Torch-on felt is one of the oldest flat roof coverings still in regular use, and also one of the most misunderstood. Many people picture the thin, single sheet of black felt that blistered on 1970s garages. Modern torch-on systems are a different thing: two or three layers of bitumen reinforced with a polyester or glass fleece, melted together and onto the deck with a propane torch to form one continuous waterproof skin. Done well, it is a sound and economical roof. Done badly, or on the wrong building, it can fail early or, in the worst case, start a fire. This guide explains what the material is, how it is laid, where it suits and where something else does the job better.
What torch-on felt actually is
Bitumen is the black, sticky by-product of refining oil. On its own it is brittle in the cold and soft in the heat, so manufacturers modify it with polymers and reinforce it with a fleece. Two families dominate. SBS felt is modified with a rubber-like polymer (styrene butadiene styrene) and stays flexible in cold weather, so it copes well with a roof that moves. APP felt is modified with a plastic-like polymer (atactic polypropylene), which tolerates heat well and has a firmer feel. Both arrive in rolls with a thin plastic film on the underside. The torch burns that film off and melts the bitumen beneath it, and as the roll is pushed down it fuses to what is below.
The felt gets its protection from its surface. A cap sheet is the top layer, and it usually carries a coating of mineral granules, often slate-grey or green, which shield the bitumen from ultraviolet light. Without that cover, sunlight would harden the surface within a few seasons.
How a torch-on roof is built
A typical domestic system has these layers, working up from the structure:
- The deck. Boards of OSB or plywood on timber joists, laid to a fall so water runs to an outlet.
- A vapour control layer. A sheet that stops warm, moist air from the house reaching the cold insulation or deck.
- Insulation. Rigid boards, in a warm roof, laid above the deck so the structure stays warm.
- A base layer. The first sheet of felt, usually nailed or bonded, which provides a stable surface for the next.
- A cap sheet. The mineral-faced top layer, torched to the base so the two fuse.
At the edges and at walls, the felt is turned up to form an upstand, the vertical part of the covering that runs up the brickwork. The cap sheet is dressed up the wall and finished under a flashing, a strip of metal or membrane lapped over the top edge to shed water. At the roof edge a metal trim guides water into the gutter. Every lap, upstand and corner is a joint that has to be melted and sealed by hand.
Where torch-on felt suits a roof
Felt is at its best on plain roofs where the budget matters and the shape is simple. A porch canopy, a shed or garage roof, a small dormer top or a modest rear extension can all be covered competently in felt. It is typically the cheapest of the three main flat roof coverings: a consumer guide puts felt at a typical UK range of £60 to £100 per m² (supply and fit), and a trade-facing guide gives a lower typical range for modified bitumen of £45 to £75 per m². Our flat roof cost guide compares these with EPDM and GRP, and your fixed quote follows the survey.
It also has a long track record. Roofers know how it behaves, repairs are straightforward, and a skilled crew can lay a roof in a day. A mineral-faced felt copes with occasional maintenance traffic better than a bare rubber sheet, and it is easy to patch.
Where felt is the wrong choice
Felt struggles where the roof is complicated. Every extra upstand, kerb or pipe adds hand-formed joints, and the more of those there are, the more chances for a leak. A roof with a large lantern, several vents and awkward corners is usually better in a material that can be formed in one piece. It also has a shorter life than the alternatives. Modern felt is a far cry from the old thin product, but of the three common coverings it generally lasts the least time. Our guide to EPDM, GRP and felt compared weighs the options on life, repair and cost.
Felt is also a poor fit for a roof people use. A terrace or a roof with regular foot traffic needs a surface that can take weight, and paving slabs placed on bare felt can crush or tear the cap sheet unless there is protection beneath them.
The fire question
The word torch is the whole story on safety. A flame of several hundred degrees is held against bitumen at the edge of a timber building, and heat travels. It can pass through gaps and joints, ignite old sarking (the boards or felt laid over rafters under tiles), dry timber or insulation hidden in a cavity, and smoulder for hours before anyone notices. Most torch-on fires start after the roofers have left, not while they are working.
