Question answered
Blisters on a felt roof: should you worry?
The short answer
A few small blisters on a felt roof are not an emergency. A blister is a pocket where moisture or air is trapped between the layers, and an intact one can stay unchanged for a long time. Worry when one splits, when they multiply, or when the deck underneath feels soft. Then the choice is a cut-and-patch repair or, where the damage is widespread, replacement.
A bubble in the felt looks alarming, yet many sit quietly for years. What matters is whether it is intact, whether it is spreading and what lies underneath it.
Why felt bubbles in the first place
Built-up and torch-on felt are made of bitumen-coated layers stuck together. If a little moisture or air is sealed between those layers, or between the felt and the deck, the sun does the rest. Heat on a dark surface makes the trapped vapour expand and lifts the top layer into a dome. At night it cools and the dome may settle, then rise again next summer.
The moisture usually arrives during the original work. Boards that were damp, felt laid over morning dew, or a shower that came before the job was closed up can all leave water behind. Joints in the deck boards, and the areas around vents and outlets, are common places for the adhesion to fail first.
A blister is therefore a symptom of how the roof was laid, not a sign that a storm has hit it. That is also why the cure depends on what is inside the blister, not on how it looks from the ground.
Leave it, watch it or open it
A small, firm blister with the mineral surface still intact is often best left alone and watched. Photograph it, note its size against a coin or a tape, and look again after the next winter. Cutting open a sound blister can create a leak where none existed.
The picture changes when the surface cracks, when the granules have worn off the dome, or when pressing the blister makes water squeeze out. Several blisters appearing in a row, or close to an upstand or an edge, also deserve a closer look, as does a damp patch on the ceiling below.
Do not climb up to prod them. Felt can be slippery with moss, and the skin of a blister is thin, so a boot can split it. If you can see the roof from an upstairs window, take pictures from there and send them to us. A survey from the roof and from inside the ceiling tells you much more than any view from the ground.
What a proper cut-and-patch involves
Where a blister needs opening, the repair is slow work rather than clever work. The felt is cut in a cross, the flaps are folded back and the exposed layers are allowed to dry fully. Sealing water in is the mistake that makes the blister come back.
Once the felt and the board beneath are dry, the flaps are stuck back down and a patch of cap sheet is fixed over the whole area, reaching well past the cut so the edges shed water. Where the deck underneath has gone dark or soft, the damaged board is replaced before any felt goes back on.
Two shortcuts are worth avoiding. Pricking a blister and daubing it with mastic or bitumen paint traps the damp and hides the evidence. A patch laid straight over a wet blister behaves the same way, and the patch tends to fail early. If a patch cannot be made to hold, that tells you something about the roof around it, and the next step is a wider look at the whole covering.
When blisters mean the roof is finished
Felt does not last for ever. Less (updated March 2026) puts torch-on felt at 10 to 20 years, and says that a well-laid roof can reach the upper figure while careless work may give out inside ten. MyBuilder (updated 21 August 2026) puts the typical life of a flat roof between 15 and 30 years. A roof towards the end of that range, with blisters across much of the surface, has moisture spread through its layers.
Other warning signs are a sagging deck, pools that stay for days after rain, and repairs that keep reappearing in new places. At that point the sensible comparison is between repeated patching and a new covering. An overlay is possible only where the old layers and the deck beneath are dry and sound, and that has to be checked, not assumed.
What a new roof costs turns on size, access and what is found underneath. Our page on flat roof cost lays out the typical ranges for each covering, and you get a firm written figure only once the survey is done.
What we look for when we survey the roof
We start from the cause. Is the blister a one-off, or one of a line running along the deck joints? Are the outlets clear, and does the roof have enough fall to clear water? Is the ceiling below stained, or the insulation damp? Water can travel a long way inside a flat roof build-up before it shows, so the visible blister is sometimes the least of it.
We then show you what we found. Pictures taken on your own roof arrive alongside our written quote. They let you weigh up whether a small patch will do, and we recommend the lightest repair that cures the cause.
If the answer is a full renewal, the season matters, and our page on replacing a flat roof in winter explains what the cold does to the work. How felt, single-ply and fibreglass roofs compare overall is covered on our flat roofing page.
Felt bubbles: two quick questions
Can I pop a blister myself and seal it with bitumen paint?
It is best not to. Piercing the blister lets the trapped water settle into the layers, and paint over the top seals it in. The blister then returns, often larger. A dry cut-and-patch repair, or a survey first, is the better route.
Will a new layer of felt over blisters fix them?
Only if the old roof is dry and flat underneath. New felt laid over damp, blistered layers tends to blister in the same places. The survey decides whether an overlay is sensible or the old covering has to come off.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.