Built-up felt is one of the oldest flat roof systems still laid on shops, offices, workshops and warehouses, and a great many of them are still working. The principle is simple: several layers of bitumen-based sheet are fused to one another over a deck, so that no single seam or pinhole carries the whole weight of keeping the building dry. In practice the result depends on the layers underneath, the way the laps are made, the finish on top and the state of every edge. A roof of this kind rarely fails across its whole surface at once. It fails at a lap, a rooflight kerb, an outlet or a parapet upstand, and the skill lies in reading which. This guide describes what a built-up system consists of, how long one can be expected to last, where leaks start, and how we decide between patching, overlaying and stripping on a commercial building around Bath. It sits within our wider commercial roofing work.
What built-up felt actually is
The name covers a family of systems rather than a single product. A built-up roof has at least two layers, often three, and each is a sheet of reinforced bitumen. The reinforcement is polyester or glass fibre, which gives the sheet strength and lets it stretch a little as the building moves. The bitumen is the waterproofing. Older roofs used hot-poured bitumen mopped between layers of fibre felt. Most work now uses torch-on sheets, where the underside of the roll is softened with a gas flame and pressed down to fuse with the layer below.
The layers do different jobs. The base layer is usually nailed or stuck to the deck and acts as an anchor. Any intermediate layer adds thickness and a second line of defence over the joints of the first. The top sheet is the cap sheet, and it carries a finish that protects the bitumen from sunlight: either a factory-applied mineral granule surface or a coat of reflective paint or chippings added on site. Historic Churches guidance, the conservation reference we draw on for this, ties the life of a built-up roof to the quality of that solar finish, and that matches what we see on commercial roofs where the cap sheet has been left bare for years.
The point of building the roof in layers is redundancy. A pinhole in the cap sheet does not necessarily mean a leak, because the sheet beneath is fused to it and has its laps in different places. That is the real difference from a single membrane, where one failed seam is the whole story. It is also the reason a built-up roof often gives warning before it lets water in, if someone is looking.
The build-up from deck to finish
What sits beneath the felt matters as much as the felt. Starting at the bottom, a commercial flat roof has a structural deck, which may be concrete, timber boarding or plywood, or profiled steel. Over the deck there is normally a vapour control layer, which keeps warm moist air from inside the building away from the cold upper layers, where it would condense. Above that sits the insulation, and above the insulation the felt layers.
This arrangement is known as a warm deck, because the insulation sits above the structure and keeps the deck at room temperature. It is the common form for commercial work. The older alternative, a cold deck, puts insulation below the deck with a ventilated void above it. Cold decks are harder to keep dry in practice and are the usual starting point for a thermal upgrade, because changing to a warm deck takes the problem out of the void altogether.
That is where Building Regulations come in. Repairs and re-covering are normally outside approval while they touch under 25 per cent of the building envelope and under 50 per cent of the roof. Stripping most of a commercial roof passes both limits, and the roof can then be treated as a thermal element to be upgraded, which is a statutory reason to look again at the insulation while everything is open. The Planning Portal sets out the re-roofing rules. We raise them at the survey, so the quote does not have to be revised later.
Falls are the last element of the build-up. BS 6229:2018 recommends a finished fall of at least 1:80, and manufacturers advise designing to 1:40 to absorb building tolerances and sag in the deck, which would otherwise leave hollows. A felt roof with no fall holds water, and standing water speeds up the ageing of the bitumen.
How long a built-up roof lasts
Published figures disagree, and the disagreement is informative. One cost guide gives torch-on felt a typical life of 10 to 20 years, noting that careful laying can reach 20 while poor laying can fail within ten years. Stirling City Heritage Trust says around 20 years. Historic Churches guidance allows up to 30 years, and the roofing contractors' confederation, CORC, suggests 30 or more with good upkeep. The cautious reading is 10 to 20 years as a typical life, stretching further for a well-made three-layer build, with exposure, workmanship and maintenance deciding where a given roof lands. None of these is a promise about your roof.
What separates a long life from a short one is rarely the felt brand. It is usually one of four things.
- The number of layers. A two-layer system has one fewer sheet to cover a defective lap.
- The quality of the details. Upstands, flashings and outlets take more movement than the open field of the roof.
- The finish. A cap sheet with intact mineral granules, or a maintained reflective coat, keeps sunlight off the bitumen.
- The attention it gets. A roof that is cleared of leaves and checked after storms ages more slowly than one nobody visits.
Age alone does not condemn a roof. A twenty-year-old system with sound laps, a few patched details and a decent finish may have years left. A ten-year-old one with open laps on a south-facing slope and blocked outlets may not. Each roof is judged on its own condition, with the findings reported area by area.
Where built-up felt fails
Leaks on felt roofs cluster in the same places, and knowing them helps a building manager brief a roofer sensibly.
