Indicators of SPF Performance
by James R. Kirby, AIA
Professional Roofing Magazine

Several physical characteristics of spray polyurethane foam (SPF) contribute to acceptable performances of SPF-based roof systems. Two of these characteristics are density and compressive strength.

Density usually determines the yield (number of board feet of coverage per pound) of polyurethane foam and affects foam's compressive strength. Compressive strength determines foam's impact resistance and how durable an elastomeric coating and SPF are under compressive loads. Generally, the higher the compressive strength, the more resistant SPF is to damage from foot traffic, dropped tools, etc.

The effectiveness of details also is a measure of performance of SPF-based roof systems. How SPF-based roof system details are installed will affect the details' effectiveness.

Minimum properties

NRCA and the Spray Polyurethane Foam Alliance (SPFA) suggest minimum physical properties for density and compressive strength. NRCA suggests SPF's compressive strength meet or exceed 40 psi (280 kPa) and density meet or exceed 2.8 pcf (45 kg/m³). SPFA recommends SPF's density be 2.8 to 3.0 pcf (45 to 48 kg/m³) and compressive strength meet or exceed 40 psi (280 kPa). It is apparent the minimum physical properties of SPF are not in question.

NRCA and SPFA generally agree about configurations for SPF-based roof system details. At this time, however, NRCA and SPFA disagree about the necessity of metal counterflashings used with SPF-based roof system details. NRCA recommends the use of metal counterflashings at the top edge of a foam's coating, but SPFA details do not require metal counterflashings.

Recent research

The SPF segment of the roofing industry, through the National Roofing Foundation, has funded an independent study of SPF-based roof systems. Phase I of the study is complete, and a report, "A Field & Laboratory Assessment of Sprayed Polyurethane Foam-based Roof Systems," has been written. The field work, laboratory work and report were done by Structural Research Inc., Middleton, Wis. Phase II of the study currently is under way.

The purpose of Phase I was to "establish and/or verify existing performance attributes for SPF roof systems." Phase I found the average field sample's compressive strength was 58.6 psi (404 kPa) and density was 3.22 pcf (52 kg/m³). Both characteristics were found to be well above NRCA's and SPFA's recommended levels. The study shows that SPF-based roof systems regularly are being installed with physical characteristics in excess of recommended minimums.

Phase II is examining the flashing issue in-depth. The primary focus of Phase II is to "observe and review the field performance of SPF flashing details with or without metal counterflashing. … This includes penetrations, perimeter and wall flashing details, as well as expansion joints."

There are, of course, differences between membrane roof system flashings and SPF-based roof system flashings. For example, membrane roof systems typically use the same material for flashings (except built-up roof systems, which typically use a modified bitumen sheet for flashings). These membranes (modified bitumen, thermoplastic and thermoset sheets) generally have the ability to accommodate minor substrate movement. SPF-based flashings typically are configured with the elastomeric coating installed higher than the foam's top edge, which potentially creates a weak point in the coating. And it is possible that SPF and elastomeric coatings may not have the ability to accommodate minor substrate movement. This is one issue Phase II's research may resolve.

Phase II should be complete—and a report issued—by the end of this year. A goal of the research is that the conclusions from Phase II will help minimize the disparity regarding the necessity for metal counterflashings with SPF-based roof system flashings.

James R. Kirby is an NRCA director of technical services.