser than modern standards may prove beneficial to performance.

The International Building Code (IBC) now has requirements for masonry properties such as compressive strength and performance in shear, though that was not always the case. Current codes are written for current construction, and do not always include previsions for how historic construction ‘works’ structurally. The International Existing Building Code (IEBC) includes provisions that allow historic buildings to remain, or repairs to occur, using original or like materials, but the structural engineer and code officials must still evaluate the structure’s capacity to withstand its loads and remain safe.

Structural engineers can use both non-destructive and destructive methods to determine masonry’s strength and ability to withstand stresses (Figure 5). It is important to specify testing appropriate to both the structure being evaluated and the goal of the evaluation. While a historic mass masonry wall may not meet the letter of the current code requirements, it may have capacity that exceeds its in-service loads with an acceptable factor of safety comparable to the code. That said, one concern with mass unreinforced masonry is it typically does not perform well during seismic events. In areas of higher seismic activity, greater care must be exercised in its evaluation.

Current requirements for energy efficiency mandate the building enclosure to have a specified resistance to heat transfer. While mass masonry walls typically have a lower R-value than modern insulated wall assemblies, they have an advantage—their bulk provides thermal mass unmatched by newer assemblies comprising several thinner layers of different materials sandwiched together. This thermal mass allows the wall to absorb and dissipate heat more slowly than modern walls, slowing the interior of the building’s reaction to changes in the exterior temperature and reducing the need for supplemental heating or cooling.

Changes to the thermal properties of a mass masonry wall, such as adding insulation to the interior, or significantly increasing the interior moisture load, may affect brick performance. Uninsulated historic masonry typically allows moisture to move through the wall (i.e. ‘breathe’) while remaining above the dewpoint, since the interior heat warms the wall.

The addition of interior insulation will reduce the wall’s temperature during the colder months. Water absorbed by the brick’s exterior wythe may go through freeze-thaw cycling as a result of the wall now being colder, and interior moisture that migrates through the wall assembly may condense on the inboard side of the masonry wall, because this side of the wall now is on the ‘cold side’ of the insulation.

Historic masonry may have two advantages that will reduce the likelihood of these two events occurring.

1. Historic brick typically is more porous than modern brick. This greater porosity may allow the brick to ‘drain’ rainwater more quickly than modern brick, and the larger pores may allow the absorbed water more room to expand without causing damage.
2. Historic lime mortars are more absorptive and permeable than modern mortars, and these properties may allow the mortar to wick water rather than having it remain on the wall.

However, it must be stressed the reaction of mass masonry to the installation of interior insulation is still a topic of study among engineers and preservationists. Further, there are currently no established guidelines for insulating walls, only various opinions on the matter.

Many designers of renovation projects may equate strength with durability and specify masonry testing with this thought in mind. Great care must be exercised when considering insulating mass masonry walls, and testing of the masonry’s porosity, absorption, permeability, expansion, and relative strength (both of brick and of mortar) should be performed. Additionally, laboratory testing and evaluation to determine the relative durability of the brick, as well as its resistance to freeze-thaw damage, are a crucial part of this kind of study.

It is also critical to evaluate test results in light of numerous factors, such as the type of building/occupancy, building use, and general quality of the surrounding construction. If a sampling of brick test as SW grade (suitable for severe weathering per ASTM C216, Standard Specification for Facing Brick [Solid Masonry Units Made from Clay or Shale]) that does not necessarily mean the wall assembly in question has the level of durability required for the specific application. SW brick on a large, clear wall area will likely provide suitable performance, but the same brick installed in a shaded location (i.e. minimal drying) below a window that experiences leakage (i.e. excessive wetting) may undergo premature degradation regardless of the brick grading.

The most important question to ask when evaluating a historic masonry building is: “How has it performed thus far?” If the building shows no obvious signs of distress after several decades or even centuries of use, its testing and evaluation must begin from a position of “How does it work?” as opposed to one of “Does it meet the code requirements for modern construction?”

This understanding will include site observations and possibly onsite or laboratory testing, and research into historic construction methods and materials. Ultimately, this approach to evaluating historic masonry may lead to a more efficient and lower cost project that also can maintain the building’s character. Regardless of testing, designers who take this approach much understand when the use of the building or other characteristics of the enclosure are changed as part of renovations, the prior performance of the building may not be a suitable predictor of long-term durability.

