Torch-on felt is the name given to bitumen roofing sheets that are glued to the roof by warming its underside with a gas torch. It has been a standard flat roof covering for a long time, but the modern product is quite unlike the old tar-and-paper felt that gave the whole category a poor name. Today's sheets are reinforced with a polyester core and modified with polymers, which makes them stronger and more flexible. They are also used in layers, so the finished roof is a built-up system rather than a single piece of material.
This guide covers the layers, the role of the cap sheet, how the sheets are fused, how long the system lasts according to the sources, and how the naked flame is handled safely. For blistering, which is a separate fault with its own causes, see our page on blisters on a felt flat roof. The wider service is on our flat roofing page.
What built-up means: two or three sheets, not one
A built-up roof is one in which the waterproofing is made of several layers glued together. The reason is simple. A single sheet has one chance to be perfect, and any pinhole or poor lap goes straight through. With two or three layers, the joints are staggered and a flaw in one sheet is covered by the sound body of the next.
The usual build-up
Starting at the deck, a typical warm-roof system has a vapour control layer, which is a sheet that stops moist air from the house rising into the insulation. Over that sits the insulation, then a base layer of felt, and finally the cap sheet. The base layer is often fixed mechanically or glued, and the cap sheet is torched onto it. A cold roof leaves out the insulation above the deck and relies on ventilation instead, as explained in our guide to warm and cold flat roofs.
Why polyester and polymer-modified
The reinforcement is what gives the sheet its tear strength and lets it cope with a little movement in the deck. The polymer modification keeps the bitumen supple through a wide range of temperatures, so it neither runs when hot nor cracks when cold. Older oxidised felts did neither job well, which is part of why they split and slumped.
Two systems, one roof
Some roofers lay a torch-on cap over an older roof, and others strip back to the deck. Which is right depends on the state of what is underneath, and our page on overlaying a flat roof explains how we assess it.
The cap sheet and why it looks speckled
The cap sheet is the top layer. It is the one that sees sunlight, rain, hail, frost and foot traffic, so it is the one built to take it. The upper surface is covered in mineral granules, usually in grey, green or a slate colour, which shield the bitumen beneath from ultraviolet light. Bare bitumen left out in the sun would harden and crack far sooner.
What the granules do
Besides protecting the bitumen, the mineral finish reflects part of the sun's heat and gives a slightly rough surface for safer footing. On a roof you can see, it is also the visible face, and a neat, even mineral finish is part of a tidy job.
Laps and side joints
Cap sheets are laid in overlapping strips. Each strip is positioned so its side joint falls clear of the joints in the layer below, which is what lets a built-up system shed water over its laps without relying on one joint alone. At the end of each strip the overlap is torched so a bead of softened bitumen is squeezed out along the edge. That bead is a visual check that the join has fused.
Upstands and details
At walls and kerbs, the felt is turned up and finished under a flashing, a lead or metal strip let into the wall or fixed over it. The detail is made in the same way as on other flat coverings, with an extra reinforcing piece in the corners. Flat roof details at walls and abutments are where leaks most often appear, so we photograph them once the work is finished.
Fusing the sheets: how the torch is used
The underside of each sheet carries a layer of bitumen that softens under heat. The roofer unrolls the sheet into position, rolls it back on itself, and heats the underside and the surface beneath together with a gas torch as the roll is pushed forward again. The bitumen melts into a bead that flows ahead of the roll, and the sheet is pressed down into it. Done properly, the whole sheet is glued to the layer below with no gaps.
Judging the heat
There is a skill in knowing how much heat is enough. Too little, and the sheet does not fuse; too much, and the bitumen is burned or the reinforcement is damaged. Experienced roofers read the surface: the sheen of the melted bitumen, the size of the bead, the way the film on the underside changes colour. These are signs, not measurements.
Bonding to the substrate
The surface beneath has to be clean and dry, or the bond is poor. Before the base layer goes on, the deck may be primed with a bitumen primer so that the melted bitumen has something to grip. Insulation boards must be of a type that can sit under a flame-applied layer, and the manufacturer's data decides which.
