Roof underlay, felt & breathable membrane
In short
The underlay is the secondary barrier beneath the slates or tiles, and on most older local roofs it is a bitumen felt that has gone brittle and torn. It can only be replaced with the covering off — so it is a re-roof component rather than a standalone job, and choosing breathable over felt is one of the real gains when that happens.
Also called: Roofing felt · Sarking felt · Breathable membrane · Torn roof felt
Signs you need this
- Felt visibly torn, sagging or brittle when seen from the loft
- Daylight through nail holes or laps in the underlay
- Water dripping behind the gutter rather than into it
- Damp, mouldy loft with a bitumen felt underlay
- Rotting fascia and rafter feet at the eaves
- Felt that crumbles when touched
A pitched roof has two lines of defence. The covering sheds the great majority of the water; the underlay beneath catches what gets past in driving rain and carries it down to the gutter. On a well-built roof the underlay is almost never doing much — but 'almost never' is exactly why it matters when the wind is in the wrong direction.
On any local roof more than about twenty-five years old, that underlay is bitumen felt. Felt has a finite life: it embrittles with heat and UV, it tears at the nail lines where the battens are fixed through it, and it sags between rafters and holds water in the trough it forms. From the loft, brittle felt with tears at the laps and daylight showing at the nail holes is one of the more reliable signs that a roof has reached the end of its serviceable life even though the covering looks even.
Bitumen felt has a second problem which matters at least as much: it is effectively vapour-closed. Moisture rising from the house into the roof space cannot pass through it, so it condenses underneath. That is a large part of why so many older lofts are damp, mouldy and full of matted insulation, and it is why replacing felt with a breathable membrane at re-roof is a genuine improvement rather than an upsell.
Breathable membranes let water vapour out while keeping liquid water and wind-driven rain out. That reduces condensation risk substantially, and in some build-ups it changes the ventilation strategy — though it does not remove the need for ventilation altogether, and any roofer who says a breathable membrane means you can block the eaves is wrong.
The honest limitation is that underlay cannot be replaced from underneath. It sits beneath the battens, which sit beneath the covering, so getting at it means taking the roof off. There is no meaningful way to renew it as a standalone job, and anybody offering to spray or paint the underside of a roof as a substitute is selling something else entirely.
What can be done short of a re-roof is a repair where the felt has torn locally — commonly at the eaves, where it has rotted at the drip edge and no longer discharges into the gutter. Felt support trays at the eaves are a small, cheap job that fixes a genuinely common failure: water running down the felt and dripping behind the gutter into the fascia and rafter feet instead of into the gutter.
What the job covers
- Underlay condition assessed properly from within the loft
- Eaves felt support trays fitted where the felt has failed at the drip
- Breathable membrane specified in place of felt at re-roof
- Ventilation strategy considered alongside the membrane choice
- Correct laps, tensioning and support over rafters
- An honest statement of what can and cannot be done without a strip
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
Can the felt be replaced without taking the roof off?
No. It sits under the battens, which sit under the covering, so reaching it means stripping the roof above it. That is a straight fact and it is worth being clear about, because it is the point at which a lot of homeowners realise that an underlay problem is a re-roof conversation.
What about spraying the underside of the roof?
Spray foam applied to the underside of a roof is a different product with real problems attached: it can trap moisture against the timbers, it makes the roof structure impossible to inspect, and a number of lenders have taken a hard line on it at mortgage and valuation stage. It is not a substitute for underlay and it should not be sold as one.
Is breathable membrane worth it over felt?
Yes, at re-roof. It substantially reduces condensation risk by letting water vapour out, it is lighter and easier to lay properly, and it does not embrittle the way bitumen felt does. It does not remove the need for ventilation, and any claim that it does should be treated with suspicion.
Water is dripping behind my gutter. Is that the felt?
Very often, yes. The felt should overhang into the gutter so water running down it discharges correctly. When the drip edge rots away, that water goes behind the gutter instead and straight into the fascia and rafter feet. Eaves support trays fix it and it is a small job compared to the joinery it prevents.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.