A blister is the coating lifting off the foam, or one foam lift separating from the next, with air or moisture in between. In 115°F surface heat it grows. Some are cosmetic for years; some are a leak waiting for the first storm.
| What you see | Likely cause | Action |
|---|---|---|
| Small (under a fist), firm, coating intact | Coating applied over damp or dusty foam | Watch; coat over at next recoat |
| Large, soft, moves underfoot | Moisture between foam lifts or under coating | Cut out, dry, refoam, coat — $400–$900 [est.] |
| Split open, foam showing | Blister failed; UV now on bare foam | Repair this season; it is a leak |
| Soft spot with no blister, dark stain | Saturated foam from an old pinhole | Core sample; cut out the wet area |
A roofer cuts a 2-inch plug, looks at the foam colour and squeezes it. Dry foam is light and springy; wet foam is dark and dense. Takes five minutes and decides whether you're buying a patch or a section. Ask for it on any roof with soft spots.
Blisters cluster on roofs that were coated in a hurry or over dew. The prevention is a dry roof and a primer — both reasons to recoat in April–June rather than after a storm. Recoating →
Small, firm blisters with intact coating are cosmetic and get coated over at the next recoat. Large soft ones or any that have split are leaks in progress and should be cut out and refoamed.
Don't. Opening it exposes foam to UV and water. A roofer cuts it out cleanly, dries the area, refoams and coats in one visit.
Coating over damp or dusty foam, foam sprayed over a wet substrate, or moisture entering through a pinhole and expanding in heat. Phoenix surface temperatures make them grow faster than anywhere.
LAST REVIEWED 2026-11-01 · Prices on this site are published Valley rates and estimates, not a quote.