February 8, 2017
by Steven Heinje
What are the differences between paints and roof coatings, and what are liquid-applied membranes? Which material is ideal when an existing roof is not in a leak-free condition? This article seeks to define these products and systems in order to establish guidance for their correct selection and use, and to identify what factors favor specifying a liquid-applied solution versus a prepared roofing solution.
Paints, coatings, and liquid-applied membranes are all polymer-rich, and share a few key attributes. They are usually based on resins possessing very good weathering resistance, and are frequently cross-linked in some fashion to confer toughness and chemical resistance. They are somewhat over-engineered to compensate for the vagaries incumbent with field application.
In basic terms, a paint is a finish where cosmetics are typically a primary attribute. Surface protection may also be a key selling point, but paint must look good. Sold in specific sheen levels and in a broad range of colors, it usually has limited flexibility, but its hardness provides for exceptional weathering and colorfastness in a thin film.
Coatings, on the other hand, are films used to provide surface protection or other resistance properties. Color range and appearance are determined completely by performance considerations. A coating generally has an elongation greater than 100 percent and good low-temperature flexibility, is high in solids, and requires a thicker film to achieve long-term weathering and waterproofing.
The third category, liquid-applied membranes, are fully reinforced systems comprising a fabric and one or more coatings or resins. They are used to encapsulate and adhere the reinforcement.
In roofing, paint applications are almost exclusively used on steep-slope metal systems for aesthetic or reflective purposes. When metal comes painted from the factory, it is called an original equipment manufacturer (OEM) paint; these baked metal finishes are usually more durable than an aftermarket maintenance coating. Although not intended to improve the water shedding of the roof system, a metal roof may still benefit from the corrosion-inhibiting, temperature-reducing, and eye-appealing properties of a paint job.
Steep-slope metal roofs are often important architectural details. Bare metal, despite its luster, has only moderate solar reflectivity and fairly poor emissivity, so it gets very hot. All paints, even dark ones, allow more heat to escape the metal, lowering the roof’s peak temperature. Paints with deep colors can employ special pigments that absorb less heat from the sun. Most white roof paints are rated for their solar reflectivity and emissivity. Together, these values will produce a Solar Reflective Index (SRI) that can predict the peak surface temperature.
In terms of cool roofing, there can be no better improvement for metal than applying a white coating to a rusting roof. Often, these roofs are not well-insulated—as they rust, they become less reflective, holding and conducting heat into the building. This dramatically increases the load on air-conditioning. Ultimately, paint can be seen as the first step in a long-term maintenance program that might later involve coatings or membranes.
Today, most coatings are based on acrylic, silicone, or urethane resins—all seek to extend the existing roof’s service life and, as a rule, prevent water intrusion. This requires a much higher degree of crack-bridging than found in paint. A coating must be able to protect a number of transitions subject to cyclical movement, such as:
This implies there must be an appropriate degree of low-temperature flexibility, substantial elongation, and a thick film. A coating should never be used to overcome a structural issue and, while they must tolerate movement, coatings cannot overcome the limitations of an improperly engineered roof system.
Although many coatings are sold as part of a cool roof solution, it is important to recognize they should not be selected in the same way one would choose a paint (i.e. solely considering appearance or reflectivity). A coating should provide robust barrier properties, starting with increased water resistance, and often include some combination of chemical resistance (e.g. plasticizers, oils, and stack emissions) and resistance to abrasion, impact, or vapor intrusion (e.g. water, oxygen, and carbon dioxide [CO2]).
Typically, coatings properties include tensile and elongation, which are usually reported at standard temperature and humidity. Crack-bridging is the key to performance; it is a function of elongation at a low temperature, tear strength, and film thickness.
When a material is tested to an ASTM standard, that protocol will include tear strength, a low-temperature flexibility test, and some weathering values.
