Tag Archives: B2080.50−Exterior Balcony Walls and Railings

Spontaneous Glass Breakage: Why it happens and what to do about it

Photo © Wes Thompson. Photo courtesy PPG

by Michael L. Rupert

The past few years have seen several highly publicized incidents involving window and balcony glass breaking spontaneously and falling from high-rise buildings in Toronto, Chicago, Las Vegas, and Austin, Texas. While such episodes are rare, the danger they pose has forced building code writers, architects, government officials, and related industry professionals to reconsider which types of glass should be specified for glass applications where strength and protection of passers-by are paramount.

For architects and specifiers, it is important to have an overview on the potential causes of spontaneous glass breakage, including some common misconceptions about its actual spontaneity. The term ‘safety glazing’ generally refers to any type of glass engineered to reduce the potential for serious injury when it comes into human contact. In addition to balcony glass, safety glazings are commonly required for:

With an increase in spontaneous glass breakage incidents, the glazing industry is looking at new ways to make assemblies safer. Images courtesy PPG Industries

sliding glass doors;
shower doors;
patio furniture;
skylights;
oven glass; and
automobile windshields.

Tempered, laminated, and heat-strengthened glass
The most common type of safety glass is tempered glass, which is made by heating pre-cut panels of glass to about 650 C (1200 F), then cooling them rapidly through a process called ‘quenching.’ By cooling the outer surfaces of the panel more quickly than the center, quenching puts the surfaces and edges of the glass in compression and the center of the glass in tension.

In addition to making tempered glass four to five times stronger than conventional annealed glass, re-heating and rapid quenching dramatically changes the break characteristics of the glass. Consequently, when tempered glass is broken, it shatters into thousands of tiny pebbles—this practically eliminates the danger of human injury caused by sharp edges and flying shards.

Another type of safety glazing, laminated glass, is made by sandwiching an interlayer of vinyl (typically polyvinyl butyral [PVB]) between two layers of glass to hold the panel together if it is broken. Although laminated glass is most commonly associated with windshield glass for automobiles, it is increasingly being specified for storefronts, curtain walls, and windows to meet codes for hurricane-resistant glass.

A third option, heat-strengthened glass, is not technically a safety glazing. This is because when it breaks, it may form larger sharp shards that can cause serious injury. However, heat-strengthened glass still meets Consumer Product Safety Commission (CPSC) 16 Code of Federal Regulations (CFR) Part 1201 and Class A of American National Standards Institute (ANSI) Z97.1, Safety Glazing Materials Used in Buildings−Safety Performance Specifications and Methods of Test, for many safety glass applications when it is combined with a laminated interlayer that holds the glass together if it breaks.

As with tempering, heat-strengthening involves exposing pre-cut glass panels to temperatures of up to 650 C, but with a slower cooling process. Heat-strengthened glass is not as strong as tempered glass because the compression strength is lower—about 24,130 to 51,710 kPa (3500 to 7500 psi) compared to 68,950 kPa (10,000 psi) or greater. However, it is about twice as strong as annealed glass. For this reason, heat-strengthened glass is often specified for applications demanding resistance to thermal stress and snow- and wind-loads.

Causes of glass breakage
The incidents of spontaneous glass breakage in Chicago, Las Vegas, Austin, Texas, and Toronto occurred exclusively with tempered glass. Despite that material’s high levels of strength and capacity to meet safety glazing requirements, it is uniquely vulnerable to these types of failures. Ironically, the center tension zone engineered into tempered glass through the quenching process is also what makes it so vulnerable to catastrophic breakage.

Safety glazings are commonly required for sliding glass doors, shower doors, and patio furniture. ‘Safety glazing’ generally refers to any type of glass that is engineered to reduce the potential for serious injury.

Poor edge quality
There are many potential causes for spontaneous breakage of tempered glass. The most common is damage to the edges of glass as it is being pre-cut into panels, or nicks or chips to the edges that occur when the glass is being packaged, shipped, or installed onsite.

While such damage may not be readily apparent, stress concentrations around these imperfections can occur as the glass expands and contracts in response to in-service temperature changes, wind load, building movement, and other environmental factors. Ultimately, when those stresses cause the glass to break, the action may appear to have been spontaneous when, in fact, the circumstances for failure had been put in place months or even years earlier.

Frame-related breakage

Expansion and contraction of glass framing members may also lead to frame-related breakage—another common form of seemingly spontaneous failure. Such incidents occur when the gaskets, setting blocks, or edge blocks in a metal window or curtain wall frame are missing or do not sufficiently cushion the glass against glass-to-metal contact caused by temperature or wind-related movement. This can cause edge and surface damage to the glass as it comes in contact with the metal frame’s perimeter, producing stresses that eventually lead it to fail for no apparent reason.

