Repairing Cracked Slate Roof Tiles: Step-by-Step

A single cracked slate is easy to ignore until the first heavy rain, when a brown patch spreads across the ceiling below. Slate is tough, but it is also brittle. Foot traffic, frost, a corroded nail or a stray branch can split a tile cleanly across. The good news is that a cracked slate is usually a local problem with a local fix. The awkward part is getting the old tile out without shattering the sound ones above, below and either side of it. Done carefully, a single replacement is an hour's work, not a day's.

First, work out what you're looking at

Not every crack tells the same story. Look at where the split runs.

A crack that starts at a nail hole and travels down the tile points to a corroded nail that has swollen and forced the slate apart. You will often see rust staining around the head. Cracks near the bottom edge usually mean impact: a ladder rested against the roof, a falling branch, somebody walking where they shouldn't. Long diagonal splits across the middle tend to appear after a hard frost, when water that has crept into a hairline fault freezes, expands and does the rest.

Stand back and look at the roof as a whole before you climb anything. If neighbouring slates have slipped, if the courses have started to wave, or if the sarking shows through, you are looking at a wider failure rather than one bad tile. Moss holds water against the slate and speeds up decay; a heavy growth can lever slates out of line.

Then check what is underneath. Lift the cracked slate and look at the batten it was nailed to. Timber that is soft, dark or crumbly, or an underlay that has perished, means the slate is only a symptom. Replacing it will not stop the leak.

Tools, access and a few safety rules

Most slate repairs are done from a roof ladder, not from the slates themselves. Slate is not a walking surface: it is thin, smooth and brittle, and one misplaced boot will cost you more tiles than the crack you set out to fix. A roof ladder with wheels at the top, hooked over the ridge, spreads your weight and gives you something to hold. For anything longer than a couple of hours, hire a scaffold tower or bring in a professional.

Wear a mask when you cut or drill slate. The dust contains crystalline silica, and it is not something to spend an afternoon breathing in.

What to have with you:

• A roof ladder with wheels and a standoff bracket, or a scaffold tower
• A slate ripper — the forked bar that reaches under a tile and shears the nails
• Tin snips or a hacksaw blade for cutting nail stubs flush
• Slate hooks or tingles for securing the replacement, plus copper or aluminium nails
• A replacement slate of matching size and thickness
• Chalk, a scraper, and a bucket on a rope for debris
• Non-slip boots, gloves and, wherever there is any fall risk, a harness

Getting the cracked slate out

Work on a dry, still day. Wet slate is slick and wind makes brittle tiles harder to handle. Cold makes old slate more likely to snap, so a mild morning beats a frosty one.

1. Mark it. Chalk a small cross on the cracked slate, then set the replacement beside you so you don't lose your place halfway through.
2. Reach the nails. Slide the slate ripper under the cracked tile, between it and the slate below, until the hooked end meets the nails at the head. Pull down firmly to shear them. Two or three bites is usually enough.
3. Break it out if you must. If the nails will not cut, or the slate crumbles as you work, remove the middle of the tile with light taps of a hammer, working outwards. Slide a thin board or a spare slate underneath first so a stray blow does not crack the tile below.
4. Ease it free. Draw the freed slate out from under the course above. Take it slowly — the slate above is temporarily unsupported, so keep your fingers clear and don't lever against it.
5. Check the batten. Cut any protruding nail stubs back flush so the new slate can lie flat, and note the state of the timber and underlay while you have the chance.
6. Clear up. Sweep every fragment off the roof and lower it in a bucket. Slate shards slide down the pitch, block gutters and are sharp.

Fitting the replacement

The new slate has to sit exactly where the old one did, with the same head lap as its neighbours and no lift at the bottom edge. Slide it into position dry first and check how it sits before you fix anything.

If you can reach the batten, drill a hole in the head of the new slate and nail it through with a copper or aluminium nail — long enough to hold, short enough not to pierce the underlay.

Where the batten is out of reach, which is most of the time on a finished roof, use a slate hook. This is a strip of copper or stainless steel that hooks over the batten, runs down the roof and is dressed over the bottom edge of the new slate to hold it in place. Use two hooks on a wide slate, one at each side. Some roofers add a small bead of roofing mastic under the head to stop the tile rattling; that is a supplement to the fixing, never a substitute for it. Silicone smeared over a crack is a temporary measure at best, and it will not survive many freeze-thaw cycles.

When the slate is in, press it flat and run a hand along the course. Nothing should rock. If the new tile sits proud of its neighbours it is too thick, and the slates above it will lift over time.

Matching the slate

Slate is not a commodity. A new imported tile next to a hundred-year-old Welsh one will look wrong, weather differently and, if the thicknesses differ, sit wrong too.

• Size and thickness matter more than colour. Measure the old tile — length, width and thickness — and take it to the yard with you.
• Reclaimed is usually the right answer on an older roof. Salvage yards grade by size and origin, and a weathered tile blends in far better than a fresh one.
• Expect the colour to shift. New slate looks flat and grey for a year or two before it settles down.
• Check permissions. On a listed building or in a conservation area, changing the roof covering or its material may need consent. Ask the local planning authority before you start, not afterwards.

When to call a roofer

One or two slates on an accessible, gently pitched roof: a careful DIYer with the right ladder can manage it. Beyond that, the balance tips towards paying someone.

Call a professional if you need more than a handful replaced, if the slates are fragile and breaking as you handle them, if the roof is steep, high or cluttered with valleys, dormers and chimneys, or if you simply do not have a safe way to reach the tile. Soft battens, perished underlay or damp in the loft all point to work that is well beyond a patch repair.

A roofer

Photo: Didgeman / Pixabay