What causes blisters on a felt flat roof?

Felt is not a single sheet. A traditional built-up roof has several layers of bitumen-coated membrane, and torch-on felt is fixed down by heating the underside so it melts into the layer below. A blister forms where that join never took hold, or has since let go, and something has slipped into the gap. That something is nearly always a little water or a pocket of air.

Once the sun warms the roof, the trapped moisture turns to vapour and the air expands. Both push upwards, and the felt, which is at its softest when warm, gives way and rises into a dome. At night it cools and settles a little, then rises again the next warm day. That daily movement is what tires the felt, so a blister that looks harmless in spring can be cracked by the following winter. Where several blisters sit close together, the felt between them is being stretched in every direction.

The most common source of that moisture is the roof itself. If the boarding or insulation was wet at the time the felt went down, or rain got under an unfinished edge, the water has nowhere to go. Felt is made to keep water out, which also means it keeps water in. Condensation from the room below can do the same job if warm, moist air finds a way up into the build-up, a problem we cover in our answer on flat roof condensation.

Poor preparation is the other cause. Felt laid over a dusty, damp or uneven surface never grips fully, and a thin layer of dust is all it takes to leave a weak patch. Blisters that appear in the first year or two usually point to what happened on the day it was laid. Blisters on an older roof point more towards wear, and moisture getting in through small splits elsewhere. A line of blisters following a board joint often means the deck has moved.

From the ground you cannot tell much, so the first job is a look from a safe position. A blister that is small, firm, unbroken and sitting in a flat area with no ponding around it is a watching brief. Take a photograph of it with a coin beside it for scale, then compare a few months later. If it has not grown, the felt has probably settled into a stable shape.

Do not press on a blister to see whether it is soft, and do not pierce it. Cutting a blister open lets the trapped water out, but it also leaves a wound in the membrane that has to be dried, sealed and patched properly, which needs a roofer with a torch and fresh felt, not a quick dab of sealant. A cheap sealant over a bubble that is still moving will usually crack within a season, and then you have a leak nobody can trace.

A blister needs attention when it has split, when there is a run of them, when it sits on a seam or along an upstand (the short vertical edge where the flat roof butts against a wall), or when there are brown stains on the ceiling below. Those signs mean water is already moving through the layers. A blister over a damp spot is a symptom, so the cure is to find and dry the damp, not just to flatten the bubble.

The repair is to cut the blister out, let the exposed layers dry, check the deck underneath, and lay patches of new felt overlapping the old. Less (last updated March 2026) puts small flat roof repairs at £150 to £500 across the UK. The quote you receive is one written total, set after the survey, because the figure turns on how many areas are involved and how much damp the deck has taken on.

Blisters scattered across most of a roof tell a different story from one bubble beside a vent. Less puts torch-on felt at 10 to 20 years, with the note that a well-laid job reaches 20 while a poor one can fail inside 10. When the covering has reached the later part of that range, ponds as well as blisters, and cracks when you flex a loose corner, chasing single bubbles is money spent on a covering that is leaving.

At that point the better advice is to replace, and the survey photographs make the case. When a repair will hold, the smaller job is the one we recommend. Our page on whether to repair or replace a flat roof sets out the deciding signs, and the main flat roofing page covers the coverings we lay, including the felt systems described in our guide to torch-on felt roofing.