Can a cracked stone slate be repaired?

Sandstone is a natural material, and slabs on an old roof often carry hairline cracks, stains and chipped corners. Many of those are only cosmetic. What matters is whether the slab still does its job, which is to hold its place and pass water onto the course below it.

A crack that runs right through, so you can see light or the dark of the loft through the gap, is a leak waiting for the next heavy shower. A crack running across the fixing hole is worse, because the slab has lost its hold. A slab that has begun to peel in thin sheets, a fault called delamination, has lost its face and no longer sheds water. Each one is judged close to, not from a distance, and the ones that must come out are marked at the survey. A roofer who condemns slabs by the dozen from the ground is guessing, and one who ignores them all is doing the same. The sensible middle path is to mark what must change and leave the rest to do its job.

It is tempting to fill a crack with sealant or smear mortar across it. Neither lasts. A stone roof moves slightly with temperature and weather, so the filler pulls away from the stone along its edge, and water is drawn back into the gap by capillary action. Hard cement patches add weight and trap moisture behind them, which feeds the next round of frost damage.

You may notice an earlier repair done this way, with grey smears or blobs of black sealant on a few slabs. That tells us where past work fell short, and it is among the first things noted on a survey. Taking it off costs time, and the slab beneath is often worse than it looked. Our guide to frost damage on stone roofs explains how water sitting inside a crack does its harm.

The slab is lifted out, together with any neighbours that have to move to free it. The batten beneath gets checked, because a crack can come from a slab sitting on an uneven or failing support. A matched slab then goes in its place, chosen for size, thickness and colour so the course stays level and the diminishing courses keep their rhythm. The new slab is hung on an oak peg, as the original was.

The replacement stone is the part that takes planning. Most is reclaimed, and our page on where Horsham stone comes from today explains the sources. When a matched slab is not to hand, the survey says so, and the price you are given reflects the wait or the alternative. A slab laid in the wrong thickness stands proud of its row and channels water the wrong way, which is how a repair creates the next leak.

If a cracked slab lets water in before the repair can be done, a temporary measure can hold the roof for a while. A lead tingle, a strip of lead fixed to the batten and dressed over the slab, can stop rain running through a split. It is discreet, it does not damage the stone and it comes away cleanly when the slab is replaced.

It is a stopgap, not the repair. The first aim is to keep rain out, and the proper job is planned once the stone is sourced. While you wait, keep an eye on the ceiling below and tell us if a stain spreads. A bucket in the loft and a dated phone picture of the mark are useful to us, and to anyone else you may need to speak to about the damage. Please leave the slab itself alone: a cracked slab can fall in two when handled, and a dropped half can hurt someone standing below.

One or two cracked slabs on an old roof are routine. A scatter over a whole slope is another story, and it usually points to something shared, such as frost working into stone that has soaked up water, or a frame that has shifted. At that stage, swapping slabs one by one may cost more over the years than a proper re-lay, where the stone is graded and the weak pieces are weeded out together.

Our guide to repair or re-lay on a Horsham stone roof explains how that call is made. The result is a clear verdict, pictures of the roof and one total price in writing, and the work proposed is the smallest that cures the fault. If cracking comes with slipping too, read why stone slates slip, and for the wider service see Horsham stone roofing.