Roofing felt is one of the oldest and most widely used waterproofing materials in the UK, and one of the most confusing to buy. The word covers everything from the underlay under your tiles to the mineral felt on a shed and the heat-bonded membranes on flat roofs. This guide explains the main types of roofing felt, typical roll sizes, how long felt lasts and how it is laid, so you can choose the right product and know what a good job looks like.
What Is Roofing Felt?
Roofing felt is a bitumen-based sheet material made by coating or saturating a base layer with bitumen. The base may be fibre, polyester or glass fibre, and the surface can be sanded, covered in mineral granules or left plain. Its job is to keep water out, either as a hidden second line of defence beneath a roof covering or as the visible waterproof layer itself. It is cheap and easy to work with, which explains its long popularity on outbuildings, garages and flat roofs.
Types of Roofing Felt
Underlay Felt for Pitched Roofs
Underlay felt sits beneath tiles or slates and catches wind-driven rain and snow that gets past the covering. It is not meant to be left exposed. Traditional bitumen underlay is a non-breathable type, which is why the roof space needs good ventilation. Modern breathable membranes have largely replaced it on new work, and we compare the two in our guide to roof felt and underlay.
Mineral Felt (Shed and Outbuilding Felt)
Mineral felt has a surface of fine mineral granules, often green, grey or black, which protects the bitumen from sunlight. It is sold as shed felt or general purpose roofing felt and is used on sheds, summerhouses, lean-tos and small garage roofs. Heavier grades, sometimes sold as heavy duty roofing felt, are thicker and more tear resistant than light or budget rolls.
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Torch-On and High Performance Felt
Multi-layer torch-on systems use polyester-reinforced bitumen sheets bonded to the roof with heat, usually as a base layer and a mineral cap sheet. They are a common choice for flat roofs and are normally installed by trained roofers because of the fire risk. Our guide to torch on felt roofs covers the pros, cons and lifespan.
Self-Adhesive and Cold-Applied Felt
Self-adhesive felt has a peel-off backing and sticks to a clean, primed surface, while cold-applied felt is bonded with roofing adhesive. Neither needs a naked flame, so they are often used for small repairs and DIY work on garages and outbuildings. The bond depends on a dry, clean surface and suitable temperatures, so follow the manufacturer’s data sheet.
Roofing Felt Types Compared
| Type | Typical use | How it is fixed | DIY friendly? |
|---|---|---|---|
| Underlay felt | Under tiles and slates | Fixed to rafters and held by battens | Only as part of a re-roof |
| Mineral or shed felt | Sheds, outbuildings, small garages | Clout nails, adhesive or both | Yes, at safe heights |
| Torch-on felt | Flat roofs, extensions | Heat bonded with a gas torch | No, use a trained roofer |
| Self-adhesive or cold-applied felt | Repairs, small flat roofs, outbuildings | Peel-and-stick or adhesive | Yes, for small areas |
Roofing Felt Sizes and Roll Lengths
Most roofing felt is sold in rolls about one metre wide, with lengths ranging from a few metres on small DIY rolls to 20 metres or more on trade rolls. Weight and thickness vary by grade, so always check the data sheet rather than relying on the label alone. When working out how much you need, measure the roof area and add extra for laps, upstands, edge turn-downs and waste. A small roof with many details may need noticeably more than the plain area suggests. Ask the supplier about roll sizes before you order, since manufacturers differ.
How to Choose the Right Felt
- Where will it go? Underlay for beneath tiles, mineral felt for sheds, and torch-on or single ply for house flat roofs.
- How exposed is it? Visible felt needs a mineral or protected surface to resist sunlight.
- How long should it last? Higher grades and multi-layer systems generally outlast single-layer budget rolls.
- Who is fitting it? DIY users usually do best with self-adhesive or nailed products, and trained roofers with torch-on.
- What is the roof deck like? Felt needs a sound, dry, clean and reasonably smooth surface.
If you are comparing felt with EPDM, GRP or single ply, our flat roof materials guide shows how they stack up.
