The short answer
Torch-on felt is bitumen roofing felt laid in two or three layers, with the underside of each roll melted by a gas torch so it bonds to the surface. It is a familiar flat roof covering for garages and extensions. Published lifespans are 10-15 years from HomeBuilding and 10 to 20 years from Less, and HomeBuilding lists supply-only cost at £65 per square metre.
What torch-on felt is
Roofing felt is a fabric base, usually polyester or glass fibre, saturated with bitumen. Traditional felt was stuck down with hot bitumen from a boiler. Torch-on felt has a layer of bitumen on its underside, and the roofer heats this with a gas torch as the roll is unrolled, so it melts and bonds to the surface below.
A roof is normally built up in layers. A first layer, often called the underlayer, goes onto the deck or insulation, and one or two cap sheets are bonded over it. The top sheet may have a mineral surface, usually fine granules, to protect the bitumen from sunlight.
Details are the skilled part. At a wall, the felt turns up as an upstand and is finished with flashing, and at the edges it runs into a drip trim. Outlets, pipes and corners need cutting and heating carefully, since a poorly formed detail is where leaks begin.
Felt is a flexible membrane, so it falls under BS 6229:2025, which keeps a minimum finished fall of 1:80, with design at 1:40 according to IKO. A felt roof that ponds ages quickly.
Modern felts are higher-performance than old ones. Some are modified with polymers to make them more flexible and durable, and the grade chosen affects both life and cost.
Lifespan and cost
Published lifespans for felt are modest compared with other coverings. HomeBuilding gives felt 10-15 years, while Less gives 10 to 20 years. The two are typical ranges and nothing more, and workmanship, drainage and exposure all move the real figure.
Compared with the other coverings in those sources, felt sits at the short end. EPDM is given 25-30 years by HomeBuilding and 30 to 50 by Less, GRP 25-30 or 25 to 40 years, and lead 60-100 years from HomeBuilding. The lower price of felt reflects its shorter life.
HomeBuilding (Sep 2025) lists materials only, per square metre: £65 for felt, £105 for EPDM, £110 for GRP and £170 for lead. Labour is not in those numbers, so they are not roof prices. For a whole job, Which? (Nov 2025) gives £1,400 to £3,500, ex VAT, for felt on a 6 m by 3 m garage.
Those are typical UK figures from national guides. A roof in Farnborough is priced after a survey, taking account of size, access, deck condition and insulation, and the price comes as one total in writing.
When choosing, weigh life against cost over time. A covering with double the life at a higher price can be the better buy, particularly where access is costly.
Our page on why a felt roof blisters covers its commonest failing.
Hot work and safe practice
The torch is the main hazard. Open-flame work on a roof can ignite timber, insulation, dust, nesting material and debris, sometimes hours after the roofer has gone. A fire that starts inside the roof build-up can be hidden until it takes hold.
Safe practice means a careful approach. The work area is cleared of combustible material, flame is kept away from timber edges, eaves and any gap that leads into the roof void, and heat-resistant barriers are used at details. Fire extinguishers are on the roof, and the roofer stays on site and checks the work for a period after the last flame, so that smouldering is noticed.
Some situations rule out a flame altogether. Thatched buildings, roofs with combustible insulation directly below the felt or details with hidden voids may call for cold-applied or mechanically fixed systems. A responsible roofer chooses the method for the building.
Building control approval also comes in where a new covering has a significantly different fire performance from the old, according to the Planning Portal, so ask your roofer how the chosen felt is rated.
Your insurer may have conditions about hot work. Ask the contractor what procedures they follow, and check your policy if you are unsure.
Choosing felt, and what we recommend
Felt suits a roof where budget matters, the area is simple and a shorter life is acceptable, such as a shed, garage or modest extension. It is easy to repair, and patches are straightforward. Our page on whether a flat roof can be patch repaired explains the limits.
For a roof that should last longer, or has complex details, or sits over a living space where a leak would be costly, a longer-lived covering such as EPDM, GRP or lead may be the better buy. The right choice follows from the building and your plans for it.
Our survey looks at the deck, falls, edges and details, and the options come with their typical lives and photographs. We recommend what the evidence supports, whatever the price, and the figure is quoted as one total.
If you choose felt, we use a good-grade system, form the details with care, work with fire precautions and follow the manufacturer's instructions. The flat roofing hub describes the other coverings, and how long it takes to replace a flat roof covers the programme.
