EPDM is a durable and flexible synthetic rubber, ethylene propylene diene monomer, which is well-suited for use as a single-ply roofing material. EPDM was developed in the 1950’s and first used as a roofing membrane in the early 1960’s. By the 1980’s, it dominated the single-ply roofing market. It’s very common to find an older EPDM roof on a building today, as properly-installed and well-maintained EPDM roofs have been known to perform for up to 40 years. Currently, EPDM is used in about 20% of all low-slope (“flat”) roof installations in North America each year, including both new construction and roof replacements.
Individual sheets of EPDM come in an assortment of widths and lengths, with 10 ft x 100 ft being a typical size for large roof installations. Standard membrane thicknesses are 45-mil (0.045 inches or 1.14 mm), 60-mil, and 90-mil for unreinforced EPDM, and 45-mil, 60-mil, and 75-mil for polyester reinforced EPDM. An EPDM roof system is referred to by the way it’s attached to the building: there are ballasted systems, fully-adhered systems, and mechanically-attached systems.
Although non-reinforced EPDM tends to be easily punctured (by careless foot traffic or dropped tools, for example) EPDM rubber is noted for being hail-resistant, and EPDM roof membranes are among the easiest to repair.
Composition of EPDM Roof Systems
“EPDM roofing” refers to single-ply roof systems where sheets of EPDM rubber are joined together to make a waterproof membrane. EPDM is a synthetic rubber, ethylene propylene diene monomer (also called “ethylene propylene diene terpolymer” because it’s a polymer made from three monomers). The EPDM rubber used in roofing contains roughly 40% actual EPDM (EPDM base resin). The other ingredients typically include carbon black (around 25%), plasticizing and extending oils (around 20%), with the remainder being various additives including anti-oxidants, anti-ozonants, fire retardants, curing agents, and inert fillers. Most EPDM roofs in place today use sheets of unreinforced EPDM, but membranes manufactured with a polyester reinforcing scrim are also used where recommended or required, particularly in mechanically-attached systems.
Common components in a typical EPDM roof assembly are: the roof deck, an air or vapor barrier (if needed), one or more layers of insulation, a cover board (if needed), and the membrane itself, along with attachment components such as adhesives, fasteners, or ballast. The field membrane (the vast majority of the surface of the roof) is formed from large sheets (typically 10 feet by 100 feet) of fully-cured EPDM rubber, seamed together with adhesives or adhesive tape during the roof installation. Smaller pieces of fully-cured or semi-cured EPDM are used to form the flashings at the perimeter of the roof and at HVAC curbs and other penetrations through the roof. Uncured EPDM is sometimes used on the corners of curbs or other locations with sharp angles.
Characteristics of EPDM
EPDM is a black (most commonly), elastomeric, thermoset rubber noted for its durability. It has good resistance to moisture, UV rays, heat, hail, ozone, and alkalis and acids. It has a high tensile strength, resists tearing, and remains flexible in cold weather (down to -45°F/-43°C). It is relatively easy to puncture, and is easily damaged by oils, gasoline and other fuels, grease, fats, and solvent-based asphalt compounds such as asphalt roof cement.
Black: Sheets of EPDM are black because they contain carbon black. As a raw material, carbon black is an extremely fine black powder, almost pure carbon, which is refined from petroleum or coal. It makes up about a quarter of the EPDM material typically used in roofing. The addition of carbon black strengthens the EPDM resin, reinforcing its molecular structure and increasing its tensile strength, and it protects the polymeric chemical bonds of the material by blocking harmful UV rays.
Because almost all EPDM roofs are black, they rarely qualify as “cool roofing” unless coated with a reflective roof coating. Black EPDM absorbs almost all of the heat from the sun’s rays during the day, and an EPDM roof can reach temperatures of over 180 degrees on a hot afternoon. In warmer areas where cooling degree days outnumber heating degree days, black EPDM has generally been replaced as a single-ply roofing material by white PVC and TPO. On the other hand, in colder climates where heating degree days outnumber cooling degree days, black EPDM can actually reduce annual energy costs versus white roofing membranes.
White: White EPDM is also available and allows EPDM to be used where a cool roof is called for. White EPDM is formulated with a white pigment, typically titanium dioxide, in place of carbon black. Titanium dioxide is less UV resistant than carbon black, and as a result white EPDM tends to be somewhat less durable than black EPDM. White EPDM roofs are uncommon; their share of the cool roof market is dwarfed by that of PVC and TPO. For a good article on white EPDM, see here.
“Elastomeric” refers to the ability of a material to stretch and return to its original dimensions without being damaged. When new, the unreinforced EPDM normally used in ballasted or fully-adhered systems can typically stretch more than three times (300%) its original size without breaking, although this may be reduced somewhat as the membrane ages. The reinforced EPDM normally used in mechanically-attached systems is constrained by the reinforcing polyester scrim, which reduces the elasticity of the roof membrane as a whole, and can only stretch about 35% before breaking.
For comparison, PVC roof membranes can only stretch about 15% before breaking, TPO membranes 27%, and an asphalt built-up roof about 2%.
“Thermoset” means that after the EPDM polymer chains are cross-linked using heat and chemical curing agents during the manufacturing process, new molecular cross-links cannot be formed. The material is now heat-stable and future application of heat will either have no effect on it or will simply damage it (at very high temperatures). In other words, after it’s manufactured, you can’t use a hot air welder on it. This is why adhesives or adhesive tape is required to join pieces of EPDM together. (Thermoplastic membranes such as PVC or TPO, on the other hand, can always be welded or otherwise altered using (very) hot air.)
The completely cross-linked EPDM used as a field membrane is also called “fully cured EPDM”. Semi-cured and uncured EPDM are often used as a flashing material where the roof makes an angle change at walls, curbs, or roof penetrations such as vent stacks. These forms of EPDM are manufactured by not fully cross-linking the polymer chains. This allows those materials to more easily conform to angles after installation without trying to snap back into a flat sheet shape like fully cured EPDM does. It also means that these materials are more vulnerable to UV radiation and heat, and they deteriorate faster than the rest of the roof.
Fully-cured EPDM is one of the most durable roofing materials available. Due to its molecular stability, its exceptional resistance to UV radiation, and its non-reactivity in the presence of corrosive environmental substances such as water, acids, and ozone, the sheets of EPDM that make up a roof’s field membrane can remain functional for over 50 years. When an EPDM roof reaches the end of its service life, it’s almost always due to the failure of a part of the roof system other than the field membrane material itself.
Go to EPDM Roofing: Reference Page for more information.