From a water penetration standpoint, there are many different test methods available for masonry walls, but not all provide useful information. For example, ASTM C1601, Standard Test Method for Field Determination of Water Penetration of Masonry Wall Surfaces, determines water penetration at the surface of a masonry wall, but does not provide any information on how much water actually leaks to the interior (as opposed to water that is absorbed and stored by the masonry). Similarly, RILEM tubes can be used to provide relatively quick evaluations of the water absorption rate of a masonry wall.1

ASTM E514, Standard Test Method for Water Penetration and Leakage Through Masonry, provides for measurement of the actual amount of water penetrating the full thickness of the masonry, but this is a lab test not applicable to field conditions (although it is often specified—incorrectly—by designers evaluating existing masonry buildings). Field surface absorption tests may have limited use in qualitatively evaluating the change in absorption that results from installing a penetrating sealer, but are typically of little to no use in evaluating water leakage.

Neither of these tests will be of practical value for masonry cavity wall construction, where any water penetrating the exterior façade is collected in a drainage cavity and wept out of the system (Figure 6). Water leakage through a masonry cavity wall is more likely the result of a breach in the water-resistive barrier (WRB) behind the masonry, since masonry veneer systems are expected to allow water into the drainage cavity.

The authors have generally found the general guidelines from ASTM E2128, Standard Guide for Evaluating Water Leakage of Building Walls—as opposed to one specific standard test method—are helpful in establishing the right combination of testing and inspection to diagnose water leakage through masonry walls.

Air barrier systems
As far as building testing goes, the testing of air barrier systems is a relatively recent development.2 Just as with window and curtain wall testing, there are multiple test standards and guides for testing air barrier systems in both the lab and the field. One of the first points of confusion is the definition of an air barrier—a system of interconnected components including walls, windows, curtain walls, and roofs that act together to prevent uncontrolled airflow into and out of the building. While air barrier testing is often thought of as testing a wall air barrier membrane (one component of the system), it can encompass everything from single materials to the entire building enclosure.

Some of the most commonly tested building components are fenestration products like windows and curtain walls. For new construction, testing is most often specifi ed as a quality control measure to ensure the installed systems meet the specifi ed requirements for air and water penetration resistance. Photo © BigStockPhoto/Graça Victoria

Some of the most commonly tested building components are fenestration products like windows and curtain walls. For new construction, testing is most often specified as a quality control measure to ensure the installed systems meet the specified requirements for air and water penetration resistance. Photo © BigStockPhoto/Graça Victoria

Testing of actual materials, such as sheet- and fluid-applied membranes, is performed in the laboratory due to the very small quantity of air leakage being measured and the high degree of accuracy required in the measurement. Air barrier products are required by most codes to allow no more than 0.02 L/s.m2 @ 75 Pa (0.004 cfm/sf at a pressure differential of 0.3 in. of water). In reality, most sheet membranes (such as self-adhered rubberized asphalt products) exceed this criteria by an order of magnitude or greater—much too low to be reliably measured in the field.

Air barrier assemblies—essentially, air barrier materials in an as-built condition that includes laps, seams, and penetrations—can be tested in either the lab or the field. Laboratory testing per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, provides an air leakage rate for a pre-defined arrangement of air barrier products, penetrations, and a window opening (but not the window itself—an oft-overlooked element of the air barrier system).

While primarily a laboratory test used by air barrier manufacturers to demonstrate their products’ performance and code compliance, the method can also be applied to field-installed mockups. However, applying this test in the field is not as simple as installing a chamber on the interior and testing the exterior. Air leakage through the perimeter of a sample area (e.g. through a concrete block or stud wall perpendicular to the interior-exterior direction) is often impossible to isolate, and due to the relatively low leakage rates being measured, even a small amount of extraneous leakage can create a false negative test result. Using this general chamber testing approach on a qualitative basis is simpler and often more effective, since telling a contractor that the test result was 0.25 L/s.m2 (0.05 cfm/sf)—in other words, a failure—does not provide the same level of usefulness as telling them there were leaks at one membrane seam and two brick ties that need to be repaired.

Specifications for field-installed air barrier assemblies often include testing of the window as part of the assembly. While this makes sense from a practical standpoint (i.e. the connection to the actual window system is a critical transition in the air barrier), there are limitations to this test. From a practical standpoint, mockup testing of air barrier assemblies typically happens at the beginning of a project, often long before the windows are delivered to the site (or in some cases, before specific window products are even selected). Testing of the assembly with a ‘dummy window’ in place is possible, but results can be misleading since the actual connection is unlikely to be the same as what is intended for the project windows.

