Roofing System 11
LIQUID-APPLIED ROOF MEMBRANES
Liquid-applied roof membranes are more widely known to be used as waterproofing systems but have gained in popularity as roof systems, especially in reroofing situations and in PRM Assemblies. However, if a liquid-applied roof membrane does not have reinforcement, it typically is considered a coating system. A reinforced liquid-applied roof membrane is considered by NRCA to be a roof system. The most common and reliable liquid-applied systems are the hot-applied rubberized-asphalt systems with uncured neoprene reinforcement.
Liquid-applied roof membranes generally are installed in 70-mil, 80-mil or 90-mil finished thicknesses but can be as thick as 115-mil in some applications. Consult the specific manufacturer for recommended thicknesses. Liquid-applied membrane roof systems typically are reinforced with polyester reinforcing fabric or fleece.
STEEP-SLOPE ROOF COVERINGS
The following roof coverings are commonly used on steep-slope roofs. These coverings are water shedding, rather than waterproofing. Special underlayment provisions are required when slopes are relatively low. The NRCA Roofing Manual provides underlayment guidance.
METAL PANELS AND SHINGLES
When used on slopes greater than 3:12 (25 percent), hydrokinetic or water-shedding panel systems may be used. Hydrostatic (water barrier) systems may also be used. Structural panels may be specified if a solid deck is provided. If a solid deck is not provided, structural panels need to be specified.
Metal shingles are also available in a variety of metals and designs. The performance varies greatly depending upon the product selected.
Shingles are available with either fiberglass or organic reinforcement. Fiberglass-reinforced shingles provide greater fire resistance and are therefore recommended. Asphalt shingle products are typically tested and classified for impact resistance according to UL 2218. This standard provides for four classifications: Class 1, Class 2, class 3 or Class 4. Class 1 provides for the least measured resistance of impact resistance, and Class 4 provides for the relatively greatest level of impact resistance. NRCA suggests designers consider specifying asphalt shingles with Class 3 or Class 4 in regions prone to large-sized hail.
In addition to the traditional three-tab design, laminated (structural) shingles are available where a different appearance is desired. Product standard ASTM D3462 has limited criteria to distinguish various products in the marketplace. Therefore, warranty duration is normally used to attempt to distinguish commodity products from those that offer longer service life. However, the warranty duration is not necessarily an indication of performance. Shingles with a minimum warranty of 25 years are recommended.
Natural slates have the potential to offer several decades of service life. However, slate is heavy and very expensive. If slate is specified, a very durable underlayment is recommended, so that it does not prematurely degrade.
Specifiers are cautioned that synthetic materials are often marketed as slate. Some of these products are made from slate particles, while others are made from polymers or other materials. Synthetics should not be expected to offer a service life equivalent to natural slate.