How to Lay Roofing Felt on a Small Roof
This is a general guide for nailed or cold-applied felt on a small, low-risk roof such as a shed or lean-to. Always follow the manufacturer’s instructions for your product.
- Check the deck. Replace rotten boards and make sure the surface is clean, dry and free from nails or sharp edges.
- Let the felt relax. Unroll it flat and leave it for a short time, ideally in mild weather, so it loses its curl.
- Start at the bottom edge. Lay the first strip so it overhangs the eaves slightly, which lets water drip clear of the structure.
- Fix securely. Use clout nails or adhesive as instructed, spaced as the data sheet advises.
- Overlap each strip. Work upwards so each upper strip overlaps the one below, which sheds water instead of trapping it. Laps are commonly around 75 to 100 mm on small roofs, but check the data sheet.
- Deal with edges and upstands. Turn felt up at walls and fit trims at the edges, as our guide to roof edge trim explains.
- Finish neatly. Seal exposed edges and laps as the manufacturer recommends and check for punctures.
Common Roofing Felt Mistakes
- Laying felt on a damp or dirty deck, which can trap moisture.
- Using too few nails, or nails that are too short, so the felt lifts in wind.
- Poor laps that point the wrong way and let water in.
- Leaving felt exposed to sunlight when it needs a mineral surface.
- Skipping detailing at edges, corners and penetrations.
- Choosing the cheapest light felt for a roof that will be walked on or exposed to heavy weather.
Safety
Working at height is dangerous. Use a stable ladder or platform, stay off fragile or wet surfaces and never work alone on a roof. Torch-on felt involves open flames and hot materials, which carry a real fire risk, so leave it to a trained roofer and tell your insurer if you are unsure.
Asbestos warning. Old garage and outbuilding roofs are often corrugated cement sheets, and they may contain asbestos. If you suspect asbestos, do not drill, break, scrub or cover over damaged sheets without advice, and get a competent surveyor to check them first. The same applies to old soffits and similar materials around the roof edge. Our guide to garage roof replacement explains the issues in more detail.
How Long Does Roofing Felt Last?
As a general guide, bitumen felt typically lasts about 15 to 20 years, depending on grade, installation and exposure. Budget single-layer felt can fall short of that, while good quality multi-layer systems laid well may reach the upper end. Signs that felt is nearing the end include blisters, splits, cracking, loss of the mineral surface, standing water and leaks at laps or edges. Flat roofs that need insulation should be designed carefully, and our guide to warm roof construction explains the standard approach.
Frequently Asked Questions
What is the difference between roofing felt and underlay?
Underlay is a hidden layer beneath tiles or slates on a pitched roof. Roofing felt is the wider term, and includes visible waterproof layers such as shed felt and torch-on felt.
How long does roofing felt last?
Typically about 15 to 20 years, depending on the grade, how well it was laid and how exposed the roof is.
Can I lay roofing felt in cold or wet weather?
It is best to avoid it. Cold felt is stiff and may crack, and a wet deck can trap moisture. Check the manufacturer’s temperature limits and choose a dry, mild day.
Can I fit torch-on felt myself?
It is not recommended. Torch-on needs a naked flame and specific training, so it is usually best left to a trained roofer.
What size are roofing felt rolls?
Most are about one metre wide, with lengths from a few metres to 20 metres or more. Check the product data sheet before ordering.
Can felt be laid over old felt?
Sometimes, but only if the old layer is sound, dry and well bonded. Have a roofer check it first, as moisture or weight problems can cause failures.
Final Thoughts
Roofing felt is a simple, affordable and proven material, but the right type depends on where it will be used and who will fit it. Choose the correct grade, prepare the deck properly and follow the data sheet for laps and fixings. Leave torch-on work to trained roofers, and check any old cement sheets for asbestos before touching them.
Get a professional assessment
If you are unsure how much life your current roof has left, a local roofer can give you an honest assessment as part of a free inspection. Browse vetted roofers through our roofer directory, or get a quick replacement estimate using our free cost calculator.