Why is my slate roof leaking?

A slate is held by a single nail near the top, or two on larger sizes, and held down by the overlap of slates above. When a slate slips, it drops a few centimetres and leaves a gap at the top. Rain then runs through the gap and onto the felt or battens. Our page on slipped slates explains the signs and what usually causes them.

Cracks are the other simple cause. A cracked slate can leak slowly, especially in wind-driven rain. Cracks come from impact, from a foot or a ladder, or from frost working into a flaw. A single crack through the middle of a slate is a leak path, and from the lawn it is easy to overlook.

A sound neighbourhood of slates makes either repair quick. What matters is why the slate moved, since a failed nail usually means others are following.

Old slate roofs were often hung on iron nails, which corrode over decades. A corroded nail thins, snaps or loses its grip, and the slate it holds slips even though the slate is perfect. This is called nail fatigue or nail sickness, and it is the commonest reason an otherwise good slate roof starts to let in water. See our page on nail sickness for how to recognise it.

The signs include slates sliding in several places, a roof that needs repeated small repairs, and nails that snap as soon as a slate is lifted. Re-fixing a slope with new nails costs far less than new slate, and the slate is reused.

So at survey we sample the fixings, photograph them and only then recommend a patch or a full re-fix.

Wherever slate butts against a chimney, a side wall or a dormer, lead flashings and soakers seal the junction. Soakers are small pieces of lead bent to slide under each slate course, and flashings are the sheets that cover the join. Both cause leaks, because lead cracks, lifts, splits at its laps and can drag out of the joint it is tucked into.

MyJobQuote says lead flashing laid correctly can pass 80 years, and Leadworx says code 4 has become the go-to weight for flashing. Even so, old lead may be thin, torn or fixed with mortar that has fallen away. A stain beside a chimney breast, or on a ceiling next to a wall, often leads to a flashing and not a slate.

Repairs range from re-pointing the flashing to renewing the lead. We tell you which is needed and show the photographs.

A valley channels water from two slopes, so it carries far more than any one slate. If the lead lining cracks or the channel is blocked with leaves and moss, water overflows beneath the slates. Our page on slate valleys explains how they are built and where they fail.

Gutters are the other half of the picture. A blocked or sagging gutter at the eaves can send water back under the lowest slates and onto the fascia or wall plate. Timber at the eaves rots quietly, and the stain only appears when a lot of timber is wet.

Before assuming the slates are to blame, check that the gutters run and that the valleys are clear. We clear and inspect them during a survey and photograph what we find.

Slate is a covering, not a seal. Under it sits the felt, battens and sometimes boards, and these catch water that gets past the slates. If they fail, leaks start. On an old roof without felt, windblown rain or snow can reach the loft directly. Where boards are fitted, water can sit and rot them for years. Our page on sarking boards explains how boarding changes the symptoms.

Torching, a lime plaster layer on the underside of slates, is another older feature. When it falls away, it can let wind-driven rain and snow blow in. See torching slates for details.

Tracing a leak means working backwards from the stain, along the rafters and battens, to the slope above. We do not guess. We photograph the loft and the roof surface and tell you what we found.

Water coming in at this moment? Phone us. A wet loft jumps the queue: we stem the flow first, which may mean a temporary cover, before the survey and the proper repair are priced. Our slate roof repairs page describes the usual fixes, and the slate roofing hub shows the full range of slate work. Cracked slates get their own explanation on the cracked slate page.

Repair is right when the cause is contained. Re-fixing or re-slating is right when nails, battens or felt are failing across a slope, and the pictures from the loft and slope show which applies. A written quote follows, we clear up behind us, and ten years of workmanship cover follows the job. Slate that leaks is rarely a reason to panic: most causes are found with a careful look and put right without disturbing the whole roof.

To arrange a visit, ring 01793 626096, use WhatsApp or send the form, saying where the stain is. We plan the survey around that spot and trace the route the water takes from there.