The short answer
A blister on a felt flat roof is not an emergency by itself, but it is not cosmetic either. It means moisture or air is trapped between the layers, and the felt is lifting away from what it should be stuck to. A small, unbroken blister can be watched. One that is split, soft underfoot or growing needs a repair, and a roof covered in them is usually an ageing roof.
What a blister in the felt actually is
Built-up felt and torch-on felt are laid in layers of bitumen-coated fabric, stuck to each other and to the deck beneath. A blister forms where that bond has failed and a pocket has opened up. The pocket holds air, water vapour or liquid water, and the felt above it is pushed up into a dome.
The dome is only the visible part. The layer below the felt, whether that is a vapour layer, insulation or the boards themselves, is wet or has let go at that spot, and the felt on top has nothing sound to grip. Tap the roof around one and you will often hear a hollow note.
Our guide to torch-on felt roofs and how the layers are built up covers the system as a whole. This page deals with the single fault.
Why moisture ends up trapped under the felt
Water gets under the felt in a few ordinary ways. It can be built in: a deck that was damp when the felt went on, or rain on an uncovered deck during the job. It can arrive from below, as warm, damp air from the room rising through gaps in the ceiling and condensing in the roof build-up. Or it can come from above, through a small defect such as a split at a lap, a failed upstand or a cracked joint around a vent pipe.
Sun does the rest. On a warm day the trapped moisture turns to vapour and expands, pushing the softened bitumen upward. When the roof cools, the vapour condenses and the felt may sink back, only to lift again the next hot spell. Each cycle works the bond a little looser, which is why blisters tend to grow rather than shrink.
That is also why we look at how a roof is laid out before judging the blister. A cold deck roof with poor ventilation condenses far more readily than a warm one. Our guide to warm and cold flat roof construction explains the difference.
When a blister can wait and when it cannot
Not every blister needs a ladder the same week. These are the things we weigh up on a survey.
| What you can see or feel | What it usually means | Sensible next step |
|---|---|---|
| One or two small, firm, unbroken domes | A localised pocket, felt still sealed | Photograph, mark the edges and check again after rain |
| A dome that has grown since you last looked | Moisture is still moving into the pocket | Book a survey before winter |
| A blister with a split, or granules missing | Water can now get in and out freely | Repair promptly |
| Soft or springy felt around the dome | The deck below may be wet or rotting | Treat as a deck problem, not a felt problem |
| Blisters scattered across the whole roof | Widespread loss of bond, often an ageing system | Compare repair against replacement |
The point at which a blister stops being harmless is the point at which it is broken, or the felt around it has lost its seal. Until then the felt is still doing its job, though the repair gets larger the longer it waits.
How a blister is repaired
Cutting the top off a blister and pressing it flat is the repair most people picture, and on its own it is the wrong one. Water sealed in underneath stays there, and a flattened blister can crack along its crease within a season.
A proper repair follows a sequence. The felt over the blister is cut open and folded back so the pocket can be inspected. If there is water, the layers are allowed to dry properly, because bonding a new patch over a damp surface only builds the next blister. Any rotten or wet boards are cut out and replaced. The felt is then laid back down and a patch is torched over the whole area, overlapping the sound felt around it on every side.
A small patch repair like this belongs in the repair end of the range. Across the UK, MyBuilder's 2026 figure for a minor repair is about £100 to £300, with the final figure depending on access and how much deck has to be renewed. Your written quote from us follows a survey and covers the whole job.
If you are weighing up a patch against a new covering, see what replacing the membrane triggers on insulation before you decide, because that changes the cost of replacement.
Blisters as a sign that the whole roof is ageing
One blister on a young roof is a defect to fix. A dozen across an older one is a pattern, and the pattern matters more than any single dome. The sources quoted for felt roofs disagree about their life: Less (2026) says a well-laid felt roof may reach 20 years but a poor one can go in under 10, and MyBuilder puts flat roofs generally at 15 to 30 years. These are typical ranges, shaped by workmanship, ventilation and exposure, and never a promise.
Patch repairs on a roof that is blistering everywhere tend to chase the problem around the roof. Dry out one pocket and the next opens within the same stretch of felt. At that point the choice is a conversation about the whole covering. A felt roof can be replaced with a new felt system, or with another material such as EPDM or GRP, and our flat roofing page sets out the options we price.
What we would not do is lay a new membrane over a blistered, wet one. Trapped moisture does not go away because it has been covered, and the new layer would eventually lift in the same places.
Where the work is a repair, the guarantee we give is 10 years on workmanship, in writing. Survey photographs show the blisters we found and the deck beneath them, so the quote matches your roof.