Elongation is the measure of a sample, typically 25 mm (1 in.) long, stretched to some multiple of its original size. A 100 percent (at 23 C [73 F]) value means a 25 mm sample can elongate to a maximum of 50 mm (2 in.) under an ideally balanced load, before rupture.
Measured at the same time is the tensile strength, or breaking resistance, with that same ideally balanced load, in accepted units of force. To ensure consistency and repeatability, the samples are prepared under ideal controlled conditions. The problem with this is, in the real world, bridging most often occurs when the contraction of materials is due to very cold temperatures. This is why low-temperature flexibility is included in most ASTM material standards.
A coating that will crack under such conditions may also peel up at the edges of a joint when it ruptures. Those possessing excellent flexibility, even down to −31 C (−35 F), will maintain their protective properties in any climate. The flexibility test is normally coupled with a weathering or accelerated-aging test to confirm barrier properties will remain for many years of use.
Tensile strength interacts with a number of very important performance concerns, but does not reveal much by itself. A roof coating is not a rope or a chain to be assessed merely for breaking strength. Instead, one must know it will resist typical roofing loads. As such, tensile and elongation are best evaluated together. If both are high, the coating is likely formulated with less filler and more resin. A combination of high tensile, high elongation, and good low-temperature flexibility indicates a higher-quality material.
Tear strength is often a part of ASTM material standards because stresses on roofs are not ideal balanced loads, and coating films are never flawless. All coatings have voids and thin spots that cover stress points like seams, metal joints, or fastener heads where tears propagate, making tear strength
the limiting strength property for measuring crack-bridging capabilities.
The role of film thickness is tied to the effective elongation a coating provides. Cracks often expand much more than the 100 percent measured by the elongation of a coating, but coatings tend to ‘flow’ under strain, especially when they are thicker. A typical hairline crack might be 0.4 mm (1/64 in.) and become 4.8 mm (3/16 in.) at −18 C (0 F)—to effectively bridge this movement, a coating would theoretically need 1200 percent elongation. However, at a nominal 1016 µm (40 mils), that film is thicker than the width of the original hairline crack. As a result, the ‘stretching’ will also occur vertically through the film and extend into the surrounding material.
Most ASTM-approved coatings can bridge cracks over 6.4 mm (¼ in.) if they are sufficiently thick, which is effectively equivalent to an ‘elongation’ of 1600 percent. In today’s marketplace, white acrylic coatings may be designed to be applied at under 254 µm (10 dry mils) and used primarily for reflectance, functioning more like paints than coatings. This might work in warm climates, but not so much in cold environments. In general, coatings used over more flexible substrates need to have in-kind flexibility, and coatings in cold climates often need to be applied more thickly. As a result, there are many kinds of coatings and specifications customized for various substrates and regions to provide crack-bridging.
The current working definition within ASTM Subcommittee D08.25 on Liquid-applied Polymeric Materials Used for Roofing and Waterproofing Membranes that are Directly Exposed to the Weather includes the assumption a fabric or reinforcement be used to create a membrane ‘system.’ Why is this good practice? It returns to the tear strength and the way that mechanical stresses are distributed through the film. A typical coating may have an extrapolated tear strength of 10.5 kN/m (60 lb/in.) at a theoretical 25-mm (1-in.) thickness, but at a more realistic 20 mils that same coating will have a minimal gross tear strength of only 0.54 kg (1.2 lb). Yet, if that same coating is embedded in a reinforcement fabric, the fabric alone will often have a tear strength above 1.8 kg (4 lb)—even at its weakest point—and will average above 9.1 kg (20 lb).