Thermal stress
Another potential cause of spontaneous glass breakage is thermal stress. Thermally induced stresses in glass are caused by a positive temperature difference between the center and edge of the glass lite, meaning the former is hotter than the latter. The expansion of the heated glass center results in tensile stress at the edge of the glass. If the thermally induced stress exceeds the edge strength of the glass, breakage occurs.

Accounting for thermal stress is especially critical today, as current design trends and the desire for daylighting are driving the industry toward the specification of larger insulating glass units (IGUs) with high-performance solar control coatings. Large IGUs have inherently greater glass surface and edge areas. When they are combined with coatings designed to manage the sun’s energy, more rigorous thermal stress analyses are required.

Nickel-sulfide inclusions

A far less common—but often cited—cause of spontaneous glass breakage is nickel-sulfide (NiS) inclusions in tempered glass. Small nickel-sulfide stones can form randomly in the production of float glass. They are typically benign, even when occurring in tempered glass.

North American glass manufacturers do not use nickel in batch formulations for primary glass and go to great lengths to avoid nickel-bearing components in their glass-melting processes. Despite rigorous quality controls and procedures aimed at reducing the likelihood of nickel-sulfide stones, there is no technology to completely eliminate their formation in today’s float glass.

Nickel-sulfide particles are tiny, extremely rare, and only found randomly in float glass. This combination makes visual inspection for such inclusions highly impractical—if not outright impossible.

There is no known technology that completely eliminates the possible formation of nickel sulfide stones in float glass. Further, because nickel sulfide stones are so small, there is no practical way to inspect their presence in float glass.

Nickel-sulfide stones are quite small and their occurrence in the final glass product is covered under ASTM C1036, Standard Specification for Flat Glass, which permits blemishes (including nickel-sulfide particles) of between 0.5 and 2.5 mm (1/50 to 1/10 in.) in float glass, depending on glass size and quality.

While nickel-sulfide inclusions may be present in annealed or heat-strengthened glass, the problems they cause are specific to tempered glass because of the tempering process. Breakage is due to a volumetric growth in the size of the stone. As detailed earlier, during the annealing and heat-strengthening processes, glass is cooled at slower, controlled temperatures that enable nickel-sulfide particles that are present to complete a phase transformation (known as the ⍶ to β phase change) during which they fully expand to their final size and remain stable thereafter.

In the tempering process, this phase transformation is arrested during rapid quenching, which causes any nickel-sulfide particles present to remain confined to their shrunken, pre-transformation states. Then, when the tempered glass is exposed to higher in-service temperatures caused by solar heat gain or other high-temperature influences, nickel-sulfide particles have the potential to resume their volumetric growth. If the expansion is large enough—and the particle is located in the center tension zone of the tempered glass panel—the resulting stress may be enough to shatter the glass.

Is heat-soaking a solution?
As indicated, nickel-sulfide particles are tiny, extremely rare, and only found randomly in float glass. This combination makes visual inspection for such inclusions highly impractical, if not impossible. For that reason, some glass fabricators and glazing contractors offer heat-soaking of tempered glass as a potential solution for minimizing the risk of spontaneous glass breakage.

When tempered glass is broken (as shown above), it shatters into thousands of tiny pebbles, practically eliminating the danger of human injury caused by sharp edges and flying shards of glass.

The surface compression of heat-strengthened glass makes it approximately twice as strong as annealed glass. Heat-strengthened glass is typically used when glass is required to meet thermal or mechanical loads caused by heat, wind, or snow.

In this procedure, the glass supplier exposes an entire lot or statistical sampling of tempered glass panels to temperatures of 288 to 316 C (550 to 600 F) for two to four hours. The goal is to initiate or accelerate the phase change of any nickel-sulfide inclusions that may be present and to cause the glass to break before it is shipped to the end customer.

While this ‘break-now-instead-of-later’ procedure may eliminate defective tempered glass panels by destroying them before they are shipped, it cannot provide a 100 percent guarantee against spontaneous breakage. Even advocates of heat-soaking are careful to state the procedure can only reduce or minimize the risk for spontaneous breakage of tempered glass. They will not use words such as ‘prevent’ or ‘guarantee.’

Risks of heat-soaking
There are risks associated with the heat-soak procedure that may outweigh any perceived benefits. For instance, small, stable inclusions could undergo the beginning of a phase change during the heat-soak. While the phase change may not be sufficient to cause breakage during the procedure, the transformation could potentially continue after the glass is installed, causing it to break in-service.

