Walk round the back of a shop parade, a school extension or an old mill outbuilding and the flat roof above you is very often felt. Not the thin shed felt of a garden, but a bitumen-based membrane laid in several layers, each stuck to the one below. That layering is what the name built-up means: the roof is assembled on site, sheet on sheet, rather than rolled out as a single membrane.
This guide sets out the system itself. It covers what each layer does, the ways the layers are joined, the details that fail first, and the lifespan range published for felt. For the question of whether to refurbish a tired felt roof with a liquid finish, see our answer on liquid roof coatings. The full range of commercial work sits on our commercial roofing page.
How a built-up felt roof is assembled
A modern felt roof is a stack. Reading from the structure upward, there is the deck, which might be timber boarding, concrete or profiled steel. Over the deck goes a vapour control layer, which slows warm, damp air from inside the building from reaching the cold parts of the roof. Then comes insulation, and over the insulation, the waterproofing itself: normally a pair of reinforcement-backed bitumen felt sheets, sometimes more.
The underlayer
The first felt layer is laid across the insulation or fixed through it to the deck. Its job is to provide a stable base for the layer above and to take up some of the movement of the roof below. It is also, in some specifications, a temporary barrier that keeps rain off the insulation while the roof is finished.
The cap sheet
The top layer is the cap sheet, the surface that sees weather and sun. It is the thickest and heaviest-duty, usually with a mineral finish, for example fine granules pressed into the surface. The mineral layer shields the bitumen from sunlight, which otherwise dries it out and hardens it, and gives the roof some resistance to foot traffic.
Why layers matter
One layer can have a flaw anywhere along its length. Two layers, with staggered joints, mean a defect in one is covered by sound material in the other. That redundancy is the whole logic of the system, and it is why built-up felt has stayed on commercial buildings in an age of single-ply membranes. Our guide to single-ply membranes is a useful contrast if you are weighing the two.
Torch-on, hot-poured and cold-applied: three ways of joining layers
The felt in each layer is the same family of material. What differs is how it is stuck down.
Torch-on
The roofer heats the underside of the roll with a gas torch until the bitumen turns glossy and flows, then presses it onto the layer beneath. The bond is strong and the laps seal by melting into one another. The method depends on skill and on controlling heat near combustible materials, so the working method, fire precautions and a check after the torch is put out matter as much as the felt. Our flat roofing guide to torch-on felt flat roofs covers the domestic side of this method.
Hot-poured bitumen
In this system, layers are stuck with hot bitumen poured and rolled ahead of the sheet. It produces a thick, continuous bond, but needs a boiler on site and care at the edge of the roof.
Cold-applied and self-adhesive
Cold systems use an adhesive or a self-adhesive backing, with no flame. They are used where open flame is not acceptable, for example on buildings with combustible roof structures or where the occupier insists. They are slower and suit some details better than others. A cold-applied roof is not a lesser roof; it is the answer to a different site constraint.
Choosing between them
On commercial work the choice is made at survey. What is underneath the felt, how close the roof is to timber, what the occupier is storing below and whether heat works are acceptable at all, shape the decision. We explain the reasoning in the specification, so the reason for a method is on paper.
Falls, laps and upstands: where the system depends on detail
Felt is a continuous skin. Its weak points are the places where the skin stops and starts.
The slope of the deck
A flat roof is not flat. It needs a fall to move water to the outlets. Published guidance based on BS 6229 recommends designing a roof for a fall of 1:40 with 1:80 as the finished minimum after build tolerances and sag. A roof that falls less than intended collects water in shallow pools, and the surface bitumen that sits wet through the winter ages faster than the dry area beside it.
The laps
Where one roll meets the next, the overlap has to be sealed along its full length. A lap that was laid cold or over dust can look closed but be only touching. This is a common place to find a split after a few seasons.
Upstands and flashings
Wherever the felt reaches a wall, a parapet or a plant base, it turns upward in an upstand. The head of the upstand needs a termination: a metal flashing, a chase in the brickwork or a fixed bar with sealant. Failure at the termination lets water run behind the upstand and into the build-up. Upstands are among the first places we examine on any felt roof.
Outlets and penetrations
Each outlet, vent and pipe is a hole through a membrane. Collars and sleeves need to be dressed with extra layers and sealed. On older roofs, we often find that new equipment has been added without anyone reinstating the felt around it.
Parapets and gutters
On commercial buildings, gutters often run within the roof edge, as box gutters or parapet gutters. Where those channels are lined in felt, the welded joints and the falls inside the gutter control the risk of overflow into the building.
