Spray Foam: When and Why?

07/01/2013 | By Richard L. Fricklas

Reasons to consider polyurethane solutions for reroofing

Year in and year out, it has been estimated that two-thirds of all commercial roofing activity is related to reroofing and re-cover rather than new construction.

Decisions on what, when, and where to reroof may be based in part on leakage, although timing may also be related to an overall corporate financial plan (spend it now because funds may not be available later).

The choices for reroofing are generally categorized as:

  • Traditional bituminous built-up roofing
  • Polymer-modified bituminous roofing
  • Single ply polymeric roofing
  • Metal roofing panels
  • Sprayed-in-place polyurethane foam (SPF)

Three Considerations for Foam
A sprayed-in-place foam solution for reroofing might be the best choice for you if:

1) Your existing roof already has two membranes, in which case it has already met the maximum permitted by building code. However, SPF is extremely light (less than 1 pound per square foot) and is usually exempted from that code limitation. If the existing roof is surfaced with gravel or ballast, removal of that surfacing and replacement with SPF will actually reduce some of the dead load on the structure.

2) The existing roof system is aged, which typically means it is under-insulated. One inch of wood fiber or perlite board would have an R-factor of 2.8, so applying two inches of SPF on top would add 11.2 (in•h•ft2/BTU), and substituting a reflective coating on the foam, compared to a dark-colored gravel, would reduce air conditioning load as well.

3) The system is fully adhered. Many single ply roof systems are partially fixed to the structural roof deck, especially those using mechanical fasteners. These systems may flutter and balloon under high wind load. SPF systems are fully adhered to the substrate and can serve as an air barrier as well as the roof membrane. See Building Science.com BSI-019 Uplifting Moments—Roof Failures, Joseph Lstiburek, 2009

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