Re-exposing tempered glass to the increased temperatures of heat-soaking also has the potential to reduce its surface compression, which is the source of its strength. Ultimately, this may undermine the glass’s ability to fulfill the safety or strength requirements for which it was intended.

Further, heat-soaking adds another layer of handling to the manufacturing process, which creates more opportunities for edge damage, scratches or color changes to the low-e coating, and other imperfections that could have an impact on the tempered glass unit’s long-term durability and performance.

As these illustrations demonstrate, the heat-strengthened glass and tempered glass have distinctive breakage patterns.

Solutions for safety
In recent months, two organizations made major announcements prompted largely in response to the incidents of falling glass in Toronto, Chicago, Las Vegas, and elsewhere. Both shared a common assessment—namely, that using laminated tempered glass or heat-strengthened glass is the most viable solution to making balcony and other types of overhead glass safer.

In Canada, an Expert Panel on Glass Panels in Balcony Guards established by the Ontario Ministry of Municipal Affairs and Housing (MAH) recommended local building codes be amended to mandate the use of heat-strengthened laminated glass for any outboard guard or glazing located beyond the edge of a floor, or within 50 mm (2 in.) of the edge of a floor. For outboard glazings located more than 50 mm inward from the edge, the panel recommended heat-soaked tempered glass or heat-strengthened laminated glass.

Similarly, in December 2012, the International Code Council (ICC) passed a code change proposed by the Glazing Industry Code Committee (GICC), mandating use of laminated glass in handrail assemblies, guardrails, or guard sections. The newly approved code states laminated glass must be constructed of either single fully tempered glass, laminated fully tempered glass, or laminated heat-strengthened glass, and comply with CPSC 16 CFR Part 1201 or Class A of ANSI Z97.1.

San Francisco’s Intercontinental Hotel showcases the design trends and desire for daylighting, which are driving the industry toward specification of larger insulating glass units (IGUs) with high-performance solar control coatings. The architect of record is Hornberger + Worstell; the design architect is Patri Merker Architects. Photo © Tom Kessler. Photo courtesy PPG Industries

Fully tempered glass and heat-strengthened glass are made using the same basic heating and quenching process.

Conclusion
Given the developments and recommendations outlined in this article, it is clear a laminated glass interlayer in combination with tempered or heat-strengthened glass may offer the optimal blend of characteristics for applications where the risk of injury from glass fallout is a primary concern. For non-safety glass applications, where strength and resistance to spontaneous breakage is desired, non-laminated heat-strengthened glass should be considered due to its lower costs.

Michael L. Rupert is PPG Industries’ director of technical services and product development for flat glass. A 39-year company veteran, he is a board member for the Glass Association of North America (GANA) and chairs the Flat Glass Manufacturing Division. Rupert holds a bachelor’s degree in civil engineering and an MBA from the University of Pittsburgh. He can be reached at mrupert@ppg.com.

Ensuring Balcony Durability: Waterproofing details that stand the test of time

All images courtesy Building Diagnostics Inc.

by David H. Nicastro, PE, and Marie Horan, PE

Wood-framed balconies experience a high rate of failure: leaks, visible damage on the finishes below, and, worst of all, concealed structural damage from continued water migration. By the time structural distress becomes evident, it may be too late to implement waterproofing remedies alone—countless wood-framed balconies have required replacement because of severe rot.

The durability of wood-framed balconies widely varies. There are subtle but important differences between the construction of balconies that function for the building’s design life and those that prematurely fail.

Balconies have many of the same details as other portions of the exterior building envelope, but there are also challenging details specific to this type of construction—topping slab edges, column penetrations, door sills, and handrail connections. They are vulnerable to decay because they catch rainfall and direct it to myriad intersecting planes.

Conventional balcony construction, consisting of a concrete topping slab over a waterproofing membrane over wood framing, is prevalent in multi-family residential construction, and it is also used in houses and some commercial properties. Wood rot of balcony framing is a well-known risk, but it is even more widespread than recognized. The authors made excavations into more than 200 balcony soffits in apartments built over a 10-year span, and found undetected water damage in more than 40 percent of them. Additionally, the visible detailing was reviewed on over a thousand balconies, and destructive evaluation and water testing were performed on selected ones.

The survey showed improper perimeter flashing details were the dominant cause of water infiltration. In addition to waterproofing details, distress was found to correlate with structural design, as discussed in this article. The accompanying photos show well-built new balconies, as well as failed conditions found during investigations.

Good balcony construction begins with stepping the wood framing down from the interior floor and sloping the deck.

The door pocket flashing is installed and tied into the wall waterproofing.

