What Roofing Felt Does

Key takeaways

• Its job is drainage, not waterproofing. It carries water down and out rather than holding it back.
• Bitumen felt has a much shorter life than the covering above it, and it fails invisibly.
• A roof with no working underlay leaks in driven rain while shedding ordinary rain perfectly.
• The state of it decides whether a repair or a recover is the right answer.
• It can only be assessed from inside the loft, which is why an outside only inspection misses it.

What it is there to do

A pitched roof covering is not sealed. It works by overlap, and in driven rain some water will always get past the laps, particularly where wind pushes it uphill against the run. The underlay is there to receive that water on a continuous surface and shed it down the slope into the gutter. It is a drainage layer rather than a waterproofing one, which is why it is laid in horizontal sheets starting at the eaves with each course lapping over the one below, so water travels on top of the joints rather than into them.

The three generations of it

Very old roofs have nothing at all beneath the covering, sometimes with torching, a lime and hair mortar, applied to the underside of the slates to close the gaps. From the mid twentieth century, bitumen impregnated felt became standard, a heavy reinforced sheet that does the drainage job well when new. More recently, breathable membranes have replaced it: lighter, stronger, and designed to allow water vapour from the roof space to pass outward while keeping liquid water out. Knowing which of the three is above you is the single most useful fact about your roof.

How bitumen felt fails

It becomes brittle. Decades of heat, cold and ultraviolet exposure at the eaves drive out the volatile content, and the sheet loses its flexibility. It then cracks under any movement, tears at the nail holes where the battens are fixed through it, and splits where it sags between rafters. Because it sags in the first place, those low points hold water rather than shedding it, which accelerates the deterioration exactly where the sheet is already weakest. None of this is visible from outside, and none of it produces a leak until the covering above lets something through.

The failure at the eaves specifically

The most consequential failure is at the bottom edge, where the felt has to be dressed over the fascia and into the gutter. That is the point where all the water it has collected is discharged, it is the most exposed part of the sheet to sunlight, and it is where the material becomes brittle first. Where it has torn back or was never dressed correctly, water carried down the underlay arrives at the fascia instead of the gutter, and runs down the timber and into the eaves. Rotted fascia and soffit on a house with an otherwise sound roof usually traces back to exactly this.

Why this decides repair against recover

The condition of the underlay is what turns a small question into a large one. A roof with a functioning membrane can lose several tiles and be repaired for a modest sum, because the secondary barrier is still doing its job in the meantime. A roof with perished felt has no margin at all: every gap is an open route, and each repair addresses only the tile rather than the underlying situation. That is why an assessment that has not established the state of the underlay cannot really distinguish between a roof needing a few tiles and one approaching the end of its serviceable life.

Why it cannot be patched from below

Homeowners often ask whether the felt can be repaired from inside the loft, and the honest answer is that anything done from underneath is a temporary measure. The sheet sits above the rafters and below the battens, so accessing it properly means lifting the covering and the battens from outside. Patches applied from below cannot recreate the lap arrangement that makes the layer work, and they frequently trap water rather than shedding it. Where the felt has failed across a roof, the remedy is a strip and relay, which is also the point at which the battens and fixings get renewed.

Breathable membranes, and what changed

A breathable underlay allows water vapour to pass outward through it while resisting liquid water from above. That matters because a roof space generates moisture from the house below, and a sheet that blocks vapour requires that moisture to be removed entirely by ventilation. Using a breathable membrane reduces the reliance on air movement, which is why modern roofs sometimes have less obvious ventilation provision. It does not remove the need for it altogether, and a breathable membrane above a poorly sealed ceiling in a house producing a lot of moisture can still see condensation forming.

The mistake of using the wrong one

Because the two types behave differently with regard to vapour, they are not interchangeable without thinking about the whole build up. Fitting a non breathable felt in a roof designed to rely on the membrane for vapour transfer, or fitting a breathable membrane while blocking the ventilation on the assumption it is no longer needed, both produce condensation problems. This is a specification question rather than a preference, and it is one of the more useful things to ask about when a recover is being quoted, since the answer indicates whether the contractor is thinking about the roof as a system.

How to assess yours

From the loft hatch on a bright day with the light off, look up at the underside of the roof. A modern membrane looks like a woven synthetic sheet, usually grey or white, taut between the rafters. Bitumen felt is black and heavier, often visibly sagging in the bays between rafters, and where it has aged it looks dull and cracked with tears around the nail lines. An absence of any sheet means the covering itself is all there is, and you will be looking straight at the backs of the tiles. Photograph it, since that image is the most useful thing you can hand to anybody assessing the roof.

