Thermoset materials normally cross-link during manufacturing. Once cured, these materials can only be bonded together with a bonding adhesive or specially formulated tape. Primary membrane types in this category are:
Ethylene Propylene Diene Monomer (or Terpolymer) (EPDM): EPDM is a synthetic rubber sheet. As of 2005, EPDM enjoys the largest market share of the single-plies in service in North America. EPDM membranes are extremely resistant to weathering and they have very good low-temperature flexibility. However, EPDM is susceptible to swelling when exposed to aromatic, halogenated, and aliphatic solvents, and animal and vegetable oils such as those exhausted from kitchens. On portions of roofs where the membrane may be exposed to these materials, an epichlorohydrin membrane can be specified over the EPDM as discussed below. EPDM membranes are suitable at airport buildings, provided liquid fuel is not spilled on the membrane.
The sheets are typically available in widths of 10, 20 and 45 or 50 feet [3, 6 and 14 or 15 m], and lengths up to 200 feet [61 m]. Hence, on large roofs with very few penetrations, this type of membrane can be very economical to install. Most EPDM sheets are black, although white sheets are available. White sheets, however, are not nearly as resistant to weathering as black sheets. EPDM is typically non-reinforced. Note that reinforced sheets can begin to delaminate very quickly if water gets to the scrim because of abuse or simply from wear. Therefore, reinforced sheets are only recommended for mechanically attached and loose-laid air-pressure equalized applications. Reinforced sheets also offer some increased resistance to puncture and tearing when used in fully adhered and ballasted applications, where non-reinforced sheets are vulnerable to physical damage, especially if rounded, graded to 3/4″ minimum size, river-washed ballast is not used. If a rigid insulation cover board is included as a substrate the non-reinforced sheet is preferred.
In fully adhered applications, typically a contact adhesive is applied to the substrate and the sheet. After the adhesive dries, the sheet is mated with the substrate. Another method of application uses fleece-backed EPDM, which is set in low-rise sprayed polyurethane foam adhesive. As there may be issues with a fleece-backed system set in asphalt, designers are advised to consult with the manufacturers before specifying or detailing fleece backed system with asphalt.
Field seams are formed using either a liquid-applied adhesive or specially formulated tape. The latter is recommended. Although tapes offer performance advantages over liquid-applied adhesives, the contractor still needs to exercise care in cleaning the EPDM prior to tape application, priming the EPDM and diligently executing the seam work as recommended by the manufacturer.
EPDM roof membranes provide predictable serviceability in roof systems in all climates. The minimum sheet thickness should be 60-mils if reinforced and 90-mils if unreinforced. All lap seams shall be fabricated with 6-in. (150 mm) seam tape and stripped-in with self-adhering, semi-cured EPDM cover strips.
EPDM sheets are resistant to the effects of UV radiation and are very durable. Seaming technology and adhesives have improved reliability with the use of tape-applied adhesive. Tape applied seams should be used. Properly constructed EPDM systems are now providing 30 years or more of service life.
Epichlorohydrin (ECH): This sheet is similar in appearance to EPDM. ECH, however, is resistant to hydrocarbons, solvents and many greases and oils, so it can be used in areas of the roof that are exposed to chemical discharges that are harmful to EPDM. Because of its permeability, the ECH manufacturer recommends placing ECH over an EPDM membrane. Because it is so specialized, ECH is seldom used. Only one manufacturer produces it in North America.