Is sagging roof felt a problem?

Normal drape and a real sag

Underlay is the membrane stretched beneath the battens, which are the slim timbers carrying the tiles. It is laid across the rafters with a deliberate bit of slack between them, so that any water reaching it runs down the drape to the eaves and into the gutter. A small, even droop of that kind is intended, and it is a sign of a roof laid by someone who knew what they were doing.

A problem sag looks different. It is deeper, uneven or concentrated in one place. It might form a pocket that holds water, press against the insulation or sit so low that it touches the underside of the tiles. It might also be paired with stains, holes or damp timber around it.

The difference is shape and consequence. A gentle run toward the eaves is fine. A bag of felt that holds water, or a droop that has changed since you last looked, is not.

Why felt sags

Several things pull underlay out of shape.

• Age. Old bitumen felt stretches and loses its stiffness, and a roof laid with a sensible drape can end up with a heavy sag after many winters.
• Poor fixing. If the felt was nailed loosely, or the battens do not hold it where they should, it can slip or bunch.
• Too much slack when laid. A drape that was generous to begin with can grow into a pocket.
• Moisture. Felt that has taken on water from condensation or a leak gets heavy and droops, and wet felt rots.
• Rafter movement. If the rafters have sagged or moved, the felt follows them.
• Weight on top. Trapped debris, or a leaking tile letting in water, adds load to one area.

Knowing the cause matters because it decides the remedy. Our guide to sagging roofs covers when the structure beneath is at fault.

When sagging felt matters

Look at three things in the loft: where the water goes, what the felt touches and what has happened to the timber.

If the felt forms a pocket, water can sit in it instead of running to the gutter. That water presses on the felt, finds a nail hole or a lap and drips through, often onto insulation or the ceiling below. Wet felt in contact with timber also keeps the batten damp and encourages rot.

If the felt rests against the tiles, it can cause moisture to wick back into the covering, and stop air moving beneath it. If it touches insulation, damp can spread into the loft floor.

Check the surrounding timber too. Dark staining, soft battens or mould around a sag tell you the problem has been going on for a while. A sag with none of that, and with felt that is still flexible and unbroken, is usually just aged underlay doing its job.

Do not push the felt back into place and leave it. That relieves the symptom and hides the cause.

Breathable membrane and old felt behave differently

Modern breathable membranes are lighter and more flexible, and the fitting instructions specify how much drape to leave between rafters. A membrane that sags more than that, or touches the tiles or insulation, was fitted without enough support or has been disturbed. Breathable membranes also need air to move freely over them, so contact with insulation or covering undermines how they are meant to work.

Older bitumen felt is stiff when new and brittle when old. A sag in an old felt is often a sign it has stretched, and if it is also hard, cracked or shedding grit, it is spent. Our page on how long roof underlay lasts gives the typical ranges and the signs of failure.

Many older houses have no underlay at all, so there is nothing to sag. Chinks of daylight in an unfelted loft are simply the covering showing through, not a defect in any felt.

Is it safe to leave?

If the felt has a slight, even sag, is dry, whole and flexible, and the timber behind is sound, you can leave it and keep an eye on it. Check again after the next storm and again in spring.

If the sag holds water, touches insulation or tiles, has holes, or sits near rotten timber, deal with it soon. A loft that is damp from above will damage insulation, rot timber and may eventually show on the ceiling. Dealing with it early is a smaller job than waiting for the timber to fail.

What you do next depends on how far the problem spreads. If only a small patch is affected and the surrounding felt is sound, a local repair may be possible, lifting a few tiles to free and refix the felt. If the felt is brittle or the sag runs across the roof, the underlay has to be renewed, and that means lifting the covering. Where the fault is local, we repair. Where it is widespread, we tell you straight.

What we do about it

We look from the loft first, in daylight if possible, taking photographs of the sag, the felt's condition and the timber behind it. That tells us whether we are dealing with a bad fit, an old felt or a structural movement.

Where tiles are sound, the work is to lift them, renew the underlay and any damaged battens, and lay everything back with the right drape. Our clay plain tiles page covers the small tiles that need the most care to lift and relay. If the sag is linked to a slipped covering, see why tiles slip.

If you can get into the roof space safely, a torch photograph of the sagging felt between two rafters, with any damp patches below it, helps us judge how much of the slope needs lifting. Everything else we do on tiled roofs is on the pitched roofing page.