Roofing felt means two different things, and mixing them up matters, since one is a covering and the other is hidden. On a flat roof, felt is the actual waterproof covering. On a pitched roof, “felt” is really the underlay hidden beneath the tiles. This guide covers the flat-roof kind: what it is, the types available, and how to lay it.
What is roofing felt
On a flat roof, such as a shed, garage or small extension, felt is the covering itself. It is a bituminous sheet material, laid in layers and bonded or heat-welded at the joints. On a pitched roof, though, felt now more accurately means a roofing underlay, sitting under the tiles as a secondary barrier. Our roof underlay guide covers that use in full.
Types of roofing felt
Mineral felt is the traditional, cheapest option. It is a fibreglass or organic base, coated in bitumen with a granule top layer, suited to sheds and low-traffic garage roofs. Torch-on felt, also called high-performance felt, is heat-welded rather than nailed. It gives a more reliable seal, and is the standard choice for anything more than a basic shed. Self-adhesive felt bonds without a naked flame, which suits DIY use or a spot where a torch is not practical.
How to lay roofing felt, step by step
The steps below suit a straightforward shed or garage roof. The sequence matters as much as the materials. Most felt failures trace back to skipped preparation or a rushed lap join, not a faulty product.
- Strip the old felt entirely and check the timber deck for rot, replacing any damaged boards first.
- Fit a drip edge trim around the perimeter, so water sheds away from the fascia.
- Lay the underlay layer parallel to the fall, with every lap overlapped by at least 75mm, checking the specific product’s instructions.
- Lay the top layer, offsetting its laps from the layer below, with every lap again overlapped by at least 75mm.
- Bond or torch every lap fully. A cold, unsealed lap is the single most common reason a new felt roof leaks within its first year.
- Dress the felt up any upstands and seal carefully around pipes or rooflights, since these junctions fail more often than the flat area itself.
Roofing felt vs modern membranes
For anything bigger than a shed, such as a garage with a room above or an extension, modern materials usually win. Single-ply membrane, EPDM rubber and GRP fibreglass all outlast felt, and are worth the extra cost. Our flat roof materials guide compares them directly. Our flat roof vs pitched roof guide is worth a look too, if the roof shape itself is still open for discussion.
Felt still suits small, low-stakes structures, though, where a 10 to 15 year life is perfectly acceptable. For anything bigger, get a flat roofing specialist involved rather than a general roofer. Installation quality decides how long any flat roof material actually lasts.
Costs, checks and older roofs
See our roof repair cost guide for realistic UK price ranges, since a re-felt job varies a lot by roof size. Our types of roof guide covers how felt compares with the pitched coverings above it. A quick check on our how often to inspect a roof guide helps catch a failing felt roof before it actually leaks.
One caution: an older corrugated garage or outbuilding roof is sometimes asbestos cement rather than felt. If yours looks that way, read our asbestos roof removal guide before anyone starts stripping it, since the two need very different handling.
Find Trusted Roofers connects UK homeowners with local flat-roofing specialists, since anything beyond a straightforward shed re-felt is worth a specialist’s eye. That includes flat roofers in Preston and roofers in Cheltenham and Falkirk, plus roofers covering Plymouth. See our flat roofing guide for the full service. Tell us about the job and we will match you with a roofer covering your postcode.
Roofing Felt: What It Is & How To Lay It — FAQs
How long does roofing felt last?
Basic mineral felt on a shed or garage typically lasts 10 to 15 years. Modern torch-on felt can last up to 20 years, with correct installation. Compare that with EPDM or GRP, which run 25 to 30-plus years — see our flat roof materials guide.
Can you lay roofing felt yourself?
On a small structure like a shed, yes, since it is a realistic DIY job with basic tools. On a house extension, or anywhere a mistake risks water damage, use a roofer instead. Correct falls, laps and edge detailing matter more than they look like they should.
How much overlap does roofing felt need?
As a rule of thumb, allow at least 75mm on every lap, and more at the ridge. [IKO's shed felt installation guidance](https://ikogroup.co.uk/wp-content/uploads/2024/09/Technical-Data-Sheet-TDS-IKO-Shed-Felt.pdf) specifies a 75mm overlap between sheets and 150mm where the ridge capping dresses down over them. Always check the specific product's own instructions, since the exact figures vary by felt type and manufacturer.
What's the difference between shed felt and roofing membrane?
Shed felt is a lightweight mineral or fibreglass-based bituminous felt, suited to small, low-traffic structures. A breathable roofing membrane sits under tiles on a pitched roof to manage moisture instead, which is a different product for a different job. See our roof underlay guide.
