Why Is My Flat Roof Blistering?

A blister is a pocket. Somewhere under the surface layer, the bond to the layer below has let go, and something has filled the gap. On a felt roof it is often a bubble of air or water vapour between two layers. On a liquid-applied or resin roof it can be gas or solvent that could not escape while the coating cured. On any of them it will swell on a warm day and flatten a little in the cool.

Press a blister gently from a safe position and you can tell a lot. A soft, spongy bubble that moves usually holds air or vapour. A bubble that feels heavy, or that leaves damp on a glove, suggests liquid water trapped in the build-up below. Do not cut it open to find out. That turns a sealed pocket into an open hole.

Blisters are most often found where the roof gets the most sun, along ridges of the deck, or just over a joint in the boards underneath, because that is where movement and moisture both collect.

Moisture gets under a covering in a few ways. The deck or insulation may have been damp when the roof was laid, perhaps after rain or a frosty morning. Water vapour from the house can rise through a ceiling and settle in the build-up, which is why our answer on condensation under a flat roof matters here. Or an old leak has soaked the deck, and a new covering has been laid over the top of it.

When sun heats that trapped dampness, it turns to vapour, expands and pushes the covering upwards. Solvent works in a similar way. Adhesives, primers and some coatings give off vapour as they cure, and if the next coat goes on before that has escaped, the gas is sealed in and the covering bubbles later.

The cause decides the cure. Air in a lap can be dealt with locally. Wet insulation or a soaked deck cannot be fixed from the surface, and any covering put on top will blister again. A survey that finds where the water is, using photos of cut-outs, tells you more than a guess from the ground.

Not every bubble is an emergency. A single small blister, with the covering unbroken and no stain on the ceiling below, can be photographed, dated and checked again after the next rain. Many owners do exactly that, and a roofer would advise it on a young roof where the cause is clearly local.

Act sooner if you see any of these: a blister that has split or cracked at the top, one that has grown since you last looked, several close together, a blister on a lap or near an upstand, or a stain on the ceiling inside. Splits let water in, and once water is in, the deck is at risk.

Foot traffic is the other trigger. Walking on a blister can burst it, so keep people off the roof and tell anyone who needs to get up there, such as a gutter cleaner or an aerial fitter. If the roof is already letting water in, a temporary cover can stop the worst while the proper repair is planned. Once the survey is done, plan the proper repair once the survey is done. Our guide on how to find a flat roof leak helps when the source is unclear.

A repair suits a limited problem: one or two blisters over dry boards, with the rest of the covering sound. The roofer cuts out the affected patch, lets the deck dry, checks it, then lays a new piece that overlaps the old and is sealed with the right material for that covering. It is a small job, and one written price covers it.

A re-cover suits a roof where blisters are spread out, the deck is wet, or the covering has reached old age. Felt typically lasts 10 to 20 years and GRP 25 to 40 years, per the Less guide of March 2026. A roof beyond the middle of those spans with blisters in several places is often better renewed than patched. Any soaked boards or insulation are taken out and replaced first.

Some owners ask about covering the old roof with a new layer. That can work on a dry, sound base, but it hides trapped damp if the base is wet. Read our answer on putting GRP over felt before you decide, and see the flat roofing page for the options.