The reinforcing properties of the fabric will dominate the performance characteristics of the membrane system, distributing stress in the same way a thicker coating does, but much more effectively. For example, an acrylic coating meeting ASTM D6083, Standard Specification for Liquid-applied Acrylic Coating Used in Roofing, applied at 635 µm (25 mils) will have an elongation of over 100 percent but a gross tear strength of only 0.7 kg (1.5 lb). When reinforced, elongation may drop down to about 35 percent, but tear strength will improve to about 9.5 kg (21 lb). As part of a composite, the fabric does not interfere with low-temperature flexibility or adhesion of the liquid, and as the roof moves, the stress does not rupture the membrane. Instead, the force is conveyed into the system, maybe 255 mm (1 in.) or more away from the crack or joint, via the fabric. That 35 percent elongation of movement extends over 25 mm on each side, allowing for 17.8 mm (0.7 in.) of movement capacity, far exceeding the 6.4 mm (0.25 in.) provided by the thicker coating described above. The dynamics of a membrane require the coating/resin remain flexible (making good low-temperature flexibility a must), and the fillers do not interfere with the action of the fabric. These design factors are why most membrane systems rely on premium coating/resins as the binder.
As simple as it is, the use of fabric in liquid-applied systems can be called a game-changer in terms of reliable performance. Most often, this approach has been applied to flashings, details, and repairs within typical coating applications. Coming into broader use are liquid-applied roofing systems, or liquid membranes, that are fully reinforced. Not only is long-term crack-bridging better with these systems, but hail and impact resistance are also often greatly improved. Liquid-applied membrane systems may be used to retrofit leaking or otherwise-compromised roofs when a coating application would not be indicated, and can perform an important role in new roofing when complex layouts of rooftop equipment, protrusions, or irregular shapes make a seamless system preferable. One unique feature of many liquid-applied membranes is they are ‘breathers’—that is, they have a permeance well above 1 US perms, indicating they allow the release of water vapor. They are frequently user-repairable and do not require special staging or lift equipment, making them perfect for in-house maintenance crews or high-rise applications.
Repairs, retrofits, and liquid-applied roofing
How are roof coatings and reinforced liquid-applied membranes best used when an existing roof is not in a leak-free condition? A good definition of a roof coating is a treatment used to extend the life of an existing roof cover, and a reinforced liquid-applied membrane is an additional roof cover.
From a code and enforcement perspective, this places a coating in the category of a component or accessory, which may not trigger the need for a permit, inspection, or code-approved product. Confirmation of a fire rating per UL 790/ASTM E108, Standard Test Methods for Fire Tests of Roof Coverings, is generally the most that is required by a building official. However, a liquid-applied membrane is often viewed as a separate ‘roof.’ Therefore, it may trip the two-roof limit and many other code-based requirements, such as:
The reality is the majority of liquid-applied projects lie somewhere between the two categories. For the most part, they are primarily coating applications, since installation is almost always over an existing roof cover. However, in more cases than not, they also involve some level of repair to the underlying roof cover, which gives them elements of a re-cover.
As most roofing professionals have experienced, few owners consider roof maintenance until they have some sort of problem—usually, a serious leak. As a result, even most liquid-applied membrane installations are actually premium-quality coating applications, since the objective is to extend the service life of the existing roof, and the new membrane system is seen as longer-term insurance against future leaks.
Repair and retrofit guidelines
With roof coatings and reinforced liquid-applied membranes, code enforcement becomes entirely discretionary on the part of the local code official, as the perspective of the code is mostly geared toward new construction and not maintenance. This issue is not unique to coatings—the entire push for sustainability implies the reuse, repurpose, and recycling of many roof materials that in the past were replaced at regular intervals. The use of liquid-applied roofing is growing, and sustainability is a big driver in the specification
Here are some guidelines for repair and retrofit:
Most liquid-applied membranes are capable of functioning independently as roof covers, so retrofitting that might involve the removal and replacement of wet insulation and cover boards is practical. For sprayed polyurethane foam (SPF), a good example of recent trends in roof assembly repair can be found with ASTM D6705, Standard Guide for the Repair and Recoat of Sprayed Polyurethane Foam Roofing Systems.