How long built-up felt lasts, according to published sources
Published lifespans for felt vary a lot, and the spread itself tells you something.
| Source | Stated life | Comment |
|---|---|---|
| Less (cost guide, 2026) | 10 to 20 years | Less puts a good job at up to 20 years and a poor one at under 10 |
| CORC | 30 years or more | If well maintained |
| MyBuilder flat roof guide (2026) | 15 to 30 years | For flat roofs generally, any covering |
Why the figures disagree
These are guides to typical performance, not guarantees, and they describe different standards of work. A felt roof that has a good fall, well-sealed laps and sound upstands, kept clear of debris, sits towards the upper end. One with ponding, moss and split details sits at the bottom. The quality of the original installation is the largest single factor, and the second is whether anyone looked at the roof afterwards.
What the range means for planning
If a felt roof is 15 years old, it is not necessarily nearly finished, and if it is 25 it is not necessarily failing. Age is a prompt to inspect, not a verdict. We pair age with condition: the state of the surface, the laps, the upstands and the insulation beneath.
Warranties are not lifespans
A manufacturer's product warranty covers the material under stated conditions. It does not predict how long the roof will last, and it does not cover failed details, a blocked outlet or later damage by other trades.
How built-up felt fails
Felt roofs seldom fail all at once. They give signs, usually at the same places on each building.
Blistering
Blisters are pockets where moisture or air has been trapped between layers and expanded in the sun. Small ones that are intact can be left under observation; those that have split or sit on a lap should be opened, dried and repaired. The fault is treated at greater length for domestic roofs in our flat roofing answer on blisters.
Surface crazing and loss of granules
With age, the cap sheet's surface cracks in a fine pattern, and the mineral finish washes into the gutters. Once the bare bitumen is exposed, the weathering speeds up. A gutter full of grit is often the first clue.
Splitting at laps and upstands
Movement of the building and thermal cycling flex the membrane daily. Joints that were not fully sealed open, and the heads of upstands pull away from the wall.
Ponding and moss
Standing water that remains for a long time after rain suggests the deck has sagged, or the fall was too shallow from the start. Moss and algae on a flat roof hold moisture against the surface and keep outlets from draining.
Wet insulation
Water under the membrane travels. A leak inside the building may appear a good distance from the actual hole. Where the insulation beneath has become saturated, patching the felt above it does not make the roof dry, which is why a moisture survey can be valuable before deciding on repairs.
Repair, refurbish, overlay or replace
When a felt roof is showing its age, there are four routes, and the best one depends on the condition of what is under the surface.
Local repair
Where defects are few and the insulation is dry, patching the laps, renewing the upstand terminations and clearing the outlets can give years more service. It is also the lowest-cost first step if the roof's remaining life is uncertain.
Refurbishment with a coating
A reinforced liquid system applied over cleaned felt can extend life where the felt is still firmly adhered and the deck sound. We discuss when that makes sense in the coatings answer linked above.
Overlay
Laying new membrane directly on the old avoids stripping and disposal, but adds weight and traps any wet insulation. It depends on the roof being dry, on the structure being able to take the load, and on drainage details staying workable. Whether or not to overlay is a fuller question than this guide can settle, and a roofer's survey will tell you whether your roof is a candidate.
Strip and replace
If the insulation is wet, the deck is damaged or the falls are wrong, stripping to the deck is the only way to put it right. The new build-up can then be designed to the right falls, with insulation chosen for the building.
Old buildings
On older commercial buildings such as the valley mills, flat roof sections often sit between pitched ranges and carry their own drainage problems. Our guide to mill roofs in Stroud covers those buildings.
Costs, quoting and what we put in writing
In its 2026 flat roof guide, MyBuilder gives bitumen felt about £70 to £100 per m², against £90 to £130 for EPDM and £95 to £150 for GRP. That is a typical national range for a flat roof covering, and it shows where felt sits in the market, not what your roof will cost. Commercial work has more variables: access, hoisting, edge protection, insulation thickness and the amount of detail work at upstands, outlets and plant.
Once the survey is done, our written quote covers strip-out, new layers, flashings, outlets, access and disposal. VAT shows on it as well, since roofing is normally standard-rated.
What the survey records
The report carries photographs of your own roof, alongside notes on the falls, outlets, laps and felt condition and any sign of wet insulation. It names the route above that we would take.
Guarantee
Roof work carries our 10-year workmanship guarantee, put in writing, and we take our rubbish away at the end.
To begin, ring 01453 933613, use WhatsApp or the contact form, attaching pictures of any outlet or upstand that worries you. A planned approach to inspection and repair is covered in our guide to planned roof maintenance.