The short answer
Blisters form when moisture or air trapped beneath the felt expands in warm weather and lifts the surface. Small, intact blisters are often left alone and watched, while ones that have split, sit over a seam or keep returning let water in and need cutting out and patching. A roof covered in blisters is usually near the end of its life.
What causes a felt blister
Felt roofing, built-up or torch-on, goes down in layers that bond to each other and to the deck. If any damp or air is caught between layers during laying, it has nowhere to go. On a sunny day the roof surface can get very hot, the trapped pocket expands, and the felt bulges upward into a blister.
Moisture is the usual culprit. A deck that was damp when felt went on, rain that fell during laying, or water that has since entered through a small split and spread sideways between layers will all feed blisters. Condensation rising from the room below can supply moisture in the same way if the roof has no vapour control layer.
Poor adhesion contributes. Where layers were not fully bonded, there are voids for air and water to collect. The same can happen where felt was laid over an uneven or dusty surface.
Age plays its part too. Felt that has dried and hardened loses flexibility and cracks when it flexes. HomeBuilding gives felt a typical life of 10-15 years, and Less gives 10 to 20 years, so a roof of that age with blisters is often showing general decline rather than a single fault.
Blisters tend to appear first on south-facing areas and on dark surfaces where the sun heats them most.
Blisters you can watch and blisters that need action
Not every blister needs repair. A small, soft, unbroken blister in the middle of a sound sheet, with no cracking at its crown, can be left and checked every few months. If it has not grown or split, the felt over it is still doing its job.
Act sooner when a blister has split or cracked, because water can now enter. Treat as urgent any that sit over a lap or seam, near an upstand or outlet, or on a part of the roof where water stands. Large blisters that have grown, and groups of them, also deserve attention.
Do not pierce a blister to let the air out. A hole in the top layer opens the felt to water, and the pocket often refills. The proper repair cuts the blister open in a controlled way, dries the area, then bonds the layers back down and patches over the top with new felt that overlaps the cut generously.
That work needs dry weather and a dry deck. Patching over a damp blister traps the moisture again and the bubble returns within a season. If you notice repeated blistering in the same area, there is probably a wet deck or a source of water that must be found first.
Our page on whether a flat roof can be patch repaired covers the limits of local repair.
Ventilation and vapour from below
Some blistering starts from inside the building. Steamy air from kitchens and bathrooms drifts up into the roof build-up and condenses if it meets a cold layer. Where there is no vapour barrier beneath the insulation, or a cold deck has blocked ventilation, the moisture finds its way up to the felt and collects under it.
The current moisture code is BS 5250:2021, which is where guidance on vapour and ventilation in roofs comes from. In a cold deck roof, the void above the insulation has to be ventilated so moisture can escape. Where vents have been blocked by insulation or by later work, moisture builds up.
If blisters keep returning after repairs, check both sides of the roof. Look at the ceiling below for stains or mould, at the loft or void for wet timber, and at the vents for blockages. Our page on condensation under a flat roof explains how a roof builds up damp from inside.
Solving the moisture source is what stops the cycle. A new felt laid over a wet or poorly ventilated build-up will blister again, however well it is bonded, so any replacement should address the cause as well as the covering.
Where the roof is being re-covered, upgrading the build-up to a warm deck with a continuous vapour layer is often the most reliable cure.
Mend it or renew it
The decision rests on the extent and the age. One damaged area on a roof that is sound elsewhere, and not yet old, is a good candidate for repair. A roof with blisters across many areas, hardened and cracked felt, and a history of earlier patches is telling you that the whole covering has aged out.
Look at what is under the felt as well. If a cut-open blister shows a wet or rotten deck, the repair has to include replacing the boards, and at that scale a new covering is often the more sensible job.
The Planning Portal's approach to re-roofing also bears on the choice. If you replace the whole covering you are renewing a thermal element, and the energy standard in Approved Document L applies. Our page on insulating a flat roof when replacing it explains that.
We inspect the roof in person, cut a test section where needed, and share the photographs and measurements. If a repair will hold, we recommend it; if the roof has had its day, we explain why. The figure we give is a single written total.
The flat roofing hub sets out the replacement coverings, from felt to EPDM, GRP and lead, with their typical lives.