Cold applied and self-adhesive options
Where the flame is not suitable, the same base-and-cap approach can be built with self-adhesive or cold-applied sheets. These are slower and generally more expensive, but they remove the naked flame from the job. We discuss that choice at the survey where the roof is close to combustible material.
Working with a flame on a roof: fire safety on site
Hot work, the trade term for work with a naked flame, has a record of causing roof fires, and a fire can smoulder inside a roof for a long time after the torch is switched off. That is why careful practice matters more than any single rule.
Before the flame is lit
Before lighting up, we find out what the roof is built from and what lies alongside it. Timber boarding, old felt, dry debris, thatch, insulation and combustible cladding all change how and where a torch can be used. The edges are checked for gaps where a flame or hot bitumen could travel into the roof void, and any gaps are protected or closed. Fire extinguishers are kept within reach of the working area.
Choosing a different method
On some roofs the right answer is not to use the torch at all. Where the details are tight against timber, where a stone or brick parapet hides a void, or where an old roof has combustible build-up we cannot see into, a cold-applied or self-adhesive system, or a different membrane such as EPDM, may be a safer choice.
After the last flame
The risk does not stop when the torch is out. Heat stays in the roof, so the working area is checked and watched after work stops, and we do not leave a site with hot work only just finished. According to the Planning Portal, a new covering that behaves very differently in fire from the old one needs approval, so check the fire classification when swapping system.
How long a felt roof lasts: the sources disagree
Felt has the widest spread of lifespans of any flat covering. Less gives a good felt job around 20 years in its flat roof cost guide, against under 10 years for poor work. CORC says 30+ years for a well kept roof.
| Source | Stated felt life |
|---|---|
| Less | 10 to 20 years (good work up to 20; poor work under 10) |
| CORC | 30+ years when well kept |
| MyBuilder, flat roofs in general | 15 to 30 years |
What explains the gap
Some of the gap is the product. Old oxidised felt and modern polymer-modified felt are not the same thing. Most of it is workmanship and care. How well the laps are fused, whether the falls hold water, how often the roof is cleared and how carefully the details are made all show up in how long the roof lasts. The Less line is a fair warning: the same system, laid differently, can sit at either end of the range.
Reading these as ranges
These are typical figures from third-party guides. They are not promises, and a warranty from a manufacturer is a separate thing from how long the roof will last. Our written 10-year workmanship guarantee sits alongside this and covers the installation.
Maintenance that moves a roof up the range
Clearing leaves from the surface and outlets, and re-sealing any lifted edge or flashing, keeps water moving and stops small faults becoming large ones. For wider thinking about the age of a roof, see flat roof drainage, because water that lingers is the commonest enemy of a felt cap sheet.
Cost, repairs and where felt suits
MyBuilder's flat roof guide, updated August 2026, gives a typical UK range of £70 to £100 per m² for bitumen felt, which sits below the rubber and fibreglass figures in the same guide. Checkatrade's repair guide, last updated June 2026, quotes bitumen flat roof repair at £100 to £140 per m², with VAT and access costs left out. These are national ranges. After the survey the quote covers scaffold, strip, deck repair, insulation, layers of felt, details, labour and waste.
Repairs
A localised split or lifted lap on a felt roof can be cut out, dried and patched with a new piece of cap sheet, fused in with a torch or glued cold. The patch must extend over sound felt on all sides, and the damaged area underneath must be dry. Wet insulation under a patch is a fault that returns. For a closer look at one of the common faults, see flat roof blisters.
Where felt is a good fit
Built-up felt suits larger, plain roofs and roofs where a familiar, widely understood system is wanted. It suits many extensions, and our guide to flat roofs on extensions covers the way they are put together. It is less suited to roofs with many tight corners right beside timber, where we would look first at a cold-applied alternative.
Paperwork
A full re-cover that touches more than 50% of a flat roof's surface can bring the roof under the thermal rules in Approved Document L, as covered in the guide to flat roof insulation upgrades. We explain what applies and sort the building control notification if one is due. When we leave, the roof area is clear and the waste has gone.