How is a single cracked slate replaced?

Old slate is rarely uniform. It weathers to a patchwork of greys and greens, picks up lichen, and shows fine surface lines that mean nothing. A crack that matters runs right through the thickness of the slate, so that daylight shows from the loft or water finds a path down the line. From the ground you may spot one half of a slate sitting lower than the other, or a ragged edge where a corner has dropped away.

Binoculars from the garden help, as does a look in the loft on a bright day. Pinpricks of light through the roof usually mean a missing or broken piece. Damp patches on the underlay or battens are another giveaway. What you cannot judge from below is whether the crack is the result of a knock, such as a fallen branch or a footstep, or whether the slate cracked because the fixing under it has moved. That second reading decides how much work follows.

Because slates overlap, the nails holding any one of them sit out of sight beneath the next course up. The roofer slides a slate ripper, a long flat tool with hooked blades at the end, up beneath the slate until the blades catch the nail shanks. A sharp downward tug cuts them. The cracked pieces are then withdrawn, and any fragments lodged between the courses are cleared so they do not hold water or prop up the new slate.

This is done by feel, and by lifting the slates above only enough to get at the work. Careless prying cracks the neighbours, which is how a one-slate repair turns into a six-slate one. The battens beneath are inspected as the gap opens. If a batten is rotten where the slate sat, it is cut out and a new section is fixed alongside, because a fresh slate on a soft batten will be loose in a year. Our page on slipped slates shows how that kind of weakness spreads.

A replacement has to match in size, thickness and tone. Older roofs often use slate that has lost its first dark finish and now looks grey-brown or streaked, and a brand-new slate beside it can stand out for months. Reclaimed slate from a roof of similar age blends in faster. We check the stock against the existing slates before we start, and our page on reclaimed roof slates explains the trade-offs.

Size matters for more than looks. Each slate has to lap correctly over the one below and under the one above, which is set by the headlap. A slate that is a few millimetres short leaves too little cover and invites a leak. On a listed or conservation-area roof the match is also a consent matter: a straight swap is judged differently from a change to how the roof looks. Where the original slates were a particular local stone or colour, say so when you ring so the right match is sourced rather than guessed.

Mastic, bitumen, mortar and rubber sealants all promise a quick fix for a cracked slate. They rarely last. A crack opens and closes as the slate heats and cools, and a filler stuck across it splits as well. Water then runs under the filler and carries on down the slate, hidden from view. A smeared repair can also trap damp against the slate, and it makes a proper replacement harder because the surface is contaminated.

Roofers do sometimes use a stopgap, such as a temporary lead sheet slid under the crack, to get through a storm. That is a short-term measure, and it should be replaced with a new slate as soon as it can be done. A leak is made safe before anything else, and you are told which repairs are built to last and which only get you through a wet night. A job that has been patched four times with different products is a good sign that someone should have replaced the slate at the first visit.

For a single slate, the visit is short. Most of the time goes on getting to the spot, checking the slates around it, and deciding whether a ladder will do or the position needs scaffold. Checkatrade's slate guide, last updated September 2026, gives slate roofers as £280 to £360 per day, and its roof repair table lists natural slate repairs at £150 to £220 per square metre excluding VAT. Those are typical UK figures; our own quote, in writing, comes after we have seen the roof.

Before anyone arrives, stay off the slope. A cracked slate is weak, and the ones near it may be too. Should rain be getting in, set a bowl under the drip and shift anything valuable out of its path. Our slate roofing page explains the wider range of work, from small repairs to a full re-slate, and our recommendation is whichever job is smallest while still curing the fault for good, because a bigger one is not always better.