Start with the structure
There are a few types of waterproofing membranes that can be used underwater, but most work best when they are well drained. Even a small imperfection or nail hole becomes a leak when the membrane has standing water on it. Therefore, sloping the wood structure to promote drainage is recommended.

The Residential Sheet Metal Guidelines by the Sheet Metal and Air-conditioning Contractors’ National Association (SMACNA) suggests 20 mm per 1 m (1/4 in. per foot) minimum slope. This is also about equal to the maximum slope (two percent) allowed for landings outside accessible doors, which will govern for many residential balconies.

Regardless of codes and standards, most people do not want their balcony surface sloped more than two percent because patio furniture would have a noticeable tilt. It would be difficult to build less slope on the topping’s top surface than at the bottom—for these combined reasons a nominal two percent slope of the structure is practical (Figure 1).

The balcony structure should also be stepped down to permit the topping slab at its highest point to be lower than the interior floor level. This is especially important to prevent water infiltration at door thresholds.¹ For the majority of multi-family construction, this step will be limited to 13 mm (½ in.)—including the threshold height—because the Fair Housing Act and other codes and standards require balconies to be accessible with few exceptions.

The authors found much less distress in covered balconies than in those fully exposed. Clearly, the amount of impinging rain on a balcony impacts its durability—even after accounting for materials, design, and construction. Therefore, the authors recommend protecting balconies with roofs or stacked construction when possible.

It is important to note the survey involved balconies framed with non-treated wood. Preservative-treated wood can enhance durability at additional cost—for the wood deck, columns, dimensional lumber, and specially coated fasteners to resist corrosion induced by the preservative. However, treated wood trusses are not commonly used because of load reductions and corrosion protection required for the connection plates. The best strategy is to keep water away from the wood rather than trying to accommodate it.

In the survey, wood rot below balcony doors was common. At this example, defects include physical damage to the metal flashing (tearing and saw-cut) and an unsealed lap seam.

Perimeter flashings are installed and shingled behind the wall waterproofing.

Flash the openings
After the wood framing is constructed, the waterproofing installation begins with flashing the door openings (Figure 2). One of the most common locations the authors found wood rot was directly below doors. The three-dimensional door pocket presents many intersecting planes that need to have integrated waterproofing.² Pocket flashing should run past the door jamb and tie into the wall flashing behind the cladding.

In the survey, the observed construction defects that caused leaks at doors included:

thresholds not bedded in sealant;
exposed top edges of flashing (not captured under counter-flashing);
door flashing not integrated with wall flashing (i.e. ‘back-laps’ and gaps); and
open flashing seams.

Damaged flashing was also observed, probably from construction traffic; doorways are used by many workers before the thresholds are installed (Figure 3).

Flash the perimeter
The next construction step is to install flashing at the balcony perimeter, and the perimeter of any columns framing into the balcony. Ideally, sheet metal should be installed first for robustness, but may be omitted where little wetting is expected. Seams should be well-lapped and fully bedded, with sealant exuding from the seams.

The sheet metal should be covered with self-adhered flashing (SAF) or liquid-applied membrane (LAM). For maximum durability, the authors recommend detailing all edges and seams of SAF with LAM so water, heat, and age do not overcome the initial adhesion (Figure 4).

The perimeter flashings must be installed before the water-resistive barrier (WRB) on the walls, so the WRB will be shingled over the flashings. Flashings (and subsequently the membrane) should be installed in the largest pieces manageable to minimize laps, seams, and edges.

In the survey, the most common perimeter flashing defects encountered were related to out-of-sequence construction—water could flow off the topping slab surface into the open top edges of flashings that were back-lapped on top of the wall’s WRB. Also common were metal flashing seams that were not adequately sealed and shingled in the direction of water flow, allowing water to run into the seams. The materials should be installed from the low point upward, always lapping to promote drainage out of or away from the seams.

Another common location for wood rot was at drip edges, installed at the outer perimeter of the balconies before the perimeter flashing. In some cases, the drip edge did not cantilever far enough off the balcony edge; where not corrected before installing the wall cladding below, this condition allowed water to drain into the top of the cladding (Figure 5). More commonly, wood rot was found at the ends of the drip edges, where water could migrate through the discontinuity.

The designer should provide a detail cut through each transition between horizontal and vertical substrates, and an isometric view of each three-dimensional corner. Even better is to show the assembly of these critical details with step-by-step instructions, and provide an extra layer of waterproofing at these complex intersections. Simply requiring the materials be installed per the manufacturer’s recommendations is not enough instruction given the numerous intersections occurring on even the simplest balconies. Unfortunately, the authors have investigated failures on projects with excellent drawings that were ignored—construction monitoring is also essential.