In cases where the dummy window is put in temporarily with sealants and sprayed-applied foam insulation, the actual leakage rate may be much lower than what will occur when the project windows are installed, giving a false positive result for the test. In the case of the project windows being available at the time of testing, the specification of pass/fail criteria for the air barrier assembly test becomes more important. The performance criteria for air barrier assemblies are based on a window perimeter being included, but not the window itself. Since most windows will experience significantly higher leakage (on an area basis) than air barrier materials and assemblies, leakage through the window may far exceed the air barrier assembly leakage criteria, even if the assembly itself, minus the window, would pass on its own. For this reason, it is important to clearly specify how and if the window is to be included in the test, and, if so, some increase in allowable air leakage needs to be included to avoid requiring a result which is not attainable in the field.

The third type of air barrier testing is whole-building testing, using blower door or similar equipment and ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, procedures to measure overall air leakage through the entire building—including all walls, roofs, and windows. Different codes and standards require different overall leakage rates, but 0.02 L/s.m2 @ 75 Pa (0.4 cfm/sf at 0.3 in. of water) is typical for most building and energy codes.

While this test provides a single number to describe air leakage that can be easily compared to other buildings, it has many limitations that must be considered before requiring a certain level of whole-building performance. The first major issue is at the time when the air barrier is substantially complete to the point where testing can be performed, it is also likely to be concealed by cladding and other materials that can make the detection (using IR thermography or tracer smoke) and repair difficult or impossible.

Second, the testing itself can be difficult to perform, especially on large/more complex buildings, due to the need for multiple fan systems that must all be linked together for measurement or adjustment. In tall buildings, internal fans may be needed to equalize pressure differentials over the height of the building. While most testing firms can easily come to a site and perform standard window or curtain wall tests, large-scale testing of whole buildings requires specialized equipment and a fair degree of experience and expertise to successfully execute.

Lastly, there is some debate over what is achievable in terms of air leakage through whole buildings. A designer can certainly specify the overall leakage rate needs to be 0.5 L/s.m2 (0.1 cfm/sf), but achieving that level of airtightness requires an exceptionally well-designed air barrier, as well as carefully planned execution of the construction.

This is a common mistake with all types of air barrier testing—specifying a high level of airtightness without providing the corresponding design detailing is a futile effort. As mentioned, once the building is physically ready to be tested, it is often far too late to practically implement repairs, which brings up the difficult question of “What do we do now that we failed the test?” While the industry is still working on answering that question, specifiers can help avoid problems by specifying reasonable levels of airtightness appropriate for the building design.

As with the other previously described tests, visualization techniques such as infrared thermography and tracer smoke can be used to take advantage of the pressure differential created during a whole building test and qualitatively identify air leakage sites (Figure 7).

Conclusion
While the wide variety of available testing standards means there is almost always a standard for the designer’s specific need, finding the right standard can be difficult when one does not have a firm understanding of the actual goals. Designers and specifiers should first evaluate the question of ‘why’ when it comes to testing, as the answer will typically guide them to the correct test method to follow.

Specifying both the appropriate testing method and the appropriate pass/fail criteria are necessary to provide meaningful test results and avoid the time and expense of unnecessary testing or inappropriate testing which leads to ambiguous results. A little more time spent researching test methods during the design phase and specifying appropriate methods and performance criteria can go a long way toward reducing confusion and disputes during the construction process in the field.

Notes
1 For more on RILEM tubes, see The Construction Specifier articles, “Testing the Test: Water Absorption with RILEM Tubes,” by Adrian Gerard Saldanha and Doris E. Eichburg, and “Durable Waterproofing for Concrete Masonry Walls: Redundancy Required,” by Robert M. Chamra and Beth Anne Feero in the August 2013 and July 2014 issues. (back to top)
2 For more on air barriers, see the article “Wind Load and Air Barrier Performance Levels,” by Maria Spinu, Ben Meyer, and Andrew Miles, in the July 2014 issue. (back to top)

Sean M. O’Brien, PE, LEED AP, is an associate principal at the national engineering firm Simpson Gumpertz & Heger (SGH), specializing in building science and building enclosure design and analysis. He is involved in both investigation/forensic and new design projects. O’Brien is a member of the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), co-chair of the New York City Building Enclosure Council (BEC-NY), and a frequent speaker and author on topics ranging from building enclosure design to energy efficiency. He can be reached at smobrien@sgh.com.

