Roofing Felt Explained: Types, Uses & When to Replace It

What is roofing felt and what does it do?

Roofing felt is the layer you never see. It sits directly beneath the tiles or slates, draped over the rafters and held down by the tile battens, and it does a job that the tiles alone cannot.

Tiles and slates are not watertight. They are water-shedding — they throw the great majority of rain off the roof by overlapping and gravity, but in driving rain, in snow, and in high wind, water and dust will always get past them. The felt catches whatever gets through and channels it down the roof slope and out over the fascia into the gutter.

That makes it a secondary weatherproofing barrier, and it does three further things beyond keeping water out. It resists wind uplift by reducing the pressure differential across the tiles in a gale. It keeps wind-blown snow and dust out of the roof void. And, in the case of modern breathable membranes, it manages moisture from inside the building by letting water vapour diffuse outwards rather than condensing in the roof space.

The word “felt” is also used more loosely for flat-roof and shed coverings, where a bitumen-based sheet is the actual waterproof surface rather than a hidden underlay. Both meanings are in everyday use, which is a common source of confusion when people search for it.

Types of roofing felt

Breathable membrane is the modern standard for pitched roofs. It weighs a fraction of traditional felt, comes in wide rolls that cover a roof quickly, and lets vapour out while keeping water in. Quality membranes are three-layer constructions — a waterproof outer surface, a breathable waterproof middle, and an abrasion-resistant inner layer — and carry BBA approval.

Traditional bitumen felt, the “1F” and “1B” grades, is what you will find under most UK roofs built before the mid-2000s. It is heavy, non-breathable, and prone to sagging between rafters and cracking with age. It still has a place in some repairs and specific details, but it is not what a modern roof is built with.

Torch-on felt is a different product entirely — a bitumen membrane bonded to a flat roof deck with a gas torch, forming the actual waterproof covering. Shed felt is the simple nailed mineral-surfaced sheet used on small outbuildings, effective and cheap but the shortest-lived of the family.

Breathable membrane vs traditional felt

The short version: breathable membrane is lighter, easier to fit, and lets moisture out of the roof space, which often removes the need for additional high-level ventilation. Traditional bitumen felt is heavier, cheaper per roll, and traps moisture — meaning the roof needs to be ventilated another way, usually at eaves and ridge.

The consequence of getting this wrong is a roof that appears to leak in cold weather when the tiles are perfectly sound. Warm moist air rises from the house into the roof void, hits the cold underside of a non-breathable felt, condenses, and drips. Homeowners call a roofer for a leak; the roofer finds a ventilation and vapour problem.

For any re-roof or new roof, a breathable membrane is almost always the right specification. We cover the full comparison — cost, weight, lifespan, ventilation, grammage and certification — in our dedicated guide to breathable roofing membrane versus felt.

How long does roofing felt last?

Most roofing felt lasts 20–30 years, but the spread across types is wide.

Several things shorten those figures. Sun exposure is the biggest — all underlay membranes have a limited exposure window before installation, typically around four months for a UV-stable breathable membrane, and leaving a roof “dried in” on felt over a long winter degrades it. Poor ventilation shortens felt life by keeping it permanently damp. Sagging between rafters causes water to pond and eventually find the nail holes. And any physical damage from foot traffic or maintenance work creates a weak point that will fail first.

Fitting felt support trays at the eaves is a cheap way to add years. They stop the felt sagging and drooping behind the fascia, which is where ponding water and rot most often start.

How to tell if your roof felt needs replacing

Felt fails out of sight, so the signs are indirect. Work through this checklist — several of these together strongly suggest the underlay has reached the end of its life.

• Torn or hanging felt visible at the eaves when you look up under the tile overhang, or from the gutter line
• Daylight visible through the roof when you are in the loft with the light off, in places where you shouldn’t see it
• Damp patches, staining or tide marks on the loft insulation or on the rafters
• Water dripping into the loft in wind-driven rain even though no tiles are missing or slipped
• Black mould on the underside of the roof structure, or a persistent musty smell in the loft
• Felt that crumbles, flakes or tears in your fingers when you touch it — brittleness is terminal
• Sagging felt visible between rafters, holding water in pockets
• Debris and grit collecting in the gutters that turns out to be bitumen granules from the felt
• Slipped or dislodged tiles across several areas, often a sign the battens and felt beneath have deteriorated

A useful test: go into the loft on a bright day and turn all the lights off. Give your eyes a minute. Pinholes of daylight in the field of the roof mean the underlay is perforated. Some light at the eaves and ridge is normal in a ventilated roof; light in the middle of a slope is not.

