Torn or perished felt under the tiles: does it matter?

1What the felt does beneath the tiles

Underlay is a membrane stretched across the rafters, beneath the battens that the tiles are fixed to. Houses of earlier decades were usually covered in bitumen felt, which is why the name stuck, and a good deal of it is still up there doing its quiet work. Newer roofs use plastic-based membranes. Its job is secondary but real: wind-driven rain and fine snow can slip past a tile, and the underlay stops it reaching the loft and drains it off at the eaves.

It also shapes how moisture behaves. Warm air from the house reaches a cold covering, and the underlay decides whether the damp escapes or sits on the timber. Marley's advice, which follows BS 5250, treats high-resistance and low-resistance underlays differently, and the type fixes how much ventilation the roof needs. A few old roofs have no underlay whatsoever, and that is a separate question from felt that has torn.

2How old felt tears and perishes

Bitumen felt dries out. After decades under tiles it turns brittle, splits along the laps and crumbles at a touch. Heat building up in a poorly ventilated loft hurries that along, and you may find flakes lying on the insulation or caught between the rafters. Felt in that state cannot be lapped or patched, because it breaks again beside any new join.

Tears usually have a mechanical cause. A slipped tile exposes the felt to sun and rain. A boot may have gone through it on an earlier repair. A rafter that has shifted can split it, and the felt can sag along the gutter line into a belly that holds water instead of shedding it. That fault gets its own page under eaves felt support.

Birds and rodents also pick at felt where it meets the wall plate. The gap they open lets draughts and damp into the roof space, and it tends to get bigger each year, so a small hole there is worth attention early. If you think bats or nesting birds use the eaves, mention it before work begins, because both are protected.

3When a tear is a repair

A clean tear in otherwise sound felt, found when tiles are lifted for some other job, can be patched. The torn area is cleaned back, a fresh strip is slipped beneath the course above and lapped over the damage so water falls onto the sheet below, and the battens under it are checked for damp. A torn section at the eaves can be put right by adding a proper support strip.

The same goes for a short length of damaged felt beside a chimney or valley, where the covering is open already.

What matters is the condition of the felt around the tear. If it is supple and the battens are dry, a local repair is reasonable. If it crumbles when touched, or the timber shows staining, the tear is a symptom and the slope as a whole needs a rethink. Damp that has reached the timber is described in our answer on rotten roof battens.

4When it signals a re-roof

Underlay sits beneath the battens, so renewing it across a slope means taking the covering off. Once you are paying for that, it makes sense to correct the battens, the fixings and any ventilation shortfall at the same visit.

Brittle felt on several slopes, felt absent where tiles have been disturbed and damp that returns to the loft after each rainy spell all describe a covering that is wearing out. The Planning Portal says a building control application is not normally needed while under 25 per cent of the building envelope and under 50 per cent of the roof is affected. Pass either figure and the work needs approval, and the roof's insulation may need raising to current levels.

On a roof of clay peg tiles, the tiles are often worth keeping, so the job becomes lifting, renewing the underlay and relaying them. A modern breathable membrane also suits a lived-in loft, and its ventilation needs are covered in roof tile vents.

5How the survey goes

The visit begins in the loft. We note how the felt lies and laps, whether it has dropped, and whether the timber behind it is dry. Pulling a small sample free tells us a lot about how brittle it has become. We record daylight, stains and the airflow at eaves and ridge, then go outside to look for the slipped or broken tiles that may have caused the damage.

Photographs of your own underlay and battens reach you alongside the written quote, which gives one overall price. When a tear is local, the quote is for the smaller job and the reason is explained. When the felt has perished, you will see exactly where and how far.

Tiles lifted for an underlay repair must be fixed again to the accepted standard, which we explain in roof tile clips. The whole range of tiled and slate work is set out on the pitched roofing page.