David Artigas, PE, is senior staff I–building technology at SGH, specializing in building enclosure design and investigation, building science, and historic preservation. He can be reached at djartigas@sgh.com.

Introducing Siphonic Roof Drainage: Common in Europe, now gaining traction stateside

All images courtesy Zurn Industries

All images courtesy Zurn Industries

by William Verdecchia

All roofs are subject to the destructive effects of seasonal weather changes, environmental conditions, loading, and air pollutants. Alternate cycles of wetting, drying, freezing, and thawing caused by water lying on the roof leads to expansion, contraction, and rotting—this risks damage to the roof and even the building substructure.

Drainage is a significant component of roof design itself. Roof collapses typically occur because water accumulation exceeds the roof’s structural capacity. With proper water drainage in place, many major causes of failure are eliminated.

For this reason, siphonic roof drainage is coming into its own in the United States. First developed in Finland by engineer Ovali Ebeling in 1968, these systems are used around the world—in Europe, they account for one-fifth of commercial projects. This sustainable technology crossed the Atlantic in 1999 with the Boston Convention Center’s installation as the first major example, and acceptance has steadily grown.

Precipitation rate maps help designers create the best drainage systems for an area given expected rainfall.

Precipitation rate maps help designers create the best drainage systems for an area given expected rainfall.

Siphonic roof drainage differs from conventional gravity drainage in what is called ‘full-bore flow.’ Unlike conventional drainage, a fully engineered siphonic roof drain system prevents air from entering, allowing the pipes to be completely full of water. The unique component of a siphonic drain that sets it apart from conventional gravity drains is the air baffle, which prevents air from entering the piping system at full flow and protects against debris.

In October 2013, a new standard developed by the American Society of Plumbing Engineers (ASPE) was approved by the American Standards Institute (ANSI) as ASPE/ANSI 45-2013, Siphonic Roof Drainage. (The testing standard remains American Society of Mechanical Engineers [ASME] A112.6.9-2005.)

Considerations for roof drainage design
The most basic functions of a roof drainage system are to carry off rainfall, directing it to an underground piping system or drainage ditch, thereby removing the possibility of water penetration into the membrane or building envelope. This rainwater management carries another market expectation: a sustainable approach to issues related to water conservation, stormwater runoff, and rainwater-harvesting.

When designing a robust roof system, the following factors come into play:

  • building location;
  • roof assembly/type of construction;
  • roof pitch/slope;
  • volume of expected rainfall (i.e. precipitation rate measured in inches/hour);
  • desired rate of drainage; and
  • roof load requirements.

Additional considerations for architects, design engineers, and specifiers are:

  • drain size and features;
  • drain placement and location;
  • overflow safety requirements;
  • building and plumbing code requirements;
  • vandal-proofing; and
  • aesthetics.

Each project location has its own historical rainfall data that include records of accumulation, intensity (i.e. duration and frequency), drop size, and terminal velocity. Rainfall intensity plays a significant role in determining the type, quantity, size, and placement of roof drains to be installed for optimal system design. (As every project is different, consultation with the roof drainage manufacturer is essential.)

Siphonic roof drainage systems provide a number of benefits to a building owner, including lower construction costs, self-cleaning capability, water conservation, lower energy consumption, and reduced natural resources depletion. This 381-mm (15-in.) diameter main roof drain has a clamping collar and low-silhouette poly-dome.

Siphonic roof drainage systems provide a number of benefits to a building owner, including lower construction costs, self-cleaning capability, water conservation, lower energy consumption, and reduced natural resources depletion. This 381-mm (15-in.) diameter main roof drain has a clamping collar and low-silhouette poly-dome.

A 363-mm ( 14 9/32-in.) diameter siphonic overflow roof drain with standard deck. The key to the operation of a siphonic system is eliminating all air from entering the piping system. This is achieved by placing an engineered secured baffle into the base of the sump that breaks up the Coriolis Effect of rotating water.

A 363-mm ( 14 9/32-in.) diameter siphonic overflow roof drain with standard deck. The key to the operation of a siphonic system is eliminating all air from entering the piping system. This is achieved by placing an engineered secured baffle into the base of the sump that breaks up the Coriolis Effect of rotating water.

