Essentials

Installing spray polyurethane foam (SPF) -based roof systems is a viable low-slope roofing option. However, many building owners, designers and roofing contractors are unfamiliar or unaware of SPF's performance attributes. Considering that SPF-based roof systems have been around since the late 1960s, SPF has managed to only capture a small share of the current low-slope roofing market.

For years, SPF-based roofing materials suffered from bad press that publicized problems associated with early installations, such as blistering, inadequate bonding between foam and coatings, and poor workmanship. However, there now are minimum standards for foam properties, improved application equipment, stringent applicator training, better design and application guidelines, and improved coatings. These improvements resulted from advanced technology, a better understanding of roofing philosophies and aggressive public relations efforts by the SPF industry.

Definition

SPF-based roof assemblies typically consist of three primary parts: roof deck and/or substrate, polyurethane foam and protective surfacing. A roof deck is the structural substrate that supports a roof system. SPF can be applied to various types of substrates, including wood, concrete, metal, rigid insulation and some existing roof membranes.

Polyurethane foam is generated by mixing two components, Component A and Component B, which form an adhered roof system's base. Component A is composed of high molecular weight polymeric isocyanate. Component B, commonly called polyol resin, is a mixture of polyols, blowing agents, catalysts, flame retardants and surfactants.

The A and B components have a mixing ratio of 1-to-1 measured by volume. When combined, they create an immediate chemical reaction and expand 20 times to 30 times their original liquid volume to form a closed-cell foam. As the foam rises and becomes solid, a skin forms on the surface. Each application made to obtain a desired foam thickness is called a lift, and each lift will form its own skin. Optimum lift thickness should be between 1/2 of an inch (13 mm) and 1 1/2 inches (38 mm).

Protective surfacing provides long-term weatherproofing characteristics and ultraviolet protection. In addition, protective surfacing increases SPF's resistance to mechanical damage and abuse and is necessary to achieve a fire-resistance rating for the roof assembly.

An atypical system

SPF-based roof systems are unique for various reasons, and it is because of some of those reasons that SPF is a viable roofing option. For example, SPF can be applied in varying thicknesses to provide slope to drain roof systems or meet a specified thermal resistance. Additionally, SPF is self-flashing, which means wall flashings and fixed penetration flashings can be sprayed in place and result in unified, continuous, self-adhered flashing details.

Other distinctive SPF features are it can be installed over most existing low-slope roof systems without additional fasteners or ballast and provides extra thermal resistance while adding minimal weight to a roof structure. However, a designer must evaluate an existing roof system to determine whether it is an appropriate substrate to be re-covered with SPF.

Although SPF-based roof systems possess many positive attributes, it is important to mention a significant issue that typically is a cause for concern when specifying SPF—weather. Weather always is an issue for roof system installations, and SPF applicators must especially be aware of environmental conditions during applications. Humidity, temperature, wind and moisture can affect SPF-based roof system installations.

For example, if moisture is on a roof deck or in the atmosphere in the form of rain, mist, fog or snow, the moisture will react with the foam components and may result in off-ratio foam. Off-ratio foam has unacceptable physical properties that may affect the foam's adhesive and/or cohesive strengths. The latter condition usually results in blisters or delamination problems in a roof system.

If you are considering installing an SPF-based roof system and want more information, contact the Spray Polyurethane Foam Alliance at (800) 523-6154; refer to the SPF-based Roofing section of The NRCA Roofing and Waterproofing Manual, Fifth Edition; contact NRCA's technical services section at (847) 299-9070 or fax (847) 299-1183; or access NRCA's Web site, www.nrca.net.

Joan P. Crowe is an NRCA manager of technical services.