The short answer
Blisters form when air or water vapour gets trapped under the covering and expands in the sun. On felt they are the commonest sign of moisture in the layers below or a poor bond between them. On GRP they usually come from trapped air or damp in the deck during laying. Small, dry, stable blisters can be watched. Large, soft or cracked ones let water in and need repair before they split.
What a blister is
A blister is a bubble where the covering has lifted away from the layer beneath. Inside it sits air, vapour or both, and often a small amount of water. Heat makes the trapped gas expand, which is why blisters swell on sunny days and sometimes appear to shrink overnight. The pressure stretches the surface a little more each summer, and each cycle of heat and cold works at the edge of the bubble.
Not all blisters are equal. A small one the size of a coin, firm to the touch, with no cracking, is a cosmetic flaw. A large soft one that feels spongy, or one with splits around the edge, is a leak waiting to happen.
Why felt blisters
Torch-on and built-up felt are made of layers bonded together with hot bitumen. If the deck or the layer below was damp when the felt went on, the moisture turns to steam under the torch and leaves a pocket. If the layers were not fully bonded because the surface was dusty, cold or poorly heated, the gap lets air in. Water vapour rising from the house can also collect under the membrane where there is no effective vapour control layer.
Once a blister exists, thermal cycling opens it. A cap sheet that has been hot all afternoon and cool all night moves, and the felt at the edge of a blister gradually tears. Cutting one open and sticking it down is a short-term fix at best, because the moisture that caused it is often still in the layers. Our guide to torch-on felt roofing explains how the layers are put together.
Blisters in GRP and EPDM
GRP is a resin and glass-matting laminate. Blisters here are usually air trapped as the resin cured, or moisture from a damp deck pushing through the laminate on a warm day. A roof laid in humid or cool weather is more at risk, since the resin does not wet out the matting as well. Small blisters in the topcoat can be ground back and patched. A blister that goes through to the glass layer lets water in and needs cutting out and relaminating.
EPDM rubber sheet can bubble too, usually where the adhesive has not bonded fully or where air has been trapped as the sheet was rolled out. The bubbles tend to be soft and wide rather than hard and tight. They are more often an installation issue than a sign of age, and they should not be left to rub against edges or fixings.
How blisters are repaired
The repair follows the cause. For a dry, small blister on felt, cutting a cross, drying the layers, bonding the flaps and capping the patch can last well. For a larger area, or one where the insulation below has been wetted, the covering and the wet insulation both come off, because new material over wet layers will blister again. A blister with water inside is a clue that there is a wider problem, and our article on the signs of flat roof failure lists others to look for.
If several blisters have appeared across a roof, the underlying issue is usually widespread damp or a failed vapour layer, not a series of individual faults. In that case a strip and relay with the correct build-up beats a succession of patches. Wet insulation and rotten decking show themselves only when the roof is opened up.
Why heat and sunlight make them worse
South-facing roofs and dark surfaces take the worst of the sun, and the temperature of a black felt roof on a summer afternoon is far higher than the air around it. That heat drives the gas in a blister to expand, and the stretched surface weathers faster than the flat felt beside it. A reflective finish, such as a light-coloured solar-reflective paint or a mineral cap sheet, keeps the surface cooler and slows the cycle. It does not cure a blister that is already there, but it reduces the stress on the covering.
Telling a blister from something else
Not every bump is a blister. A ridge along a board joint is often movement in the deck. A pool of water that makes the felt look swollen is ponding, which has its own cause, covered in our page on ponding on a flat roof. Photograph the bump from the same spot over a few weeks and note whether it grows.
Our free survey includes a close look at your blisters, and we can cut a small test opening, with your agreement, to check whether the layers below are wet. We then tell you whether a patch will do or the roof needs more. The fixed written quote covers the work and the 10-year workmanship guarantee stands behind it. See the main flat roofing page for the systems we fit.
Every roof is different. A free survey with photos settles it for yours.
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