Figure 1 indicates the precipitation rates of numerous locations expressed in inches per hour and based on 15 minutes of precipitation (extrapolated from historical rainfall data collected over a period of 10 to 100 years). Reading this map, a design engineer would size a building drainage system in the Carolinas to handle a 178-mm (7-in.) hourly rainfall. Roof drain systems for most of New York State and Michigan would be sized to accommodate 102-mm (4-in.) hourly rainfall.

Traditional roof drains
Traditional roof drainage systems rely on gravity and water’s ability to spread out and flow to the lowest point. As the water accumulates, the depth increases and becomes the driving force causing it to flow through gutters to the roof outlets. Each outlet has its own down-pipe directing water underground. Unfortunately, as water enters the down-pipe, air is also drawn in, reducing the drainage system’s efficiency.

Figure 2 is a table to help size gravity roof drains by following these steps:

  1. Calculate total roof area.
  2. Determine and select the size of leader (i.e. roof drain, down-pipe, conductor, or downspout) to be used.
  3. Use precipitation map to find rainfall rate for building’s location.
  4. Cross-reference leader size with hourly rainfall in chart to obtain roof area that can be handled by each leader. For example, using a 102-mm (4-in.) leader for a location with a 102-mm hourly rainfall, each drain can handle 427 m2 (4600 sf) of roof area.
  5. Divide total roof area by area found in Step 4 to obtain the number of drains required. For example, 13,936 m2 (150,000 sf) divided by 427 m2 equals 32.6—this means 33 drains, equally spaced and symmetrically located.

Other non-siphonic roof drain systems
Regions affected by tropical storms or other severe weather phenomena make it necessary to transport large amounts of runoff water as fast and efficiently as possible. Designed for volume efficiency, high-capacity roof drains are more than 30 percent larger than standard ones. Careful consideration must be given to roof drain location and outlet pipe diameter.

Controlled flow roof drains are ideal for dead-level or sloped roofs, and for areas with restricted stormwater drainage capacity. With these drains, excess water accumulates on the roof under controlled conditions. The water is drained off at a metered rate after a storm abates. The key is to use a large roof area to temporarily store the maximum amount of water.

A controlled flow roof drain system requires fewer drains, smaller diameter piping, smaller sewer sizes, and lower installation costs. Another benefit is these systems reduce the probability of storm damage by lightening the load on combination sewers and reducing the probability of flooded sewers and backflow into basements and other low areas. The stored water on the roof also can act to temporarily improve the heat loss characteristics of the roof. To ensure success, designers and specifiers for controlled flow roof drainage design must carefully consider drain location, roof deflection, scupper sizes, overflow drains, and roof loading.

Siphonic roof drainage
Developed to operate with 100 percent full flow for increased discharge, smaller pipe diameters, and no drainage slope, siphonic roof drainage’s first commercial installation was at a Swiss turbine factory in 1972.

The theory behind siphonic roof drainage systems traces back to one of the fundamental equations of fluid mechanics—Bernoulli’s Energy Equation, named for the 18th-century Swiss mathematician and physicist Daniel Bernoulli. The energy balance equation holds when a fluid, at rest or in motion, possesses three fundamental forms of energy—static pressure, kinetic, and potential—the sum of their states is conserved and remains constant, even though the system energy states may be transferred from one to another.

The equation assumes the fluid is incompressible, that no work is done or performed on the system, and the system is adiabatic (i.e. no heat is gained or lost). It is used to determine change in flow between any two points in a drainage system. Numerous modifications of the equation have been made for specific applications. Siphonic theory itself has also undergone revisions to take into account losses due to friction in a length of pipe.

In their current design, siphonic roof drains look similar to conventional gravity roof drains; they share features such as a drain body, dome strainer, and membrane clamping device. The component unique to siphonic systems is the highly engineered air baffle.

This table assists designers with how to best size a gravity roof drain.

This table assists designers with how to best size a gravity roof drain.

The air baffle is secured into the sump of a standard drain, preventing vortex flow. In other words, it prevents the Coriolos Effect, which typically forces water to rotate around the drain and draw air down the center and into the pipe. By prohibiting air from entering the tailpipe and horizontal collecting piping, a negative head pressure is created in the collector pipe and the water is siphoned off the roof. The atmospheric pressure above the drain becomes the system’s driving force.

Location of the baffle in the drain sump is critical to the drainage system design. Locating the baffle lower in the drain body minimizes the amount of water on the roof that is required to make the drain go siphonic.

