The Short Answer
On our estimates, stripping the tiles off a typical semi, laying a new membrane and battens and putting the old tiles back costs £3,800 to £9,000 including scaffold. Re-felting a shed roof is £250 to £450, and a flat garage roof £1,000 to £2,000. A patch under a few tiles is £200 to £500.
The underlay sits under the tiles and battens, on top of the rafters, so every tile has to come off. That makes re-felting nearly as much work as a new roof. I'd only do it when the tiles are good enough to go back on; otherwise put the money towards new tiles.
Re-felting prices
| Job | Typical cost |
|---|---|
| New membrane and battens under the existing tiles, per m² of roof | £45 to £120 |
| Whole semi, scaffold included | £3,800 to £9,000 |
| Patch under a few tiles | £200 to £500 |
| Shed roof | £250 to £450 |
| Flat garage roof | £1,000 to £2,000 |
How long does roof felt last?
Often as long as the roof. Roofing Megastore, a roofing supplier, says good sarking felt can last up to 80 years or more, even when it looks worn at the eaves, but the poorer felt used on some homes around the 1950s, when materials were short, may need replacing about every 25 years.
The signs that matter are inside: damp on ceilings and walls under the roof, or exposed felt where tiles have cracked. Worn felt at the eaves on its own isn't a reason to strip a roof. Get a roofer to look from the loft and the roof before you pay for anything; a few torn patches can be repaired by lifting the tiles around them.
Old felt or breathable membrane?
Underlays come in two kinds. Old bitumen felt is "high-resistance": it stops water vapour as well as rain, so the loft relies on ventilation. Marley says that in most cases that means vents at the eaves and the ridge as a minimum. Breathable (vapour permeable) membranes let vapour through, which helps cut condensation.
Breathable doesn't mean no ventilation. Marley calls using a breathable membrane as the only ventilation a common mistake: British Standards and the NHBC still require high-level ventilation with it. Approved Document C also expects roofs to be designed against condensation, and cold roofs ventilated. Ask your roofer which membrane they're using, whether it has a BBA certificate, and how the roof will be ventilated afterwards. More on roof ventilation.
Why spray foam isn't a fix for failed felt
Building regulations
Stripping a roof back to the rafters to re-felt it counts as renovating a thermal element under Approved Document L. If you renovate more than 50% of the roof, the insulation should be brought up to standard where that's practical, which for a cold loft usually means at least 250mm of loft insulation. A roofer registered for roof coverings with the National Federation of Roofing Contractors (NFRC) or NAPIT can self-certify the work; otherwise apply to building control before work starts.
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Frequently asked questions
How much does it cost to replace roof felt?
On our estimates, £3,800 to £9,000 to replace the underlay under the tiles of a typical semi, including scaffold and refixing the old tiles. A patch is £200 to £500.
How often does a roof felt need replacing?
Often never: good felt can last as long as the roof, up to 80 years or more by Roofing Megastore's account. Poorer felt from around the 1950s may need replacing about every 25 years. A roofer should judge yours.
Is it worth refelting a roof?
Only if the felt has actually failed (damp inside, exposed felt) and the tiles are good enough to go back on. If the tiles need replacing too, I'd price a new roof alongside it: you pay for the scaffold and the labour either way.
Can I replace roof felt without removing tiles?
Not properly. The underlay sits under the tiles and battens, on top of the rafters, so they have to come off. Small tears can be patched by lifting a few tiles. Spraying foam on from the loft isn't a substitute: RICS says insulation shouldn't be used to remedy leaks.
How much does it cost to re-felt a shed roof?
£250 to £450 for a typical garden shed, on our estimates.
Is breathable membrane better than felt?
It lets water vapour out, which helps cut condensation. But it still needs ventilation at high level: Marley says relying on the membrane alone is a common mistake.
How much does it cost to reroof a 3 bed house?
That's a new roof rather than re-felting: see our new roof costs, which break it down by house and tile.
How we priced thisView sources (7, 5 official)
Rules come from the legislation and official guidance, read at source. Prices are our estimates: we check them against trade price data, retailer shelf prices and specialists' price lists, then build them up from their parts so the totals add up. Below are the sources behind the page; the trade price guides we only compare against aren’t listed. Where sources disagreed, we've said so. More on our method.
- OfficialSpray foam insulation: consumer guide, RICS. Checked 15 Oct 2026.
- IndustryRoofing underlays: what type should you use? (updated 7 March 2025), Marley. Checked 15 Oct 2026.
- OfficialApproved Document C: Site preparation and resistance to contaminants and moisture (roofs, paragraphs 6.10 to 6.12), GOV.UK. Checked 15 Oct 2026.
- OfficialApproved Document L: Conservation of fuel and power, Volume 1 Dwellings (2021 edition incorporating 2023 amendments), GOV.UK. Checked 16 Oct 2026.
- OfficialThe Building Regulations 2010, regulation 3: meaning of building work, legislation.gov.uk. Checked 15 Oct 2026.
- OfficialThe Building Regulations 2010, Schedule 3: self-certification schemes (para 16, roof coverings), legislation.gov.uk. Checked 15 Oct 2026.
- IndustryThe cost of replacing felt under tiles (updated 22 May 2023), Roofing Megastore. Checked 15 Oct 2026.
