Can a cracked slate be repaired?

A slate roof sheds water because each slate overlaps the ones below it. A crack through the slate lets water run through to the underlay, and that water often runs a good way along the batten and down the underlay before it shows as a stain on a bedroom ceiling. By then the timber under the crack may already be damp. Frost makes it worse: water in a crack freezes, widens the split and lets the lower part of the slate slip.

Where it crosses a nail line, a crack also frees the slate from its fixings. That is the more common reason cracked slates go missing in a gale. The slate is not blown off whole; the lower half drops away from a broken top half that is still nailed to the batten. Our page on why slate roofs leak sets this alongside the other usual causes.

A cracked slate near the ridge or in a valley deserves quicker attention than one low on a sheltered slope, because those spots carry more water. Nothing about the damage tells you how long it has been there, so a stain that has appeared in the loft may relate to a crack that opened years earlier.

Sealants, mastic and bitumen strips can close a hairline crack temporarily, and a roofer may use one as a stopgap while waiting for the right slate to arrive. Their weakness is movement. Slate moves a little with the temperature, the roof flexes in wind, and the sealant cracks or lifts within a few seasons. Painting a crack with a brush-on compound can also hide the problem, so you stop watching it. We treat sealing as a stopgap, for use when a roofer cannot get to the roof for a while, not as a repair we would put in writing.

Replacing the slate means a lasting result. The roofer lifts the slates above, cuts away the old nails with a slate ripper, and fits a new slate held by a lead or copper tingle strip or a metal clip, since the nails of the old slate are gone. On a roof that can be reached safely it is quick work, and once the new slate has weathered the repair is hard to spot from the ground. If the crack is in a slate under a ridge, valley or abutment, the surrounding leadwork or ridge tiles may have to be lifted first, which lengthens the visit a little.

One broken slate and a handful of broken slates are very different pictures. We check whether the damage has an obvious cause, such as a slipped ladder, a falling branch or a satellite engineer walking on the roof. Natural slate is brittle underfoot, which is why our page on walking on slate exists, and a line of cracks along a path someone has walked is a giveaway.

We also check what the slates are fixed with. If neighbouring nails are rusted, the cracks may be a symptom of wider fixing failure, and replacing slate after slate becomes false economy. Then we look underneath: the battens may be soft where water has entered, and the underlay, if the roof has one, may be torn or perished. Where the roof has sarking boards under the battens, a cracked slate is less likely to drip into the loft, which is a point covered on our sarking boards page.

Replacement is only as good as the slate you replace it with, and a poor match is the usual complaint after a slate repair. Size, thickness, colour and the way the slate has weathered all count, and a bright new slate among old ones shows as a patch for some years. Reclaimed slate of the same size is often the best match, and on a historic roof it is sometimes the only suitable choice. Our page on natural and fibre-cement slate explains why a fibre-cement substitute rarely suits a natural slate roof.

If the roof has dozens of cracks and no slate to match, the repair-or-replace question changes, and the discussion moves to whether part of the roof should be stripped and relaid with the existing slates reused. The written quote spells out that choice and shows the photographs behind it.

MyBuilder's roof repair guide, updated 21/08/2026, puts a minor repair at £100 to £300. Replacing one or two slates normally falls in that band when the roofer can reach the spot by ladder and a crawling board. Higher roofs and awkward slopes need access equipment, and a steep slope with a deep eaves overhang may need a tower rather than a ladder, and MyBuilder's scaffolding guide gives around £850 to £1,200 for a standard two-storey house, which is why several small repairs are often better done in one visit.

Checkatrade's October 2026 repair guide gives per-square-metre figures for roof repair, with natural Spanish slate at £150 to £220, excluding VAT and access equipment. These are national figures, not Skyline's prices. We would rather fit one slate that cures the leak than propose a bigger job that is not needed. After the visit you hold pictures of your slope and a price in writing. Repairs carry a ten-year workmanship guarantee on paper, and the broken slates leave with us when the job is done. See slate roofing for the full range.