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:
• Why is my roof sheathing sagging between the trusses?
• Why is granule loss a problem for an asphalt shingle roof?
• What are the mistakes to avoid when doing attic improvements?
• What causes roof shingles to curl up at corners?
• What causes shingles to buckle along a line on the roof?
• What causes leaks at a fake roof dormer?
• What causes a sagging roof ridge line?
• What causes bubble-like blisters in a built-up and gravel roof?
• Why does it cost so much more to replace a steep roof than a low slope roof?
• What is "ponding" on a flat roof?
• Is an attic required to have a light by the building code?
• How can I inspect my roof for hurricane damage?
• Why is premature curl of roof shingles a problem?
• How can I tell if a roof has more than one layer of shingles?
• What are the common problems with attic insulation?
• What is the life expectancy of an asbestos cement shingle roof?
• What's the average lifespan of a roof?
• Why is it a mistake to replace a roof and not replace its flashings?
• Why is there no attic access hatch in the house?
• What is the building code requirement for an attic access hatch, scuttle, or door?
• Does a roof with multiple layers of shingles last longer?
• What can I do to prevent roof leaks?
• Are roof trusses better than roof rafters (stick framing)?
• Why is a popped nail in a shingle roof a problem? How do I fix it?
• What are the most common problems with wood roof trusses?
• What causes a lump or dip in the roof?
• If my roof is not leaking, why does it need to be replaced?
• How can I be sure my roofing contractor got a permit?
• How many layers of roofing are allowed on a home?
• What are the dark lines running parallel to shingles on my roof?
• Can metal roofing be used on a low slope/pitch roof?
• How can I make my roof last longer?
• What are the warning signs of a dangerous attic pull-down ladder?
• How can I find out the age of a roof?
• Should I buy a house that needs a new roof?
• Should I buy a house with an old roof?
• What are those metal boxes on the roof?
• What does "lack of tab adhesion" in an asphalt shingle roof mean?
• Why do roof edges start leaking?
• Why do my dormer windows leak?
• Do home inspectors go on the roof? Do they get in the attic?
• Should I put gutters on the house?
• How much of a roof truss can I cut out to make a storage platform in the attic?
• What's the difference between an "architectural" and a regular shingle roof?
• What does a home inspector look for when examining a roof?
• Do stains on the ceiling mean the roof is leaking?
• How can I tell if the house needs a new roof?
• Why does my homeowner's insurance want a roof inspection?
• What are the hazards to avoid when going into an attic?
Visit our ROOF AND ATTIC page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.