In the survey, wood rot was common at drip edges. Here, the drip edge is not visible below the T-bar, so water is being directed behind the stucco.

The membrane is installed over the flashing, and all seams and edges are detailed.

Install deck membrane
If flashings and surface preparation are done properly, installing the membrane across the field of the deck is probably the easiest part (Figure 6). In the survey of completed balconies, some back-laps were found, but a more common condition was punctures. Placing the concrete topping soon after installing the membrane should limit damage from construction traffic.

Install T-bars
Before placing the concrete topping, metal T-bars are installed along the outside balcony edges to serve as screeds and as permanent forms (Figure 7). T-bars generally come in two styles:

those with weep holes, which must be stripped into the edge flashing so water is directed into the T-bar and its weeps; and
more commonly, those spaced above the flashing so water can drain out below them.

Placing a folded piece of SAF under each T-bar fastener location provides an adequate drainage gap and helps to seal the fastener holes (Figure 8). In the survey, the absence of this critical gap correlated with wood rot at the outside edge of balconies—water built up on the membrane until it found a weak point to leak through to the wood structure.

The T-bars are installed at the outside edges, and railing post embed plates are installed and stripped into the waterproofing.

A spacer, consisting of a folded piece of self-adhered flashing, is placed under each T-bar fastener to allow drainage under and to help seal the fastener penetration.

Install railing anchors
In the survey, railing posts were common locations of wood rot. Failures correlated with simplistic connections consisting of steel plates bearing on the topping slab fastened through (and damaging) the membrane. A better design is to secure railing anchors to the deck, stripped into the waterproofing, with an embed plate for welding a railing post, or with a stub extending above the concrete to mate with a hollow railing post (Figure 7).

Place concrete topping
A best practice is to perform a pre-covering inspection of the membrane, and seal any discovered holes just before placing the concrete. As a quality assurance (QA) measure, a water flood test can also be performed at this stage (Figure 9, ).

The concrete topping might not be considered part of the balcony waterproofing system, but it serves an important role in shedding water off the surface to protect the membrane and flashings—but only if it is properly sloped. Unfortunately, the topping subcontractor has little control over the slope. The lowest point is determined by the T-bar, and the highest point is set by the door threshold (with whatever step is permitted). Still, the contractor should use reasonable care in concrete finishing to ensure water sheds off the entire surface without ponding (Figure 10).

Conventional concrete quality control (QC) measures should also be used to minimize cracks, which allow excess water to reach the waterproofing layer. If patterned correctly and formed early, control joints limit cracking in the field of the concrete, which can be both an aesthetic and durability issue.

The balcony is flood-tested by filling with water.

The concrete topping slab is placed with careful screeding to ensure uniform slope. Control joints will be scribed into the fresh concrete to limit cracks in the field of the deck.

Conclusion
To avoid concealed decay later, it is important to construct each layer of wood-framed balconies in accordance with best practices. These include:

slope the substrate;
flash all transitions;
protect corners with additional waterproofing;
provide drainage from the membrane; and
slope the concrete topping and limit cracking.

With attention to detail, the durability of wood-framed balconies can match the design life of the building.

Notes
¹ See the article, “Understanding Why Doors Leak,” in the May 2013 issue of The Construction Specifier. (back to top)
² See the article, “Waterproofing Balconies,” in the August 2012 issue of The Construction Specifier. (back to top)

David H. Nicastro, PE, F.ASTM, is the founder of Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and resolving disputes arising from them. He is a licensed professional engineer, and leads the research being performed at Building Diagnostics’ testing center, The Durability Lab, at The University of Texas at Austin. He can be reached by e-mail at dnicastro@buildingdx.com.

Marie Horan, PE, was formerly a senior engineer at Building Diagnostics, specializing in the investigation of problems with existing buildings and designing remedies for those problems. She can be reached by e-mail at mhoran@buildingdx.com.

Structural Safety of Wood Decks and Deck Guards: Multi-family Balconies

by Joseph R. Loferski, PhD, and Frank E. Woeste, PE, PhD

Multi-family balconies are often framed with cantilevered wood, with a concrete covering and gypsum or vinyl ceiling. Structural drawings for a project typically contain a note describing the quality of lumber materials assumed and used by the structural engineer in the structural design process. For example, a materials note may read:

All framing lumber shall be No. 2 (minimum) D-FIR (or HEM-FIR) S-Dry.

