The short answer
Roof underlay typically lasts between about 15 and 40 years, depending on the type and how the loft behaves. Old bitumen felt tends to fail at around 30 to 40 years, and one trade guide puts modern breathable membrane at 40 to 50. That guide is a single source, so treat the figures as a rough guide. Underlay frequently fails before the tiles do, which is why a sound-looking roof can still leak.
Felt and membrane: what is under your tiles
Underlay is the sheet resting across the rafters and tucked beneath the battens that carry the tiles. Its job is to catch the small amount of water that gets past a tiled covering, usually wind-driven rain or snow blown up under the laps, and guide it down into the gutter.
Two broad types turn up in Bath roofs. The older one is bitumen felt, a tarred fabric that was standard on pitched roofs for most of the twentieth century. It is solid, so it does not let moist air pass through. The newer type is a breathable membrane, a plastic fabric that blocks liquid water but lets vapour through, which helps the roof space dry out.
Many older houses have neither. Roofs from before underlay became common were simply tiled over battens, sometimes with lime torching, a layer of lime and hair plastered on the underside of the tiles. We cover that kind of roof on our page about roofs without felt.
Which type you have decides how long it lasts, and what happens when it ages.
Typical lifespans, and how much to trust them
The honest answer is that nobody has published a firm, independent figure for underlay life. The only source we found that gives numbers is a single trade guide, BestBuilders' 2026 piece on tile lifespans, which we treat as low confidence.
That guide says underlay lasts 15 to 40 years, that old bitumen felt fails at around 30 to 40, and that a modern breathable membrane lasts 40 to 50. We could not find a manufacturer or BBA figure to back it up.
The safer way to use these numbers is to treat them as a range, not a promise. Life depends on how the roof was put together and how it has been treated. Sunlight reaching felt through gaps in the covering, condensation sitting on the underside, poor laps and heat in a poorly vented loft all shorten it. A felt that has stayed cool, dry and covered can outlast a neighbour's by many years.
What the guide does support is the general point that the underlay often fails before the tiles. A clay or concrete covering can reach the end of its own life long after the underlay beneath it has gone brittle.
Signs the underlay has failed
You rarely see the felt from outside. The clues turn up in the loft, or as damage in the rooms below.
- Brittle or cracked felt. Old bitumen felt hardens with age. Press a corner gently. If it snaps or flakes, it has finished its working life.
- Tears and holes. Rips near the eaves, at laps or around the ridge let water straight through.
- Daylight where there should be none. Bright patches in a felted roof suggest a missing tile and a torn felt below it.
- Stains and drips. Dark rings on the underside of the battens, or water marks on ceilings under a slope with no obvious tile fault, point to felt that is no longer shedding water.
- Damp rafters and battens. Soft, dark timber under an apparently sound covering is a strong indicator.
- Moisture under breathable membrane. Droplets on the surface or mould on the timber mean the loft is not drying as the membrane should allow. Our page on pitched roof problems covers how that damp builds up.
A single sign is worth watching. Several together mean the felt is past repair.
Why felt fails early in some roofs
Age is only one cause. A few conditions bring forward the end.
Heat is one. A slope that takes the afternoon sun, or a loft with no ventilation, bakes bitumen felt until it dries and splits. Condensation is another. Warm moist air from kitchens and bathrooms rises into the roof space, condenses on the chilly underside of the covering and wets the felt from below. Over time the fabric rots, and it takes the battens with it.
Poor installation matters too. Felt laid too tight, without a slight sag between the rafters, can tear when the roof flexes. Laps that are too short or placed the wrong way let water creep through. Nail holes through the felt where it was fixed with clout nails, instead of the battens holding it down, give water a path in.
Lastly, damage from repairs counts. Every time someone lifts a tile and walks across the underlay, the felt takes a knock. An old roof that has been patched many times may have a felt full of small tears.
Replacing underlay: what is involved
Underlay cannot be changed in isolation. The tiles come off, the old felt and battens are removed, new underlay and battens are fitted and the tiles are relaid. That is the work described on our page about re-felting cost, which sets out typical UK price ranges and what moves them.
On a small roof, or a roof where only one slope has failed, it may be sensible to renew a single section. On a roof where the felt has aged evenly all over, doing the whole thing at once saves paying for scaffold twice.
Choosing a replacement depends on the roof. A breathable membrane suits most pitched roofs that have a warm, humid loft. In some roofs, a traditional felt with proper ventilation at the eaves still works. The ventilation requirement differs between the two. Marley's guidance, which follows BS 5250, gives a non-breathable felt with insulation at ceiling level 25 mm of continuous eaves ventilation at 15 degrees or less and 10 mm above 15 degrees. A breathable membrane can need far less: 3 mm at the eaves in Marley's example of a well-sealed ceiling and an air-permeable covering, if the product has UKAS third-party certification. Pitch, span and the ceiling all change the figure, so it is worth discussing with your roofer.
Getting the roof checked
If your roof is old, or you have never seen what lies under the tiles, a loft inspection is the starting point. Look at the underside of the slope with a good torch, and make a note of anything brittle, torn or stained.
Our survey costs nothing, and it comes with pictures of your loft and of each slope, then a firm written price. Where repair works, we repair. When the underlay, battens and fixings have all reached the end together, we tell you so and give the reasons. If you have the house deeds or old invoices, a date for the last re-roof gives a useful clue to the age of the underlay.
If you are weighing what to put back on top of new underlay, our comparison of clay or concrete tiles sets out lifespan, weight, look and cost side by side. The wider pitched roofing service page covers the rest of the roof.