What good looks like on a new roof

If a roof is being recovered, the underlay work is worth understanding because it is entirely hidden afterwards. Sheets should run horizontally, starting at the eaves and working up, each lapping over the one below by the specified amount so water runs across the joint rather than into it. At the eaves it should be dressed over the fascia and into the gutter, usually with an eaves carrier or support tray to stop it sagging back. At the ridge, hips and valleys it should be lapped correctly around the detail. Photographs taken during the work are the only evidence you will ever have that this was done properly.

What it means for a leak diagnosis

The presence and state of the underlay changes the interpretation of everything else. A leak on a roof with sound membrane points at a specific defect: a junction, a flashing, something that has breached both layers. A leak on a roof with perished felt may have no single cause at all, since water is getting past the covering in the ordinary way and there is nothing to catch it. That distinction is why a roofer asks to see the loft before forming a view, and why a homeowner who already knows the answer can shorten the conversation considerably.

Where the sheet sits in relation to everything else

Understanding the order removes most of the confusion about what can be done to it. The underlay lies directly on top of the rafters. The battens are fixed through it into those rafters, which means the sheet is pinned beneath the timber that carries the covering. The tiles then hang from the battens above. So the underlay is the middle layer of a sandwich, accessible from above only by removing the covering and the battens, and visible from below only in the gaps between rafters. That single fact explains why it cannot be replaced without stripping the roof, why it cannot be properly repaired from the loft, and why its condition is invisible from the street.

What it costs to get wrong

Because the underlay is buried, the consequences of skimping on it arrive years later and are difficult to attribute. A cheaper grade of membrane, or one laid with insufficient lap, produces a roof that performs acceptably in ordinary rain and admits water in the conditions that matter. Underlay not carried properly into the gutter puts water onto the fascia for the life of the roof. And a covering relaid onto old felt, which is the classic omission on a low quoted recover, delivers a new roof with no functioning second barrier from the day the scaffold comes down. In each case the owner has paid for a roof and received a covering, and the difference only becomes apparent when the weather finds it.

Frequently Asked Questions

What does roofing felt actually do?

It catches water that gets past the tiles in driven rain and carries it down the slope into the gutter. It is a drainage layer rather than a waterproof one, which is why it is laid in horizontal sheets from the eaves upward with each lapping over the one below.

How long does roofing felt last?

Considerably less than the covering above it. Bitumen felt becomes brittle over decades as heat, cold and sunlight drive out its flexibility, then cracks, tears at the nail holes and splits where it sags. The failure is invisible from outside and produces no symptom until the covering lets something through.

Can felt be repaired from inside the loft?

Only as a temporary measure. The sheet sits above the rafters and below the battens, so proper access means lifting the covering from outside. Patches applied from below cannot recreate the lap arrangement that makes the layer work and frequently trap water rather than shedding it.

How do I tell what underlay I have?

Pick a bright day, open the hatch, kill the electric light and let your eyes settle before looking up. A modern breathable membrane is a taut synthetic sheet, usually grey or white. Bitumen felt is black and heavier, often sagging between the rafters. Nothing at all means you can see the underside of the tiles and daylight between them.

Why does the underlay decide whether I need a new roof?

Because it determines the margin. With a functioning membrane, lost tiles are a modest repair since the secondary barrier still works. With perished felt there is no margin at all and every gap is an open route, which is why an assessment that has not established its condition cannot distinguish a small repair from an ageing roof.

Is a breathable membrane always better?

It performs better and it is not a substitute for ventilation. Vapour can pass outward through it, which lessens how hard the ventilation has to work. That is not the same as removing the requirement, and a house pushing moisture upward through an unsealed ceiling will still produce condensation beneath one. The build up has to be considered as a whole.

My fascia is rotten but the roof looks fine. Why?

Most likely the underlay at the eaves. It has to be dressed over the fascia and into the gutter, and that edge is where the material becomes brittle first. Where it has torn back, water collected by the underlay arrives on the timber instead of in the gutter and stays there.

Can I see the underlay condition from outside?

No, which is exactly why an inspection carried out entirely from outside misses the fact that most influences the recommendation. It can only be judged from the loft, and any assessment of whether a roof needs repair or recovering should include somebody having actually looked at it from underneath rather than forming a view from the pavement, because that single observation changes the recommendation more than anything visible from outside.