The short answer
A small, even drape in roof felt between rafters is normal and is deliberate, because it lets water run down to the gutter instead of collecting on the batten. Felt that sags a long way, touches the insulation, forms a pocket that holds water or is torn is a problem. It can block the ventilation gap, soak the timbers and let rain through. Whether it is serious depends on how far it has dropped and what condition it is in.
Why felt is laid with a drape
Underlay is the waterproof layer under the tiles. It is laid across the rafters and held down by the battens, with a slight dip between each pair of rafters. That dip, called a drape, means that any water getting past the tiles runs down the felt and does not sit behind a batten. Manufacturers give a recommended drape, usually small, and a loose fit also allows for movement.
So a modest, even wave seen from the loft, with the felt hanging gently between rafters, is nothing to worry about. The question is whether yours is gentle or excessive, and whether it is in good condition.
When sagging becomes a problem
Trouble starts when the felt hangs low enough to touch the insulation or the ceiling. A gap should remain between the felt and what is below it, so air can move. Where the felt presses on insulation, moisture has nowhere to go, and condensation collects. Over time that damp can rot timbers and spoil the insulation.
A second problem is a pocket. A deep sag can form a pool where water collects, either from a small tear or condensation. Water held in a pocket finds its way through the felt eventually, or soaks the batten above it. A third problem is age: felt that is brittle, cracked or torn at the sag points no longer protects the roof, whatever its shape.
Why felt sags
The most common reason is simply that it was laid too loose, or the supports have moved. Old felt stretches over decades as it dries and loses its elasticity. Battens that have bowed or slipped let the felt follow them. Where a roof has been re-covered but not re-felted, the old underlay may have been left to flap. Moisture in the roof, from a leak or from condensation, softens the felt and makes it droop.
Modern breathable underlays are different in behaviour. Some are designed to be laid taut, and some need a specific gap, so follow the manufacturer's instructions. Our guide on breathable roof membranes explains the types.
How to check from the loft
Go up with a torch, in daylight, when you can. Look for felt that touches the insulation, dark stains on it, pools or lumps, and tears at the nail points. Check the timbers underneath for damp, and for black spots that signal mould. Do not step between the joists, because the ceiling will not hold you. If you see daylight, that is a sign of a gap in the tiles or the felt.
Photograph anything that worries you and send it across. It is a lot cheaper to fix a felt problem than a rotten rafter. See our answer on whether a pitched roof is cheaper than a flat one if you are weighing overall options.
Felt, ventilation and the loft
The felt is part of a system that includes the batten gap, the loft air and the insulation. If insulation has been added over the years and pushed up against the felt, it reduces the air gap and raises the chance of condensation. That is a more common cause of damp than a leak in the covering. The remedy is to pull the insulation back from the eaves and fit a vent tray so air can flow.
If you have recently had loft insulation laid, a damp smell or new stains under the felt can follow. Mention it when you call, because it changes where we look first.
Old felt and modern standards
Felt laid decades ago was often a bitumen product with no vapour permeability. It is stiffer, more brittle and more likely to tear when the battens are lifted. If a roof is being re-covered, it is normal to fit new underlay, and the choice between breathable and non-breathable types depends on the roof build-up. Where only the sagging section is bad, a localised repair is possible, but it can be difficult to blend new underlay into old.
What the fix looks like
If the felt is sound and only slightly low, the fix may be to adjust the insulation so that it clears the underlay. If it is torn or perished, it needs replacing, which means lifting the tiles, fitting new underlay and re-laying them. That is a bigger job, so it is often combined with other repairs. We will show you what we find and tell you plainly whether a patch will do. More on our tiled-roof service is on the pitched roofing page.
Every roof is different. A free survey with photos settles it for yours.
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