For that reason, good practice is to keep the flame away from anything combustible, to use heat shields at upstands, to work with a fire extinguisher at hand and to stay on site afterwards to watch the roof. Detailing around dormer cheeks, timber walls and eaves is where we are most cautious. On those roofs we often choose a cold-applied or self-adhesive felt, or switch to EPDM or GRP, so that no naked flame is needed. It is part of the reason the right answer for a roof varies from one house to the next.
Cold-applied and self-adhesive alternatives
Felt does not have to be torched. Self-adhesive felts have a sticky underside, protected by a film, that bonds to a primed surface when rolled down. Cold-applied systems use a bitumen adhesive or a bonding compound, and mechanically fixed systems use fasteners and heat-welded or adhesive seams at the laps. These methods remove the open flame, though they are generally slower, more weather dependent and a little more costly. For roofs near timber walls, thatch, or tile-hung cheeks, they can be the sensible choice.
Weather, timing and preparation
Felt is less fussy about temperature than a liquid resin or an adhesive-bonded rubber sheet, but it still needs a dry deck. Moisture trapped under the first layer turns to vapour in sunshine and pushes up the blisters mentioned below. A careful crew checks the boards with a moisture meter, waits out a damp spell and covers the work if rain threatens. Cold weather makes the rolls stiff and harder to unroll without cracking, so in winter they are stored somewhere warm before use.
Preparation matters as much as the laying. The deck is swept, any loose or damaged boards are renewed, the edges are fitted with trims, and the surface is primed where the base layer is to be bonded. Skipping the primer is a classic shortcut: the felt appears stuck, but the bond is weak, and the first strong wind or hot summer lifts it at the laps.
Felt on garages, sheds and outbuildings
Many of the felt roofs we meet are on garages and garden buildings, where a mineral felt is a common and sensible choice. The roofs are often simple, the access is easy and the cost matters. Our garage roofing pages cover those buildings in more detail, but the principles above apply: dry deck, proper falls, solid edges and a careful approach to fire. Felt shingles, a different product made of small bitumen tiles for sheds and garden rooms, are covered in a separate guide for outbuildings.
How felt fails
When felt gets old, it shows. The surface cracks in a network of fine lines as the bitumen hardens, the granules wear off and expose the black underneath, and the laps and edges open. Blisters appear where moisture trapped in the roof expands in sunshine. Splits form at upstands where the roof has moved. Water ponding in a dip speeds the process, since standing water softens and swells the surface. Our answers on blisters on a flat roof and the page on the signs of flat roof failure help you read what you are seeing.
A good deal of felt failure is down to workmanship, not the material. Laps that were not fully fused, an upstand too low, a base layer laid on a damp deck, and an edge trim fixed without a drip are common causes of a roof that fails within a few years. That is why the surface can look fine while the detail at the edge is the real fault.
Repairing felt
Felt is easy to repair. A split can be cut back, dried and patched with a new section torched or bonded over it. A lifted lap can be re-fused. A worn surface can be refreshed with a reflective or bitumen-based coating, provided the layers beneath are sound and dry. What a patch cannot do is rescue a roof whose whole surface has hardened or whose deck is wet. When a roof is past that point, the money is better spent on a new covering. Our guide to flat roof repair or replacement explains how we decide.
What to ask a roofer about felt
If you are comparing quotes, a few questions separate a well-planned felt roof from a risky one. Ask how many layers are proposed and whether the cap sheet is mineral-faced. Ask what is being done about the deck and the falls, and whether insulation is included. Ask how the upstands and the edge trims will be finished. Ask which method of fixing will be used near timber, and how the roof will be monitored after the work. A contractor who can answer in specifics knows the material.
We choose felt only where it fits the roof, and say so when something else would serve better. A free survey with photographs of your own roof comes first, then a fixed written quote, and our workmanship is covered by a guarantee that lasts ten years. For the wider range of work see flat roofing in Basingstoke. To talk it through, the number is 01256 213956.
