Blisters on a flat roof: should you worry?

When a membrane bonds to a deck, any air or damp left underneath has nowhere to go. In warm weather the trapped air expands and the vapour pressure pushes the covering up into a dome. Felt roofs show it most: the bitumen softens, the surface lifts and a soft bubble appears, sometimes the size of a coin and sometimes the size of a dinner plate.

Blisters also form on single-ply and liquid systems if the surface was not dry or clean when laid, and GRP can bubble where resin trapped air during curing or the deck gave off damp. The mechanism differs but the cause is the same: something is under the covering that should not be.

Sunshine on a dark surface speeds all of this up, which is why blisters often show after a warm spell. Roofs without a vapour layer are prone to it, but a blister can show up anywhere that moisture or air was sealed in.

The usual causes are practical rather than mysterious.

• A damp deck. Timber boards or an existing layer that was not dry when the new covering went on release moisture as the roof warms.
• Trapped air at the bond. Poor adhesion leaves gaps that fill with air and lift with heat.
• Moisture from inside. On a roof without a vapour control layer, warm air from the house passes up through the deck and condenses under the membrane.
• An old leak. Water that has got in at an upstand or edge and spread under the surface forces the covering up.

Which one applies shapes the repair. A cosmetic blister from trapped air is a different problem from a bubble filled with water from a leak. Our page on why a flat roof feels spongy covers what a soft, wet deck means.

Sometimes. A small, firm blister in the middle of a roof, with no cracking and no sign of damp inside, can be watched. Mark its edge with chalk and check it after a few weeks, photographing it from the same spot each time so you can compare its size and shape across the seasons. If it has stayed the same size, it is probably stable.

Leave it alone only while it is intact. The covering is stretched across a blister, so it is thinner and more brittle there. Foot traffic, a hailstone or a winter freeze can crack it, and then rain gets straight into the layers below. Blisters on seams, laps and upstands are more urgent because the joint is already the weakest part of the roof.

Please do not pierce or press a blister to let it down. That makes a hole in the waterproofing, and the bubble often returns.

Repair makes sense when a blister has split, is wet inside, sits on a joint, is growing, or comes with ceiling stains below. Several blisters in one area suggest damp over a wider patch, which is usually a sign that the deck has wetted through.

For felt, a repair means cutting out the affected area, drying and checking the layers beneath, then patching in new sheets lapped over the sound felt. For single-ply or GRP, the damaged area is cut out, prepared and patched with a compatible system. A repair is only as good as its preparation: patching over damp simply creates a new blister next summer.

If blisters cover a large share of the surface, the better answer may be to strip back and replace. Our guide to the minimum fall for a flat roof explains why a poor fall makes damp worse, since water that stays on the surface finds the weak points faster.

A localised repair costs less and disturbs less, but only suits a roof whose remaining covering is sound. Where the covering is nearing the end of its expected span, which Less puts at 10 to 20 years for torch-on felt and MyBuilder at 15 to 30 years for flat roofs generally, repairs buy time but add patches.

Replacement is the proper answer when the deck is soft, the blisters are widespread or the membrane has failed along the edges. It also gives a chance to add a vapour control layer or insulation, so the cause is removed rather than patched. Our page on liquid flat roof coatings explains one option that is sometimes used over a sound, dry surface.

Wet insulation or decking under a blister has to come out. Covering it again just seals the problem in.

On site we examine the blister itself, feel the ground around it, look at the underside from the ceiling side where we can, and take photographs. The written quote that follows may be for a patch, or it may be a recommendation to renew the covering, and the pictures show you why. We would rather mend one bubble than sell a roof that does not need replacing, though a widespread problem calls for the bigger job.

Either route comes with a written 10-year workmanship guarantee, and we sweep up after ourselves. Phone 01793 626096, message on WhatsApp or send the form to book a visit to your flat roof. If a ceiling is already stained, mention it when you call, because a wet ceiling moves you up the list and we deal with the water coming in before anything else.