Why Is the Roof Felt Flapping in the Wind?

Most pitched roofs have a layer of felt or a plastic breathable sheet laid over the rafters, with timber battens fixed on top and the tiles hung from those. That layer is called underlay. It is not the waterproof skin of the roof; the tiles do that job. The underlay catches wind-blown rain and snow that works its way under the tiles, and lets moisture from the house escape.

If you hear a slapping or rattling noise in a gale, or see a grey or black strip fluttering at the edge of the roof, the underlay has lost its fixing. Sometimes the material is visible from the garden. Sometimes you only hear it from inside, and the loft shows it clearly when you shine a torch along the rafters.

Be cautious about the source of the noise. A loose ridge tile, a piece of fascia or a cable clipped badly to the wall can all sound alike. A short look from the ground, and a look in the loft, usually separates them. Never go on the roof to check, as wet felt and tiles are slippery and fragile, and a loose sheet can catch a foot.

Wind does its worst at the edges. Along the eaves, down by the gutter, underlay is meant to lap into the gutter or over a fascia support tray. If it was cut short, has dried out or has been pulled by a gutter replacement, its edge can lift. Birds and squirrels sometimes tear it too.

At the verge, the felt tucks under the end tiles and is held by the pointing or a dry verge system. If that edge has cracked or crumbled, wind gets in underneath and lifts the underlay like a sail. Older felt becomes brittle with age, with small tears growing as the sheet moves.

Another cause is simple damage. A storm may have lifted or stripped tiles, leaving the underlay uncovered. Felt left exposed to sun and wind does not last, and a tear can grow across the roof with every gust. If the flapping started after a gale, look for missing tiles as well; our answer on roof tiles blown off covers what to do about those.

A loose strip of underlay does not mean an instant leak, but it removes your second line of defence. Rain driven sideways, snow blown under the tiles and water splashing back up from a poor gutter can then reach the battens, the rafters and the loft insulation, and none of it will be visible from the garden. Timber that stays damp can rot slowly, and soaked insulation stops doing its job.

Draughts are the other result. Cold air blowing through the gap makes the loft colder, may carry dust into the house and can stir the loft insulation. In an exposed spot, constant fluttering also works the nails loose, so the damage spreads along the edge.

Check the loft after rain for daylight, damp patches or drips on the insulation, and look at the ceilings below for stains. If you find any, put a container underneath and photograph it. For wind that is still blowing, our guide to wind speed and roof damage explains what a strong gust can do to a roof, and when work should wait for it to drop.

The cure depends on the extent. For a short loose edge, the roofer lifts the nearest tiles, pulls the underlay taut, fixes it with galvanised clout nails or staples into the battens, and sets the tiles back. A torn section is cut out and a new strip lapped under the course above and over the one below, so that water runs down and out, never behind.

At the verge or eaves, the repair often includes the edge itself: re-bedding or re-fixing the verge, fitting an eaves guard tray, or replacing cracked pointing. Where much of the felt has perished, a larger patch or the whole slope may need stripping and relaying, which is when the survey and its single written price earn their keep.

If wind or rain is due before the repair, a temporary cover can hold the edge down. Our emergency roof tarpaulin guide explains how, and why the fixings matter. We photograph what we find under the tiles, so you can see whether the felt, the batten or the edge was at fault, and the emergency roof repairs page shows what comes next.