Built-up felt roofing is a flat-roof covering made from several layers of bitumen-coated felt bonded together to form one waterproof skin. Those bubbles you see on an older roof are blisters — pockets of trapped air or moisture that have expanded under the sun and lifted the felt away from the layers beneath. They are one of the most common signs that a felt roof is ageing, and the rest of this guide explains how the system works and why it behaves that way.
What a built-up felt roof actually is
"Built-up" simply means the roof is constructed in layers rather than a single sheet. A traditional system uses three layers of felt — a base layer fixed to the deck, an intermediate layer, and a top layer called the cap sheet — each bonded to the one below. Bitumen, a sticky black material derived from crude oil, is the glue and the waterproofer all in one.
The felt itself is a fabric or fibre mat soaked in bitumen so it stays flexible and sheds water. Older roofs used a fibre called organic felt; later versions moved to a polyester or glass-fibre carrier, which copes far better with movement. The reason for stacking several layers is redundancy: if one layer is breached, the others underneath still keep the building dry.
The visible surface is the mineral cap sheet — a top layer with fine stone granules pressed into it. Those granules are not decorative. They protect the bitumen below from ultraviolet light, which is the main thing that ages a felt roof, and they give some grip and fire resistance. When you look up at a flat roof and see a grey, green or reddish gritty finish, that is the cap sheet doing its job.
Felt roofs sit on a deck — usually timber boards or a sheet material like plywood or OSB — which gives the covering something solid to bond to. The deck normally has a slight fall, a gentle slope of perhaps one in sixty, so water drains towards a gutter or outlet rather than sitting in a pool. A "flat" roof is rarely truly flat.
Why older felt blisters, cracks and lifts
Built-up felt roofing is a flat-roof covering made from several layers of bitumen-coated felt bonded together to form one waterproof skin.
Older felt fails for a handful of predictable reasons, and most of them come down to time, sunlight and water.
Blisters form when air or moisture gets trapped between layers, or between the felt and the deck, often because a layer wasn't fully bonded when the roof was laid or because moisture later worked its way in. Heat makes the trapped gas expand, the felt has nowhere to go, and it domes upward. A blister isn't always an immediate leak, but it is a weak point — the stretched felt is thinner and more likely to split.
Cracking and splitting usually follow years of sunlight and temperature swings. Bitumen hardens and loses its flexibility as ultraviolet light drives off the oils that kept it supple. Once the granules on the cap sheet wear thin, that ageing speeds up. The roof expands in heat and contracts in cold every single day, and brittle felt eventually tears along the lines of greatest stress — often at upstands, where the roof meets a wall, or at joints between sheets.
Lifting happens where the bond gives way. Wind can get under an edge that was never properly sealed and peel the felt back, or moisture under the surface can break the adhesion across a wider area. Common trouble spots include:
- Edges and upstands, where felt is dressed up against walls, kerbs or skylights and is most exposed to movement.
- Joints and overlaps between rolls, which are only ever as good as the bond made when they were laid.
- Around outlets and pipes, where water concentrates and the felt is cut and shaped.
- Low spots where rainwater ponds and slowly works at any weakness.
It is worth understanding that a few blisters do not automatically mean a roof must be replaced. A surveyor will usually look at the overall condition — how brittle the cap sheet has become, whether splits have reached the waterproof layer, and whether there is moisture trapped below — before judging whether patch repairs are sensible or whether the covering has reached the end of its life.
How a modern torch-on system differs
"Torch-on" describes the method used to install many modern felt roofs. Instead of pouring hot bitumen between layers, the installer heats the back of the felt roll with a gas torch until the bitumen coating melts, then rolls it down so it fuses to the layer beneath. The result is a continuous, welded bond rather than a glued sandwich.
The biggest practical change is in the felt itself. Modern torch-on membranes use bitumen that has been modified with polymers — most commonly SBS (a rubber-like additive) or APP (a plastic-like additive). SBS keeps the membrane flexible in cold weather, while APP improves its tolerance of heat. Either way, the membrane stays elastic for far longer than the old organic felts, so it copes with daily expansion and contraction without becoming brittle as quickly.
These systems are also usually a two-layer build rather than three: a torched or self-adhesive underlayer, then a torch-on mineral cap sheet on top. Because each layer is fused over its whole area rather than spot-bonded, there is less chance of air pockets and therefore fewer blisters in the first place. The polyester carrier inside the felt resists tearing, which helps with the splitting that plagued older roofs.
None of this makes a torch-on roof permanent. It still relies on a sound deck, proper falls, good detailing at the edges and around outlets, and a cap sheet whose granules will eventually wear. The open-flame method also calls for care — there is a fire risk during installation, and many people prefer self-adhesive or cold-applied alternatives near combustible structures. The honest comparison is that modern systems tend to fail more slowly and more predictably than the older felts, not that they never fail.
If you are weighing up an ageing felt roof, the useful questions are how old the covering is, how brittle the surface has become, and whether the underlying deck is dry and sound. Those answers tell you far more than the number of bubbles on the surface alone.
Reviewed: June 2026