Once the rainwater is drawn through the drain and into the tailpiece, it then travels to the horizontal piping, located just below the roof. In this section of the system, the water continues to depressurize, and piping size increases to prevent cavitation or the pipe walls from imploding under the negative pressure. When the rainwater reaches the vertical stack, it stays at full bore flow but continues to pressurize as it moves downward to the zero point (i.e. siphonic break). The pipe turns down into a vertical downspout and transfers to conventional gravity drainage when below grade by expanding the pipe diameter.

The driving hydraulic head of the system is the entire height from the top of the roof to the discharge point, as opposed to a conventional system where only the roof water acts as a head pressure. Due to this, the siphonic system allows for higher flow capacities and velocities than a conventional system with the same sized piping. The higher velocities also mean a siphonic system can be considered ‘self-cleaning,’ eliminating the need for cleanouts in the piping.

Ideal applications
Siphonic roof drains provide full-bore flow when used in conjunction with a fully engineered/designed piping system. The full-bore action is achieved through natural hydraulic action. The system is designed to use the full volume of the piping—the water goes siphonic when the pipes are completely full.

Siphonic systems require fewer downpipes and smaller pipe sizes and need less space. This means greater design flexibility, reduced installation times, less material resources, and cost savings. They can be used on all buildings regardless of size, height, or exposure to rainfall. However, they are most efficient on low-rise buildings with large footprints, along with shopping malls and factories.

This is because in a siphonic system, the water moves through the horizontal piping at negative pressure, but increases in pressure as it drops vertically through the leader until the point it reaches zero pressure and transitions to gravity flow by increasing the pipe size. Since this transition typically happens after dropping only a few floors, most of the drop will be oversized gravity piping on a high-rise building—this excludes the traditional benefit of reduced pipe size for the entire building height. Further, the larger the footprint of the building, the more negative pressure can build up in the piping, which allows it to drop down the side of the building further before transitioning. Most high rises have smaller footprints and small roof areas—this does not allow a lot of negative pressure to build in the horizontal piping.

CS_March_2014.indd

A siphonic roof drain system was installed on a new Volkswagen plant in Chattanooga, Tennessee. Siphonic design software provided sizing calculations so the installed system would work as engineered. The project scope was more than 46,451 m2 (500,000 sf) and included 85 siphonic roof drains.

Specifying siphonic roof drainage
In the United States, the standard siphonic drain is 380 mm (15 in.) in diameter. Outlet sizes include 50-, 76-, and 102-mm (2-, 3-, and 4-in.) no-hub mechanical joint connections. Drains and clamp collars can be made from iron or stainless steel, while domes are made from polypropylene, aluminum, bronze, or iron. Specifiers should consider using stainless steel vandal-proof hardware to help reduce corrosion in the assembly.

A siphonic system is available with many of the basic, conventional roof drain options. Mounting devices such as deckplates are recommended to help speed installation. A drain riser and adjustable extensions assist in leveling the roof drain during construction. For gravel rooftop applications, a gravel guard can help ensure proper drainage. If desired, an overflow drain can be used with standard siphonic systems to connect to a separate drain line, discharging to an outside location instead of a sewer.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains (Figure 3). First, one decides what outlet size is needed (left column) and what rainfall rate is necessary (rows across). Then, one finds the number in the box where the column and row values meet. The number is the square footage each siphonic drain can cover under those conditions.

When comparing the siphonic to the conventional roof drain sizing chart, it becomes apparent the siphonic pipe size will be roughly half of a conventional drain covering the same area. (The chart should be used for a preliminary estimate, and should not take the place of consulting a manufacturer’s engineering or technical staff for complete piping layout.)

Another important step is to check the local plumbing code to see whether a rainfall rate is dictated and if siphonic drainage systems are addressed. In many cases, plans will need to be submitted as an engineered system or variance. A licensed professional engineer must verify the system will work as designed.

Siphonic design guidelines
Siphonic design software is now used to design and calculate if minor changes during installation fall within the design’s acceptable range of pressures and velocities in the piping. The aforementioned ASPE/ANSI 45-2013 should be reviewed to determine how installation differs from a conventional drain system. One main difference is the use of eccentric reducers whenever there is a change in pipe diameter. These help maintain a flat surface along the top of the pipe, eliminating any air pockets.