Laps and seams
The side and end laps of each sheet are where it is joined, and they are the most common point of failure. Where a lap was never completely fused, or thermal movement has stretched it, it opens up as a fine line. On torch-on systems you can often feel it with a thumb before you can see it.
Upstands, kerbs and parapets
The turn-up of the felt at a wall, a rooflight kerb or a plant base takes the most strain. The felt is forced to bend through a right angle, then dries out and cracks at the fold. A parapet gutter, the lead or asphalt channel running behind a short wall along the roof edge, adds a second problem: it collects water from the field of the roof and any defect in it shows quickly. Wall tops and cover flashings, usually lead, are the other weak point, and where they have lifted the water goes behind the felt.
Outlets
An outlet is the point where water leaves the roof and enters a downpipe. A blocked or poorly sealed outlet makes the roof pond, and then the nearby laps carry water they were never designed for.
Blistering and splitting
Blisters appear when moisture trapped in the build-up turns to vapour in sun and lifts the felt. A single blister may be cosmetic. A cluster across a slope suggests wet insulation beneath, which no patch will cure. Splits across the roof, often running in straight lines, usually follow the joints of the deck below.
Repair, overlay or replace
The right call depends on three things: how widespread the defects are, whether the insulation is dry, and how much life the existing layers still have. We go through them in that order.
Localised failure, such as a split at a single upstand or a few open laps, is a repair. The area is cut back to sound felt, dried, and patched with new sheet lapped well beyond the damage, and the detail is redressed so it does not fail in the same way again. This is the right answer where the rest of the roof is in good order, and it is the answer we give where it works.
Where the existing system is sound underneath but tired on the surface, an overlay can be considered: a new membrane laid over the old one after preparation. That saves stripping, skips and disturbance to the occupants. It is only suitable when the insulation is dry and the existing layers are well adhered, because a wet or blistered build-up simply goes on holding the water. We deal with when this is and is not the right route in a separate guide on overlaying commercial roofs.
Full replacement is the answer when the insulation is saturated, when blistering and splitting are widespread, or when repairs have built up into a patchwork. The old system is stripped to the deck, the deck is checked and made good, and a new vapour control layer, insulation and waterproofing go down. That is also the moment to decide whether another felt system is the best choice or whether a single-ply membrane suits the building better, a comparison made in the guide to single-ply roofing.
One caution applies to older industrial buildings. Beneath a felt roof there can be an older deck or sheeting that was installed when asbestos-containing products were still in use, and it should be identified before it is disturbed. Our note on asbestos on industrial roofs explains how that is handled.
Felt on flats, shops and mixed-use buildings
Built-up felt turns up in more places than the industrial estate. Rear flat roofs on shops, linking roofs between buildings, plant decks, stair and lift overruns and the flat roof over a first-floor office extension are all typical. Each has its own demands, and a few are particular to older town buildings.
In central Bath, a commercial flat roof can sit hidden behind a stone parapet, with its water draining through a gutter lined in lead or asphalt. The felt and the gutter lining then have to be treated as one system. Renewing the felt and leaving a split lead gutter in place hands the leak straight back. If the building is listed, or stands in the conservation area, the choice of finish and the visible edge details may need to be agreed with B&NES, because the roof can be seen from neighbouring windows even if not from the street.
Blocks of flats bring another variable. The roof is shared, the cost falls on several leaseholders, and the work usually has to go through a formal consultation before it starts. That is covered in our guide to section 20 roof works, and it affects the timetable more than any technical detail does.
Access is the practical limit. A flat roof above a trading shop may only be reachable through the building or over a neighbour's yard, and materials have to be hoisted. Torch-on work involves a naked flame, so the roofer must take care over combustible materials near the work, and the occupier needs to know when and where it will happen. We plan the access and the hot-work precautions with the occupier before a day is booked.
What a survey of a felt roof covers
A commercial felt roof is priced on survey because no table can see what is under the surface. The only price figure we can responsibly cite is a low-confidence one: a single contractor's answer on MyJobQuote (July 2026) put torch-on at £80 to £120 per square metre, plus roughly £20 to £40 per square metre where new insulation is needed. Treat that as a typical UK range, not a quote. The written price comes after the survey.
The survey walks the whole roof, not only the area where the leak shows. We look at the laps and the finish, press on blisters to learn whether they are dry, check each outlet and upstand, and look for ponding by its stains. Where possible we look inside at the underside of the deck for stains and for signs of condensation. Your report carries images of the roof itself with each fault marked, so the decision between repair, overlay and replacement is made on evidence you can see.
The written price then sets out what is renewed and what is kept, the layers and finish specified, the upstand and gutter details, access and scaffold, and who deals with building control where the work is notifiable. The workmanship is covered by our 10-year written guarantee.