How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What is an SPF roof?
Sunday, June 24, 2018
The letters stand for Spray Polyurethane Foam, and an SPF roof is constructed by combining two chemical components, isocyanate and polyol, at the head of a spray gun. It is applied in sweeping strokes over the roof and creates a hardened closed-cell foam. SPF has a good R-rating and can be built-up several inches high to provide insulation at the roof deck. The spray thickness can also be tapered to assist in roof drainage on a flat or low-slope roof.
The material can be applied directly over an existing roof without having to remove it, as long as the surface is stable and properly cleaned. Customers that can’t resist the cost savings from leaving the old roof in place, however, sometimes end up with a strange and unsightly SPF roof at a location that is not really appropriate for the material, like the shingle mansard shown below with an SPF coating.
We don’t see this material too often on residential roofs. When we do, it is usually on mid-century modern home with a flat roof that has no attic. The SPF provides desperately needed insulation without disturbing the exposed wood beams below. The closed-cell type SPF used for roofing has an R-value of 6.5 per inch of thickness. A final coat of an elastomeric material is applied over the SPF to provide protection from sunlight UV and the elements, and mineral granules or sand are sprinkled into the surface while wet to increase durability.
Because it is applied by spraying repeated layers, even a roofer experienced in applying the material cannot get it perfectly smooth, so an SPF roof often has a slightly wavy surface. You can see it in the photo above. But we have also inspected roofs that appeared to be done by an inexperienced applicator and resembled the marshmallow topping on a Thanksgiving candied yam casserole.
SPF is also used for attic insulation and is applied to the underside of roof sheathing, creating a cool attic. See our blog post What is the average life expectancy of an SPF roof? for more.
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To learn more about roofs and attics, see these other blog posts:
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