Slate roof leaking: common causes

1Slipped, cracked and loose slates

A slate is held by a nail or hook through one small hole, so when the fixing goes the slate slides and leaves a gap. The usual cause is nail sickness: the iron nail rusts and loses its grip. The pattern shows as a few slates dropping down their courses across a slope, and it often means more are about to follow. Our pages on nail sickness and slipped slates go into it.

Cracks are the second cause. A slate can split from a footstep, a fallen branch or frost, and old slates can flake along their layers. A man-made slate becomes brittle with age, a point covered in fibre cement slate lifespan. A single replacement slate, held by a tingle, often closes the leak. A tingle is a narrow metal strap that clips a new slate down where no nail can be driven.

2Flashings, chimneys and abutments

Anywhere a slope butts against brickwork, for instance at a chimney, a gable wall or a dormer side, the junction relies on flashing: lead lapped beneath the slates at one edge and tucked into a cut made in the masonry at the other. Soakers are small folded lead tabs laid between the courses, and they work with the flashing. If the lead curls away, cracks or works loose from its chase in the wall, rain follows the masonry down inside.

Code 4 lead, 1.80 mm thick, is Leadworx's pick for general flashing work. Cement fillets, a mortar bead some builders used in place of lead, shrink and part from the wall over time. Sealant smeared over the top may hold for a season and then trap water behind it. The lasting fix is to dress or renew the lead and tuck its upper edge into the wall.

Chimneys are the commonest place for this fault, because the stack meets the roof on all four sides and exposes its mortar joints to the weather. A stain on the ceiling below a chimney breast is a cue to have the flashing checked.

3Valleys, ridges and gutters

At the meeting line of two slopes sits the valley, a trough that usually has a lead lining and takes the run-off from both. Moss or leaves can choke the channel, and in a downpour the water then spills under the slates beside it. Our guide to slate valleys shows how they are built.

Ridges and hips cap the joints at the top and corners. Many older slate roofs were finished with ridge tiles on a mortar bed, and the mortar cracks with age. Current practice adds mechanical fixings to the bedding, and ridge tiles on a slate roof sets out the choices.

Gutters cause leaks at the eaves. A blocked outlet or a sagging run overflows behind the fascia, where the water soaks the eaves timbers and the bottom course of slates. The stain looks like a roof fault, yet the cure sits at gutter level, and it is cheaper to catch early than after the timber has rotted.

4Underlay, torching, pitch and condensation

Older slate roofs often have no underlay. They rely on torching, the lime pressed under the slates, which crumbles with age. See torching on slate roofs for what that means for a leak.

Pitch matters too. Cupa Pizarras reads BS 5534:2014+A2:2018 as setting 20° as the shallowest allowable pitch for natural slate, assuming moderate exposure, a 500 mm slate and 115 mm of lap. A roof re-covered below its limit, such as a porch, can stay dry in light rain and leak when wind drives water uphill.

Not every drip is a leak. Steam from a bathroom or kitchen drifting into the loft can condense on cold slate undersides and timbers, more so when ventilation is poor. A stain that appears in cold, still weather and not in storms points to condensation, and the cure is ventilation, not slates.

5How we trace it and what repairs cost

The first look is from inside the loft, torch in hand, tracing stains and wet timber upward, and only then do we go outside to match what we saw to a slate, flashing, valley or gutter. Water often enters further up the slope than it shows, which is why a quick patch over the stain so often fails.

For scale, MyBuilder's roof repair guide (21 August 2026) quotes £150 to £1,500 as typical, with major jobs at £2,500 or more. Your figure depends on the cause, the access and how many slates are involved. Our written quote, built on loft and roof photographs, comes to one total. Rain entering today means the first visit makes the roof safe. The wider list of fixes is on slate roof repairs, and the whole subject sits under slate roofing.