S-Dry means at the time the lumber was surfaced (or planed) at the sawmill, the maximum moisture content (MC) of the individual lumber pieces was less than 19 percent. When structural engineers use a typical lumber note as presented, the intent is more inclusive than simple stating the MC of the product when manufactured, but it includes the assumed maximum MC of the lumber for the service life of the project (e.g. 50 years or more). The validity of their structural designs is conditioned on the assumption the lumber components will be protected from high MC conditions above 19 percent and liquid water.

‘Dry’ lumber that remains dry in-service is known to perform for centuries. It is a biological fact that manufactured ‘dry’ lumber that is exposed to liquid water, either continuously or intermittently, will absorb water and trigger the decay process. When lumber and lumber connections (i.e. nails, screws, and bolts) experience decay, they can no longer be relied on to function in-service as expected.

For the technical reasons cited, it is absolutely critical for the entire balcony and interface with the primary structure to be protected by a waterproofing system that protects all water exposed surfaces. For example, a guard system design detail may show a connection of the guard post to the outside balcony side(s). If the balcony sides are not protected from water entry, the guard system connections to balcony framing may be compromised due to hidden decay of the wood products.

In conclusion, waterproofing design/detailing by a professional is critical to the likely in-service performance of a balcony and balcony-structure interface. Further, field inspection of the waterproofing installations for strict conformance with the waterproofing details by the responsible design professional is recommended.

To read the full article, click here.

Structural Safety of Wood Decks and Deck Guards

by Joseph R. Loferski, PhD, and Frank E. Woeste, PE, PhD

Most residential deck-related accidents are caused by failure of the deck-to-house connection or of the guardrail system, which can cause a person to fall from the deck, resulting in serious injuries or death.

A sample of accident reports is given in Figure 1. Additionally, the authors have partial data dating back to 2001, showing the problem is widespread; accidents occur in nearly every region of the United States. The news reports often state the cause of a catastrophic deck failure was ‘over-loading.’ Based on media reports and the authors’ own investigations, however, many of the subject decks should have safely carried their load without collapsing.

Several media reports showing a sampling of deck collapses in the United States.

Why do some decks and guards fail and cause injuries? Several reasons for deck failures have been observed, including inadequate fasteners and connections, corrosion, improper materials, construction defects, and wood decay.

Decks are often designed as a collection of individual parts, rather than as a unified system of interrelated components. For example, the failure of the deck ledger connection to the house may be due to a combination of factors, including no or improper flashing leading to decay in the house band joist, and improper or inadequate fasteners. Guard failure is most commonly caused by failure of the connection of the guard post to the deck, the connection of the rails to the post, or connection of the pickets to the rails. Further, because decks are exterior structures permanently exposed to weather, long-term decay or fastener/connector corrosion is more likely, and can contribute to a failure.

This article specifically addresses residential decks, but multi-family and light commercial decks, balconies, and guards—with more demanding code requirements—can have similar failure modes, making the general concepts quite pertinent. The architect and specifier should exercise great care and diligence when specifying deck or balcony-type structures (and related guards) as they are likely to require expertise in waterproofing specifications and details. (See “Multi-family Balconies.”)

Problem overview
Deck collapses and guard failures generally can be traced to designers and contractors who focus on parts and components rather than taking a holistic approach. For example, a through-bolted connection between the guard post and the deck band joist rarely fails. However, the connection of the band joist to the deck joist often uses nails or screws inserted in the end-grain of the joists. Since the guard post is at least 914 mm (36 in.) above the deck surface, it acts as a lever, causing the band joist to ‘peel’ away from the joists.

Photo of a typical deck attached to house; the circle shows the ledger connection. [CREDIT] Photos courtesy Joseph Loferski

Photo of a typical deck attached to house; the circle shows the ledger connection. Photos courtesy Joseph Loferski

The 2012 International Residential Building Code for One- and Two-family Dwellings (IRC) does not include a prescriptive method or detail for connecting a guard post to a deck substructure. Therefore, many contractors are faced to determine what is needed for a safe guard post connection without any code guidance, except for the vague 890-N (200-lb) concentrated load requirement in IRC.

Specific language and prescriptive details in a future IRC edition on how to make a connection of the guard post to the deck that will safely resist this 890-N load is desperately needed by all parties involved—specifiers, architects, engineers, contractors, home inspectors, and the building code enforcement community.

IRC does contain prescriptive guidelines for bolt and lag screw attachment of deck ledgers to a solid-sawn house-band, based on work by the authors at Virginia Tech University in collaboration with researchers at Washington State University.¹

Deck ledger testing at Virginia Tech showing load application and deflection measurement made with a LVDT.

Design considerations for ledgers
Most often, decks are attached directly to the house using bolts or lag screws that connect the ledger to the house-band. On the other side, the deck is supported by a beam resting on columns bearing on concrete footings as shown in Figure 2.

