Flat roof blisters are raised, bubble-like pockets where a layer of the roof has come loose from the layer beneath it and the gap has filled with air or water vapor. They form where moisture or a void was trapped in the roof, and sun heat makes them swell because warm vapor expands. A small, firm, unbroken blister away from seams is usually watched rather than cut; one that is broken, growing, soft or sitting on a seam or flashing needs a repair and a check for wet material below. To have blisters looked at, call NEMA Roofing at 866-631-3366.
Why flat roofs blister in the heat
A blister is a pressure problem. Any pocket inside the roof, between two plies, between the membrane and the insulation, or under a coating, holds a little air and usually some moisture. When the sun heats the roof surface, that moisture turns to vapor and the pocket’s pressure rises, pushing the loose layer up. At night it cools and shrinks back, but the bond around its edge has been stretched, and the next hot day it lifts a little further. That daily cycle is why blisters appear or grow after the first hot weeks of the year, and why a dark roof, which runs hotter in the sun, shows them more readily than a reflective one.
Where the trapped moisture comes from
- Materials that were wet when covered. Insulation boards rained on before the membrane went down, a damp deck, or a concrete deck that had not dried. U.S. Department of Defense roofing criteria (UFC 3-110-03) require a moisture test on concrete decks before roofing (ASTM D4263, a plastic sheet taped to the deck), with the deck acceptable only if no moisture shows under the sheet after 24 hours.
- Voids left during installation. Asphalt mopped too thin or too cold, a torch-applied sheet that was not fully melted, adhesive skipped in spots. Air in those voids behaves just like moisture once it heats up.
- Leaks. Water getting in at a seam or flashing spreads under the surface and feeds new blisters away from the entry point.
- Moisture from inside the building. Warm, humid indoor air can carry vapor up into the roof. Where the design needs a vapor retarder, the federal criteria stress that it only works if it is continuous: moist air leaking through gaps defeats it.
Which flat roofs blister?
| Roof system | How blistering shows up |
|---|---|
| Built-up (tar and gravel) | The classic case: plies separate, often in low ridged mounds under the gravel |
| Modified bitumen | Bubbles in the cap sheet, often along laps or where the sheet was not fully bonded |
| Coated roofs and spray foam | Coating lifting off the surface where it was applied over damp or dirty areas; bubbles in foam sprayed onto a wet surface |
| Fully adhered single-ply (TPO, PVC, EPDM) | Less common as true blisters; loose, puffy areas where the adhesive let go, sometimes from wet insulation below |
Not every bump is a blister. A long straight ridge usually follows a joint in the insulation boards below; a wrinkle is loose membrane; and a spongy area with no raised surface may be wet insulation. Each points to a different cause, which is why the diagnosis comes before the fix.
Leave it, or repair it?
| Usually monitored | Repair now |
|---|---|
| Small and firm, surface intact | Broken, cracked or split open |
| Away from seams, flashings and drains | On a lap, at a flashing or in a drainage path |
| Not changing between inspections | Growing, or new ones appearing each summer |
| Dry roof underneath | Soft, wet or squishing water when pressed |
Photograph blisters and mark them on a sketch of the roof so the next inspection can tell whether they have grown. Never step on them or pop them: a broken blister lets water straight into the layers the blister was keeping dry.
How a blister is repaired
- The blister is cut open, usually in an X or by cutting out the loose section.
- The layers underneath are checked: if the insulation or deck is wet, the wet area is found and dealt with first. A patch over wet material just makes the next blister.
- The opened area is dried, the loose layers are re-adhered or removed, and a patch of the same system goes over the top, extending past the cut on every side.
- If blisters are widespread, a moisture survey maps the wet areas before anyone decides between repair and replacement.
A roof covered in blisters that keep coming back usually has a wet or poorly bonded layer throughout. The California Residential Code does not allow a new roof over an existing one that is water soaked or deteriorated (section R908.4), so in that case the old layers come off. For how widespread water is traced and what it damages, see flat roof water damage.
Preventing blisters on a new or existing roof
- Keep insulation and decks dry until they are covered, and test concrete decks before roofing.
- Ask how full bonding is achieved and checked for the system being installed, whether mopped, torched or adhered.
- Fix leaks promptly so water never spreads under the surface.
- On buildings with humid interiors (kitchens, pools, laundries), have the roof design checked for a vapor retarder.
- Consider a reflective surface: cool roofs lower surface temperatures, which reduces the heat that drives blisters. Reflectivity rules for reroofing are explained in energy efficient flat roofs.
For the other effects of summer heat on a flat roof, see flat roof maintenance for hot summers, and for the wider list of flat roof faults, common flat roof problems.
Blister checks and repairs from NEMA Roofing
NEMA Roofing is a licensed C-39 roofing and B general building contractor (CSLB license #1083283), bonded and insured. The company runs five Google-verified locations, in Santa Barbara, Oxnard, Woodland Hills, Tarzana and Downey, and serves Santa Barbara, Ventura, Los Angeles, Riverside and San Bernardino counties in full, so a roofer is never far from your building; the Ventura County area page names the towns there, coast and inland alike. See our flat roofing services, or call 866-631-3366 to have the blisters on your roof assessed.