Liquid-applied roofing is often selected for smaller complex roofs with many protrusions and details (e.g. above a laboratory or commercial kitchen), which may justify a greater degree of repair and subsequent cost. On larger and more open roofs, limits should be lower—perhaps 20 percent of the roof area. This is due to the fact that, at some point, the economic trade-off of the diminished R-value of the remaining old insulation and decreasing performance of aging materials makes the economics of a quality repair doubtful when the cost of a complete re-roof may be lower.
Two basic approaches to repair
It is useful to think of a project in two distinct phases:
It is often best not to use a liquid-applied repair solution if a conventional one is available. There are numerous reasons for this.
Once in-kind repairs are made, the field coating phase is greatly simplified and can be accomplished in accordance with the manufacturer’s standard published instructions. This may involve redundant detailing if deemed a condition of warranty or the desire to extend service life.
Of course, there are other cases where liquid-applied repairs make the most sense. They include cases where:
Generally, liquid-applied details involve the use of one layer of reinforcing fabric (polyester, glass mat, or a hybrid of the two), overlapped so at no point of the detail is there a butted joint leaving an unreinforced site. Illustrations are typically available from the manufacturer in architectural format, but they are all very much the same. The coating may be used all at once, in a wet-on-wet approach as is common in reactive systems (e.g. urethane, polyether, or polymethyl methacrylate [PMMA]) or in three courses utilizing coating, fabric, and coating.
The reinforced detail will typically be at least 650 µm (25 mils) thick, and have a total tear strength of at least 3.5 kg/cm (18 lb/in.). When using unreinforced liquid details (e.g. liquid flashings), the total tear strength will be similar but will require a special grade product and thicker application, perhaps 1525 µm (60 mils) or more. Consequentially, liquid flashings are often solvent-based urethane, styrene ethylene butylene styrene (SEBS), or other materials that are both tough and flexible at low temperatures.
How liquid-applied systems work and what to look for
Roof construction details generally specify tough, thick materials or use reinforcement to spread stress away from the point of movement over a broader area. In these areas, the critical characteristic of crack-bridging is primarily a function of tear strength, more so than the more commonly reported physical properties of tensile strength or elongation. Tear is a test of strength when there is a flaw or irregularity, and this is always the case in actual roofing practice.
In addition to considering tear strength, in order to function properly, detail design must allow for adequate low-temperature flexibility (LTF). If the material hardens severely as temperatures drop, it will no longer spread the stresses at the detail. When reinforced, a cold or stiff material will cause the fabric to shear, because the fibers can no longer move as intended in the composite. Details are stressed the most when materials contract due to cold temperatures, making low-temperature flexibility a key benchmark when comparing liquid-applied systems.
The overall condition and performance expectations of an existing roof provide the basis for what level of protection is needed. Only occasionally can that be satisfied with paint or a thin coating. Much of the time, the roof will need some repairs while the field of the roof is still functional, and can be improved by a coating application. More severely leaking and deteriorated roofs often indicate the need for a membrane system, as well as for extra impact resistance or a longer service life beyond what a coating can provide.
Repairs and retrofit solutions utilizing liquid-applied approaches are available for almost all forms of commercial roofing. These liquid-applied systems are not simple paint jobs, but are roofing installations done by experienced roofing professionals. Not every roof is a good candidate, due to either economic considerations or material properties limitations as a result of age. For most roofs, there a point in the life cycle when a liquid-applied solution is a good option. Given the right materials and methods, most roofs can remain in service longer with the help of liquid-applied roofing products.
Steven Heinje is the technical manager of liquid-applied systems for GAF. He has degrees in biology and chemistry, along with an MBA. Heinje has 30 years of experience in roof coatings, specializing in acrylic elastomers and urethane coatings. He is a vice president and board member of the Roof Coatings Manufacturers Association (RCMA), and leads several task groups in ASTM D08 roofing, as well as maintaining active memberships with American Society for Quality (ASQ), RCI, Reflective Roof Coatings Institute (RRCI), and the American Chemical Society (ACS). Heinje can be reached at firstname.lastname@example.org.
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