Spray polyurethane foam (SPF) roofing consists of an application of specifically designed foam covered with an elastomeric coating (typically acrylic, silicone or polyurethane) or aggregate covering to protect the foam from ultraviolet rays. Specialized equipment mixes two liquid components at the spray gun that applies the SPF to a prepared substrate. The mixed liquid expands many times its original volume in a matter of seconds, forming a rigid foam plastic that chemically bonds to the surface to which it is sprayed. Spraying the foam in ½? to 1-1/2? lifts allows the applicator to reach the desired thickness to fill in low areas, build up slope, and provide insulation.
SPF has a closed cell structure that makes it water resistant. It must, however, be protected by elastomeric coatings or other coverings (such as aggregate) to prevent ultra-violet-induced surface degradation. Such coverings can also be used for other purposes, including, inhibiting moisture vapor transmission, enhancing the aesthetics of the system, increasing the impact and abrasion resistance of the system, achieving non-flammability and meeting code requirements.
SPF roofing systems have good adhesion to a variety of substrates including metal, wood, concrete and built up roofing (BUR). Since SPF adds little weight to existing roof coverings and can build slope to fill in low areas, these systems are used frequently as a recover roofing system. Caution should be used when specifying any recover roofing system. The existing roof covering and roof deck assembly should be thoroughly evaluated by a structural engineer to verify that it can be a safe substrate for SPF roofing systems.
Hail and wind driven missiles (such as tree limbs, broken roof tile, metal flashing, etc.) can damage the SPF roofing system. However, this type of damage typically does not cause leaks and can be repaired later without compromising the long-term performance of the system.
SPF roofing systems also excel when the following conditions exist:
- Additional insulation is required
- There are severe temperatures
- The roof substrate has numerous penetrations
- The roof deck is an unusual configuration
- The roof is in an area where high winds are likely to occur
- Lightweight materials are required
- Slope must be added to provide positive drainage
Because of the energy saving characteristics and low maintenance costs of SPF roof systems, these roofs are suited to companies or organizations that own their own buildings and must pay their own energy and maintenance costs.