Timing matters too. If the roof is over 25 years old and has never been touched, and you are already having tiles refixed or a chimney reflashed, replacing the underlay at the same time is far cheaper than doing it as a separate job later — the scaffolding and tile stripping is most of the cost either way.

Can you felt over existing felt?

On a pitched roof, no. The underlay sits beneath the battens and tiles, so reaching it means stripping the covering. There is no way to add a new layer over the old one without taking the roof off, and any offer to do so should be treated with suspicion.

On a flat roof or a shed, it is sometimes possible to overlay — and often a false economy. Overlaying adds weight, hides whatever is wrong with the deck beneath, and traps moisture between the layers. If the existing covering is sound, dry, well-adhered and flat, an overlay can buy time. If there is any blistering, ponding, soft decking or trapped water, strip it back to the deck.

For a shed roof, stripping the old felt is quick work and lets you check the boards underneath, which is exactly where rot hides.

Choosing the right felt for your roof type

For grammage, 100 g/m² is a solid domestic specification. Stepping up to 115 or 120 g/m² gives a heavier, tougher membrane that handles better in wind during installation and stands up to more abuse on a large or exposed roof. Look for BBA approval — it is the practical evidence that the product has been independently assessed for UK use.

Roofing felt accessories

The membrane on its own is only part of the job. Three accessories make a measurable difference to how long the roof lasts.

Felt support trays (eaves protectors) fit at the bottom of each rafter bay and hold the underlay up so it discharges cleanly into the gutter rather than sagging behind the fascia. They are supplied in 1.5m lengths, adjust for all rafter centres, and suit new build and refurbishment alike. Skipping them is the most common reason an otherwise good roof rots at the eaves.

Treated roofing battens hold the underlay and carry the tiles. Use graded, treated timber — 25 × 50mm is the common domestic size — because untreated battens are the weak link in an otherwise sound roof.

A dry ridge kit mechanically fixes the ridge tiles instead of bedding them in mortar, and provides ventilation at the ridge. Since the 2014 update to BS 5534 this is effectively the standard for new and re-roofed UK roofs.

Frequently asked questions

What is roofing felt used for?

It is a secondary weatherproofing layer fitted beneath tiles or slates, catching any wind-driven rain, snow or dust that gets past the covering and channelling it out to the gutter. It also helps resist wind uplift on the tiles.

How long does roof felt last under tiles?

Traditional bitumen felt typically lasts 20–30 years. Modern breathable membranes, protected from UV once the tiles are on, can last 30–50 years.

How do I know if my roof felt needs replacing?

Look for torn felt at the eaves, daylight through the roof from inside the loft, damp staining on rafters or insulation, water entering in wind-driven rain with no missing tiles, and felt that crumbles when touched.

Can I put new felt over old felt?

Not on a pitched roof — the underlay sits under the battens and tiles, so the covering has to come off. On flat and shed roofs an overlay is sometimes possible but usually a false economy if there is any blistering, ponding or soft decking.

What’s the best roofing felt for a shed?

Traditional shed felt is cheapest but typically lasts 5–10 years. A lightweight breathable membrane costs a little more and lasts considerably longer, and is the better choice if the shed is insulated or used as a workshop.

Products mentioned in this guide

• 50m² Breathable Roofing Felt 1m × 50m 100 GSM BBA
• 75m² Eurovent Breathable Membrane 1.5m × 50m 115 GSM
• 20m² Rubershield Eco Extra 1m × 20m 120 GSM
• Felt Support Tray Eaves Protector 1.5m, pack of 20
• Treated Roofing Batten 25 × 50mm
• Universal Ventilated 6m Dry Ridge Kit

Related guides

• Breathable Roofing Membrane vs Traditional Felt
• Insulating a Garden Room, Shed or Van