Why Is My Slate Roof Leaking?

Slates lie in overlapping rows called courses, and rain is passed from one row to the next. If a slate slides down or breaks, water lands on the underlay beneath, the sheet membrane under the battens, or straight onto timber where an old roof has no underlay at all. One gap can soak a rafter for months before any mark appears indoors.

Why did the slate move?

Often the nails have corroded, the slate was cracked by a footstep, or frost opened a split in weak stone. A single slate lost in a blow is a modest job, and the roof around it may be perfectly healthy. Several in one patch point to something wider, and fixing them singly can cost more than dealing with the whole section.

From the garden you can spot a dark gap, a slate hanging at an angle or fragments in the gutter. Please do not climb up to look, because slate cracks under a boot and a wet slope is slippery. Our guide to slate roof repairs explains what a roofer does with slipped, cracked and missing slates, and our page on slate roofing covers the wider picture.

Wherever slate butts against a wall, a chimney stack or a roof window, a strip of lead called flashing turns the water back onto the slope. It is the commonest weak spot on a roof whose slates are still sound. MyJobQuote says lead can last for over 80 years, but only while it stays tucked into the brickwork and free of splits.

How does lead fail on a slate roof?

It cracks with age and movement, lifts where the pointing has weathered, or hides under a smear of sealant that merely postpones the leak. Chimneys are the usual trouble spot, because water finds the apron at the front of the stack and the gutter behind it.

Look for stains on the chimney breast, damp rafters around the stack in the loft, or lead that has slumped down the brickwork, and notice whether the marks appear only after rain from one direction. MyJobQuote lists patch repairs to chimney leadwork at £150 to £250 and a full set at £350 to £920, with scaffold swinging the total (Sep 2026). Our answer on how to tell if flashing has failed lists the signs.

A valley is the V-shaped line inside a corner of the roof, where one slope runs into another, and it collects the run-off from both. It is a narrow gutter doing a big job, so a split or a blockage can pour a lot of water through one spot. Roofs with extensions, dormers and bays have more valleys, and so more places for a fault to start.

What goes wrong in the channel?

Lead liners thin out and crack. Moss, leaves and silt fill the gap and shove water sideways beneath the neighbouring slates. For a full valley replacement MyJobQuote quotes around £540 as a typical UK figure (Sep 2026), and renewing a tired valley is often cheaper than chasing repeated leaks.

Look for stained slate along one edge of the valley, or green growth running down it. If the damp patch inside lines up beneath a valley or a stack, the roofer already has a trail to follow. Our answer on lead and mitred valleys sets out how a valley is built and which type suits which roof.

Clearing debris from the valley is a job for a ladder at the eaves or a roofer, never a boot on the slope.

Many older slate roofs were laid with no modern underlay. Instead lime mortar, known as torching, was pressed between the slates and the laths, the thin timber strips they rest on, to hold back wind-driven rain and dust. After decades the mortar falls away, and the roof starts letting in fine spray during a gale even though every slate looks sound.

Can I check for it myself?

Yes, from the loft in daylight. Chinks of light between the slates, or crumbs of old mortar on the insulation and floor, tell you torching has failed. It is no cause for panic, though it does deserve attention before the next winter.

If water is dripping in now, our first move is to make the roof safe with a temporary cover, and the lasting repair comes when conditions allow. Then comes the survey, where you see pictures of the slope for yourself and receive one written price. Whether the torching is patched or the slope is relaid depends on how much has gone. A photo of the stain and of the slope above it, sent to us, starts the process.