No more than 4645 m2 (50,000 sf) of roof area should be tied into one common collector pipe (otherwise, the system can be very hard to balance due to the large distance between drains and large pipes used). Further, roof areas made from different types of roof materials should not be tied into a common collector pipe. These roof materials have different coefficients of discharge, and would not be able to draw a siphon at the same time if tied into the same pipe. (Different fittings can also have dissimilar coefficient of friction ratings—software should be used to select the most appropriate type of transition.) Roof areas with extreme variation in slope, or roof areas at different roof levels, must also drain into separate collector pipes.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains.

Determining the number of siphonic roof drains needed is similar to the same process for conventional roof drains.

One must avoid not only multiple vertical drops in a single piping system, but also piping below an obstruction. Both these issues would delay the system from priming, slowing it from going siphonic.

Every roof trough must have at least one siphonic drain present in that area. When initially designing a system, drains tied to a common collector pipe should be spaced no more than 20 m (65 ft) apart, and stacks should be located no more than 20 times the building height away from the furthest drain. However, these dimensions are only guidelines—they can be increased if the system can be properly balanced. Pipe lengths longer than 20 m should be divided into smaller sections to accurately determine where the pipe diameter can increase from one size to the next.

At least 1 m (3 ft) must be maintained between the surface of the drain and the center of the horizontal collector pipe. Tailpieces attached to the bottom of the drain must be at least 0.5 m (21 in.) long before tying into the horizontal pipe.

For siphonic systems, each individual and total system rating must fall between negative 10.13 kPa (1.47 psi) and positive 10.13 kPa. It is important for overall imbalance of the system be as close to zero as possible. Cast iron or polyvinyl chloride (PVC) piping can withstand a minimum pressure of negative 90 kPa (13 psi). If pressure is too low, pipe diameter must be increased to relieve pressure.

The flow rate of the ‘zero pipe’—the first section of pipe after the siphon has been broken and the transition to gravity drainage has been made—must be less than 2.5 m (8 1/4 ft) per second to prevent damage to the storm sewer. In many cases, flow rate is reduced to around 1 m (3 ft) per second to align with traditional pipe sizes based on the roof area and the rainfall rate. (This flow rate is the minimum for ensuring the system remains self-cleaning.)

Conclusion
This system of roof drainage is beginning to gain traction in the United States. Statistics are difficult to come by, but the wide acceptance of siphonic roof drainage in Europe and other parts of the globe provide reassurance and a backdrop of success. The key for any project, including those involving siphonic roof drainage, is to ensure one deals with reliable manufacturers with track records for delivering engineered solutions. Since every project is different, consultants must be willing and able to give the time and expertise required.

William Verdecchia is the director of product management and engineering for Zurn Specification Drainage. He has more than 25 years of experience in the construction industry; with five years as a construction professional and 20 years leading innovation, product commercialization, and sales/marketing activities for Zurn Industries. In his current role, Verdecchia oversees product lifecycle management activities and application support for plumbing products. He has been awarded numerous plumbing industry patents and is an active board member for the Plumbing and Drainage Institute, Penn State Behrend’s Plastic Engineering Program, and Gannon University’s Mechanical Engineering Program. He can be reached at william.verdecchia@zurn.com.

A Light Discrepancy?

We received an e-mail from a reader regarding an article that appeared in our November 2013 issue:

I read the article titled “Rethinking Cool Roofing: Evaluating Effectiveness of White Roofs in Northern Climates,” by Craig A. Tyler, AIA, CSI, CDT, LEED AP. Although I agree with what I think is the article’s concept, the author repeatedly states ultraviolet (UV) absorption into the roof system contributes to heat gain in a building or that UV reflection helps to keep the roof cool. UV is not only not responsible for heat gain through absorption into the roof system, but it is also entirely the opposite end of the spectrum from the infrared (IR). UV exposure has long been known as a contributor to the breakdown of roofing materials.

When the author was contacted, he responded:

The reader brings up a valid point about infrared being at work here as well. Sunlight consists of infrared, visible, and UV electromagnetic radiation, known as ‘solar radiant exposure.’ Infrared, while commonly associated with heat radiation, is not the only component of sunlight that contributes to temperature rise. All sunlight will heat surfaces that absorb the solar radiation. Infrared consists of 49 to 53 percent of sunlight, with the remainder being mostly visible and UV light. UV exposure from sunlight reaching the earth’s surface can cause degradation to many types of construction materials, as well as people. Degradation can be observed as a reduction in physical properties, cracking/crazing, loss of gloss, and discoloration.