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Spray foam roofing

Spray foam roofing is rigid, closed-cell polyurethane foam sprayed on as a liquid, where it expands into one seamless layer of insulation and waterproofing. Sunlight breaks bare foam down, so it gets an elastomeric coating, often with granules, or a layer of gravel. It is used on low-slope commercial roofs, often over the existing roof instead of a tear-off.

Warranty available
Up to 20 years from some manufacturers
Foam thickness
1 inch minimum, more for insulation and slope
Seams
None; the foam is seamless
Surface
Elastomeric coating, often with granules
Attachment
Self-adhered to the deck or old roof

How it's installed

The first choice is whether the foam goes over the existing roof or onto a bare deck. Either way the work runs in three steps: prepare the surface, spray the foam, then coat it.

Over the existing roof

The foam goes onto the clean, dry old roof, such as built-up or modified bitumen; loose gravel comes off first, and single-ply usually needs a fastened board on top. Wind resistance then depends on how well the old roof is attached to the deck.

On a bare deck

On a tear-off or new building, foam can bond straight to a concrete deck, while a fluted steel deck needs treatment, such as fastened board, to give it a flat surface. Foam bonded to concrete usually fails only at around 900 psf in uplift pull tests.

Foam in passes, sloped to drain

Foam goes on in passes from 1/2 inch to about 2 inches thick, low areas first so the finished surface drains, and each area reaches full thickness the same day.

Coating and recoating

A base coat and top coat, generally 20 to 40 mils dry in total, go on within 24 hours; when the coating wears thin, it is recoated rather than torn off. Warranty length follows coating thickness: one manufacturer asks for 20 mils of silicone for 10 years and 40 mils for 20.

Pros and cons

Where spray foam wins

  • No seams, and the foam forms its own flashing at walls, curbs and penetrations.
  • Spraying it thicker in low spots builds slope to the drains and gets rid of ponding.
  • It adds insulation as it goes on, roughly R-5.5 to R-6.5 per inch.
  • It is light enough to go over many existing roofs; on a gravel roof, the gravel removed usually outweighs the new foam roof.

Where it falls short

  • The coating is all that keeps sunlight off the foam, so it needs inspections each spring and fall and recoating as it wears.
  • Weather limits the work: foam can't go on wet or damp surfaces, and wind can carry overspray onto nearby cars and windows.
  • The roof is made on site, so its quality depends on the crew's skill and how its spray rig is set up.
  • Air intakes must be shut off or sealed during spraying to keep fumes out of the building.

Spray foam roofs and hail

FM 4470 rates roof assemblies for moderate (MH), severe (SH) or very severe (VSH) hail; spray foam assemblies are generally listed MH or SH, and some are unrated. The rating covers the whole assembly, and the coating can change it: in Gaco's FM listings, its foam over a steel deck rates MH with one silicone topcoat and SH with another. In the severe or very severe hail zones of FM Global Data Sheet 1-34, ask for the FM listing that matches the exact foam, coating, coverage rate and deck.

Hail damage on foam is mostly surface damage, such as a broken coating. Because the foam is at least an inch thick, it typically doesn't leak and can usually be repaired rather than replaced, but have it checked soon after a storm, since exposed foam degrades in sunlight. A 1998 study tied hail resistance to the coating's strength and stretch and the foam's density and compressive strength, so compare those figures on data sheets.

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Sources

  1. Insurance Institute for Business & Home Safety (section by the Spray Polyurethane Foam Alliance), RICOWI Roof Guide: Spray Foam
  2. BASF, SKYTITE C3 Spray Polyurethane Foam Roofing Insulation
  3. BASF, SKYTITE Roofing Systems
  4. Gaco (Amrize Building Envelope), GacoRoofFoam F2733RHFO product page
  5. American WeatherStar, Application Guideline: Coated Spray Polyurethane Foam System over Single-Ply (Foam-Gard, rev. 9/18/25)
  6. Gaco (Amrize Building Envelope), GacoRoofFoam F2733RHFO Product Data Sheet (rev. 12/2024)
  7. BASF, SKYTITE Elastomeric Roof Coatings
  8. Gaco (Amrize Building Envelope), Application Specification SPF-P300-34-15: GacoFlex P300 Polyurethane Coating over Sprayed-in-Place Polyurethane Foam Roofing (rev. 04/2026)
  9. IIBEC (RCI Interface, July 1998), Hail Resistance and Performance Analysis of Elastomeric Coated SPF Roof Systems (Kashiwagi and Pandey)
  10. USACE / NAVFAC / AFCEC, via the Whole Building Design Guide, UFGS 07 57 13 Sprayed Polyurethane Foam (SPF) Roofing (August 2025)
  11. FM Approvals, Examination Standard Class 4470: Single-Ply, Polymer-Modified Bitumen Sheet, Built-Up Roof (BUR) and Liquid Applied Roof Assemblies (April 2022)
  12. Gaco, FM Approvals Roof Assemblies: GacoRoofFoam F2733 with GacoFlex S20 Silicone Roof Coating
  13. Gaco, FM Approvals Roof Assemblies: GacoRoofFoam F2733 with GacoFlex S21 Silicone Roof Coating
  14. NRCA, Professional Roofing, Designing for hail resistance (Mark S. Graham, May 2018)

Updated October 2026.