Deck ledger connection tests were conducted to develop bolt and lag screw spacing requirements for various commonly used deck designs.² Figure 3 shows a deck ledger connection being tested for gravity load capacity. The test used a 2×10 (nominal) to simulate the house band and a 2×8 (nominal) pressure-preservative-treated (PPT) member to simulate the deck ledger. Two simulated deck joists were attached to the ledger. The load was applied to the joists until failure. A transducer measured the ledger’s deflection relative to the band joist.

The results were used to compute fastener spacing requirements based on tested capacity of the connection. IRC Table R507.2 (Figure 4) gives the ledger connection requirements, and IRC Section R507.2.1 provides the required placement of bolts or lag screws in the deck ledger connection. All parties involved must closely review the design assumptions and limitations in the caption (e.g. live and dead loads) and footnotes to Table R507.2.1.

Proper screw and bolt installation requirements—American Wood Council’s (AWC’s) National Design Specification (NDS) for Wood Construction with 2012 Supplement—must be followed for the fastener spacing requirements listed in the table.

Data courtesy International Code Council

Another consideration is the type of lumber and posts used to build the deck. Using alkaline copper quaternary (ACQ) and copper azole (CAB) wood preservatives is now common for deck construction. These preservatives are generally known to be more corrosive to steel fasteners than chromated copper arsenate (CCA) preservatives. Therefore, corrosion-resistant fasteners such as hot-dipped galvanized steel or stainless steel must be used for reliable performance.

Some decay fungi are ‘copper limited,’ meaning they can colonize wood products treated with copper-based preservatives such as ACQ and CAB.³ Research has shown even wood treated to ‘ground contact’ treatment retention levels may experience fungal decay.⁴ So, embedding support posts into the ground may not be the best option. To improve deck post longevity, posts should be placed on concrete piers above ground-line and connected with a corrosion-resistant post-connector to the concrete piers. While not necessarily a code requirement, 6×6 (minimum) posts are recommended for aesthetic reasons and are much less likely to undergo severe warp (i.e. bow and twist).

Typical deck guard system showing posts, rails, and pickets. [CREDIT] Photo courtesy Joseph Loferski

Typical deck guard system showing posts, rails, and pickets. Photo courtesy Joseph Loferski

Properly installed flashing between the deck and house to prevent water infiltration into the wall sheathing and house band is extremely important. The typical house band joist (or engineered rim board) is not made from PPT lumber. Therefore, water intrusion into the wall section can lead to wood decay around the bolt or lag screw and thus failure of the connection. Proper flashing and caulking is needed to keep the house band dry, preventing decay. Due to the difficulty of achieving a reliable waterproofing system, a PPT house band—with compatible corrosion-resistant fasteners—at the deck attachment location is recommended.

Other system considerations include lateral bracing for both house-attached and freestanding decks. In Section R507.2.3 and Figure R507.2.3, IRC includes a prescriptive detail that can be used to meet the code requirements for lateral deck stability. The detail shows a connection from the deck joists to the house joists, and it requires a design load capacity of 6.67 kN (1500 lb). At least two such connectors are required for the deck’s lateral bracing.

Considerations for safe deck guards
A guard is a system of interconnected parts that protects occupants from falling off the deck for whatever reason. Guards are required on residential decks more than 762 mm (30 in.) above ground level. Per IRC, the guard must be 914 mm (36 in.) above the deck surface or, in the case of a bench-guard combination, the guard height must be 914 mm above the bench seating surface.

Wooden guards typically consist of many parts fastened with bolts, screws, or nails. Guard posts are typically stress-rated 4x4s bolted to the deck band joist. Two rails between the posts are usually 2×4 cross sections—one at the top of the post, and the other positioned less than 102 mm (4 in.) above the deck surface. The rails transmit the applied loads to the posts. Pickets between the rails transmit loads into the rails. A typical guard is shown in Figure 5. While not addressed in this article, numerous non-structural requirements for guards are extremely important to child and life safety, and are covered by IRC.

Tests of guardrail post to deck connections at Virginia Tech demonstrated a safety factor of 2.5 on the code required load of 890 N (200 lb). [CREDIT] Photo courtesy Frank Woeste

Tests of guardrail post to deck connections at Virginia Tech demonstrated a safety factor of 2.5 on the code required load of 890 N (200 lb). Photo courtesy Frank Woeste

The well-known building code requirement for guards is 890 N (200 lb) concentrated load applied in any direction to the top of the guard system. As stated earlier, a critical connection is the guard-post-to-deck structure. Laboratory tests of 4×4 posts loaded at 914 mm above the deck surface demonstrated the large force that is produced at the base of the post. Some commonly observed post connection details that rely on lag screws alone have been tested at Virginia Tech. For one tested case, the guard post separated from the deck band joist at a load level approximately 25 percent of the code requirement.

