The short answer
A blister is a pocket of trapped air or moisture under the top layer of felt. Heat expands the pocket in summer, and age makes the bitumen soft and the layers less well bonded. Many small blisters are cosmetic, but one that is split, wet inside or growing is a sign that water is getting into the roof. Looking and pressing is how a roofer separates the harmless from the harmful.
Bubbles in a felt roof are common, and they do not all mean trouble. What they hold and where they sit decides whether they matter.
What a blister actually is
Felt roofing is built in layers of bitumen-coated fabric, and in torch-on work the layers are fused together by heat. A blister forms where two layers have not bonded, or have come apart, and a pocket opens between them. Air or water vapour gets into that pocket, and it swells.
Why do blisters appear on warm days?
Trapped air expands when it is heated. A dark felt surface can get hot in strong sun, so a pocket that sat flat in March may stand up in July and settle again at night. That daily push and pull works on the bond, which is how a small bubble becomes a larger one over a few seasons.
Moisture adds to it. If the deck or insulation was damp when the felt went on, or if damp has crept in since, the water turns to vapour in the heat and pushes the layers apart. This is the usual route to the larger, spongy blisters.
The torch-on felt guide explains how the layers are put together and fused: torch-on felt roofs.
Age, weather and the layers underneath
A felt roof does not stay supple for ever. Less puts the life of torch-on felt at 10 to 20 years, with the caution that poor workmanship can end it inside 10 (Less, Oct 2026). Blistering tends to show up in the second half of that range, as the bitumen dries out and loses its grip on the layer below, which is why the felt may also look dull and a little brittle.
Does blistering mean the roof is old?
Not always. A young roof can blister if it was laid in cold or wet weather, over a damp deck, or without enough fusing at the laps. In that case the blisters appear early and in patches, often along the line of a join. On an older roof they are more scattered, and the felt around them tends to look dull, cracked or gritty where the mineral top has worn away.
A roof that is mid-life, with a handful of small blisters and no leaks, is a different case from one that is failing. The layout of the problem tells a roofer much more than the count does: clustered blisters on one side suggest a wet patch below, while an even spread suggests ageing.
A ceiling stain beneath a blister is the cue to call someone out rather than wait for the next shower.
When a blister matters and when it can wait
You can learn a fair amount from the ground or a ladder. A smooth, unbroken blister the size of a coin or a hand, with sound felt all around it, is often left under watch, with a photograph taken today to compare against next season. It is the ones that change that need attention.
Should I pop a felt blister?
No. Cutting or piercing one lets rain straight into the build-up, and the usual outcome is a wet roof, not a flat one. Likewise, do not smear sealant over it. The covering needs a proper repair, in which the blister is opened, dried, fixed down and patched, or an area cut out and replaced.
Treat it as more urgent if the blister has split, if it feels soft or squelchy under a light press, if it is growing between your checks, or if water has marked a ceiling or wall below. Several blisters along a seam or around an outlet also deserve a look, because those are the places where water enters.
The answer on flat roof patch repair explains what a good patch involves.
Patch, overlay or replace?
What follows depends on what the roof looks like once it is opened up. A few small, dry blisters on a sound roof can be cut out and patched. Widespread blistering over damp insulation is a stronger argument for a new covering and, if the deck below is wet, new insulation too.
What decides the cost of a felt repair or replacement?
The extent of the damage, the access, and whether the deck and insulation are dry all move it. For scale, MyJobQuote's guide to felt roof replacement gives an average of around £1,500, and £1,200 to £1,800 for SBS felt on a 15 square metre garage, SBS being a rubberised bitumen type (MyJobQuote, Oct 2026). Those are typical UK ranges, so your own figure depends on your own roof.
On our side, a survey comes first. We test for moisture instead of guessing at it, and you get to see pictures of the layers beneath the bubbles. One written total then follows for whichever route you pick, the repair or the re-cover. See also overlaying a flat roof and our flat roofing page.
More on felt roof bubbles
Will a blistered felt roof leak straight away?
Not necessarily. An intact blister is still a watertight skin, and the roof can carry on for a time. The risk comes when it splits or when the bond fails around it. Keep an eye on the ceiling underneath, and photograph the blister now so you can see later whether it has grown.
Can I cover blisters with another layer of felt?
Rarely, and only after the blisters and any wet material have been dealt with. Covering them traps whatever is inside, and heat then works on the new layer as well. An overlay is possible on some sound roofs, and a roofer checks the moisture level and the deck before saying yes.
The full picture: Flat roofing in Taunton · More short answers