By Charlie the Roofer for Roofline · Published · Checked against the sources listed below
A leaking felt flat roof can usually be patched if the damage is in one spot, the deck is dry and firm, and the felt is under about 15 years old. If the insulation is wet, the deck is soft, blistering or cracking covers much of the roof or it keeps leaking in new places, replacement is the better spend.
When is a patch repair worth doing?
A patch is worth doing when the fault is in one place and the rest of the roof is sound. Trade2base, the trade cost guide, says repair suits a localised failure on a dry, structurally sound deck where the felt still has roughly five to seven years of life left. Typical patchable faults are a split lap, a puncture, a perished flashing where the felt meets a wall, or a single blister.
The age of the felt matters as much as the size of the hole. Trade2base's 2026 felt guide says budget felt typically lasts 10 to 15 years before it cracks and blisters, while a well-laid three-layer torch-on roof commonly lasts 20 to 25 years. On an older Yorkshire garage or back extension still under budget felt, a patch is a short-term fix.
The leak has to be traced before it can be priced. Our guide on finding a roof leak explains what you can check safely from inside.
When does a felt flat roof need replacing instead?
A felt roof needs replacing when the damage has reached the deck or insulation below the felt, or when the felt itself is worn out across the roof. Trade2base lists four signs: moisture found under the membrane when it is probed, cracking or crazing over more than 20% of the surface, felt more than 15 years old (10 years for cold-applied felt), and a roof that has already had several patches.
Some of this you can judge from an upstairs window or from inside the garage:
- Cracks along the joist lines. The Department for Communities in Northern Ireland, in its maintenance checklist for felt roofs, says cracks and tears along the line of the deck joists can be caused by saturated insulation or a sagging deck.
- A chipboard deck. Bauder, the flat roof manufacturer, says chipboard is no longer recommended for flat roof decks because it becomes structurally unstable over time, especially once moisture has got in.
- Widespread cracking or blistering. The same Northern Ireland guidance says widespread surface crazing calls for re-roofing.
Blisters form when water vapour trapped below the felt pushes the covering up, which is the cause the Northern Ireland guidance gives. Do not pop one or tread on it. A split blister lets rain straight in. The proper repair is for a roofer to cut the blister, let the trapped moisture escape, bond the felt back down and then find where the moisture came from.
What does a patch cost compared with a new felt roof?
A patch usually costs a few hundred pounds, while a new felt roof on a garage or small extension costs a few thousand. Trade2base's June 2026 guide puts a small felt patch of up to 0.5 m² at £150 to £400 and a larger one of 0.5 to 2 m² at £300 to £800, both before VAT.
For a full replacement, Trade2base puts new felt at £40 to £80 per m² before VAT, and its felt guide gives £1,200 to £3,000 per job for a small garage or single-storey extension of 15 to 25 m². Replacing wet, rotten deck boards adds £15 to £25 per m². Patching over a soaked deck leaves the boards rotting, so that part of the bill tends to grow.
Ask for an itemised quote so you can see whether deck boards, insulation and upstands are included. Roofline gives free itemised quotes and sends photos of what it finds, so you can see the state of the deck for yourself.
Can a new roof be laid over the old felt?
Sometimes, but on a leaking roof the old felt usually has to come off. Bauder's refurbishment guidance says an overlay only suits a roof that is sound, dry and free from water damage, and that the old coverings should always be removed if there is doubt over the roof's long-term integrity or significant moisture is trapped in it.
EPDM rubber is a common replacement, but it cannot simply be glued onto old felt. ClassicBond, a widely used EPDM system, says its membrane should not be used to overlay bituminous surfaces such as felt and asphalt, so the felt is stripped or boarded over first. Our flat roofing page compares EPDM, GRP and torch-on felt.
Building Regulations can apply as well. Approved Document L counts replacing the waterproof membrane on a flat roof as renovation, and where more than half the roof is renovated, the whole roof should be insulated to the improved standard of 0.16 W/m²K, if that is technically feasible and pays back within 15 years. The rule covers roofs over heated rooms, so it usually catches an extension or dormer but not an unheated garage. Our re-roofing and Building Regulations guide explains how sign-off works.
Send us a photo on WhatsApp and we will tell you what we think it is and whether it needs a visit. Quotes are free and itemised.
Sources
- Trade2base: Flat roof repair costs UK (8 June 2026)
- Trade2base: Felt roof costs UK (14 June 2026)
- Department for Communities (NI): Maintenance checklist, bituminous sheet flat roofs
- Bauder: Roof surveys and diagnostics (Overlay or replace?)
- Bauder: Flat roof design considerations
- Bauder: Cold applied liquid systems
- ClassicBond (Rubber4Roofs help centre): Can EPDM be laid over existing roof coverings?
- GOV.UK: Approved Document L Volume 1, 2021 edition incorporating 2023 amendments
General information about roofing in England, checked against the sources above when published. It is not a substitute for advice about your own roof.