Due to the ‘lever’ action, the band joist can also separate from the joist ends because it is inadequately attached with nails or screws installed into the end grain of the joists.⁵ This connection detail is weak because the fasteners used to attach the band to the joists are loaded in withdrawal from end grain. The guard post is through-bolted to the deck band joist, so it appears to be a strong connection—however, it can fail at very low loads compared to what is required by the code.

Laboratory tests of 4×4 stress-rated guard posts (Figure 6) demonstrated steel connectors can adequately transmit loads produced by the code-required concentrated load on the top of the guard into the deck joists.

Figure 7 shows a schematic of a post-to-deck attachment using one or two connectors. For guardrails running perpendicular to the joists, the post can be attached directly to the connector if the post location is adjacent to a joist. Otherwise, two connectors are used to attach the band to joists, and the guard post is attached to the band with bolts between the connectors.

Schematic of post-to-deck attachment with connectors. [CREDIT] Image courtesy American Wood Council

Schematic of post-to-deck attachment with connectors. Image courtesy American Wood Council

When installing guards, one must also consider how rails are attached to the posts and the pickets to the rails. A preferred method is to attach the rails to the inside face of the posts with screws or threaded nails. The pickets are attached to the rails with screws or threaded nails. In no case should guard components be connected with smooth-shank nails, as the connection design strength is reduced by 75 percent due to in-service moisture changes.

Severely weakened post notched at the bottom. [CREDIT] Photo courtesy Joseph Loferski

Severely weakened post notched at the bottom. Photo courtesy Joseph Loferski

Guard posts should not be notched. In the past, bottom-notched posts were commonly employed to allow the post notch to sit on the deck surface, as shown in Figure 8. This practice severely reduces the post’s bending strength, and notching can worsen as cracks travel up from the corner of the notch.⁶

For all fasteners and connectors used in exterior environments, corrosion is an issue since it reduces connection strength. Therefore, at a minimum, code-recognized and approved corrosion-resistant metals or coatings must be employed in guard construction. Stainless steel fasteners and connectors are recommended by AWC DCA 6, Prescriptive Residential Deck Construction Guide, for guard systems exposed to saltwater or coastlines.⁷

Notes
¹ For more, see the authors’ co-written article with D. Carradine and D. Bender in the May 2008 issue of Structure Magazine, “Lessons Learned: Residential Deck Ledger Connection Testing and Design.” Visit www.structuremag.org/Archives/2008-5/C-LessonsLearned-DeckLedger_Carradine-May08.pdf. (back to top)
² See the authors’ co-written article with R. Caudill, T. Platt, and Q. Smith, “Load-tested Deck Ledger Connections,” in Journal of Light Construction (vol. 22, no. 6). Visit www.jlconline.com/Images/Practical%20Engineering_%20Load-Tested%20Deck%20Ledger%20Connections_tcm96-1098165.pdf. (back to top)
³ See the article by Loferski et al, “Brown-rot Decay of ACQ and CA-B Treated Lumber,” in Forest Products Journal (vol. 57, no. 6). (back to top)
⁴ Ibid. (back to top)
⁵ See the authors’ co-written article with D. Albright and Caudill, “Strong Rail-post Connections for Wooden Decks,” in Journal of Light Construction (vol. 23, no. 5). Visit www.jlconline.com/lumber/strong-rail-post-connections-for-wooden-decks.aspx. See also the co-written article with Albright from the July 2007 Structure Magazine, entitled “Tested Guardrail Post Connections for Residential Decks: Lessons Learned.” Visit www.structuremag.org/Archives/2007-7/C-LL_Wood_Post_Connections_by_Loferski.pdf. (back to top)
⁶ Ibid. (back to top)
⁷ See Woeste’s article, “Safe and Durable Coastal Decks,” from Coastal Contractor (vol. 5, no. 2). Visit www.coastalcontractor.net/pdf/2008/0803/0803safe.pdf. (back to top)

Joe Loferski, PhD, is a professor of sustainable biomaterials at Virginia Tech. He has an international reputation and experience in the areas of performance of wood and wood composites in buildings, along with the preservation of historic wood buildings. Loferski can be reached at jloferski@vt.edu.

Frank Woeste, PE, PhD, is an adjunct professor of sustainable biomaterials at Virginia Tech. He is a wood construction and engineering consultant, and a past contributor to The Construction Specifier. Woeste can be contacted via e-mail at fwoeste@vt.edu.