Question answered
Can a cracked slate be repaired?
The short answer
A cracked slate is best replaced, not patched. Sealant, mortar and tapes on the surface soon fail, because the slate flexes and weathers. A roofer lifts the neighbouring slates, slips out the broken one and fits a matching slate, held with a tingle where a normal nail cannot be used. A crack across a whole slope is a sign of a wider problem.
A cracked slate lets water through the break and onto the underlay below. Patching it rarely lasts, and the better repair is to take the slate out and put a sound one in its place.
Why patching a crack does not last
A slate is a thin plate of stone that moves slightly with temperature and wind. A crack runs right through it, and water follows the crack, wets the batten and the underlay, and carries on down. Sealant spread over the line is stuck to a surface that is always moving and always wet.
Bitumen tape, mastic and mortar all look like repairs from the ground, and most come loose in the first hard frost or the first gale. Some trap water inside the break, where it freezes and widens the gap. A patch can also hide a fault, which makes the next inspection harder.
The slate itself is still the point. A slate with a clean break is a slate in two pieces, and no filler turns it back into one. That is why every roofer worth hiring will tell you the real fix is a new slate.
How a broken slate is replaced
Because slates overlap and are nailed beneath the course above, a broken one cannot simply be lifted out. The roofer first frees it, using a tool called a slate ripper, which hooks the nails and cuts or pulls them. The broken slate then slides out in pieces.
The replacement is slid into the gap and needs a fixing, but the original nail position is hidden under the course above. The usual answer is a strip of lead or copper, called a tingle, nailed to the batten and folded over the foot of the new slate. Our guide to slate tingles covers how that works.
The headlap and the gap between the new slate and its neighbours are checked, so water has the same path down as the rest of the course. The whole job on a single slate is quick, and takes far less disturbance than re-covering. Neighbouring slates are lifted only as far as the ripper needs, and each is checked as it goes back.
Where cracks come from
Some cracks are accidents, such as a falling branch, a ladder foot, a dropped tool or a boot. Foot traffic is the most avoidable. A slate carries the weight of rain and wind comfortably but not the point load of a boot, which is why walking on slate is best left alone.
Others are signs the slate is failing. Hairline cracks and flaking edges on several slates point to the fault in our page on delaminating slates. A crack alongside a slipped slate may mean the batten beneath has softened or the headlap is thin.
Movement can crack slates, too, where the roof structure has shifted or where a valley or a flashing pulls at the course beside it. Our survey looks at the cause before it looks at the price, because replacing a slate whose neighbours are about to follow it is only half a job.
When one cracked slate is really many
If a roof shows a few broken slates in different places, one visit can usually put them right. Our wider slate roofing page covers the larger jobs. If cracks are spread across a whole slope, or several slates beside each other have broken, the slope may be reaching the end of its life or have been damaged by foot traffic during earlier work.
This is the point at which we weigh the cost of repeated repairs against re-slating a section. For some roofs, a covering of a different type is the sensible next step, which our page on changing slate to tile looks at.
We show you photographs of the cracks and of the underlay and battens we find, and a written quote follows from what we have found, as one total figure. If one new slate is all the roof needs, that is the job we recommend, because the lightest repair that cures the fault is the right one.
Matching the slate, and keeping out of trouble in the meantime
A replacement should match the old slate in size, thickness and colour as closely as possible. Slates weather over the years, so a new one can look darker or lighter than its neighbours for a time. Sound reclaimed slates often blend best, though only if they are in good order.
Until the repair is done, keep an eye on the ceiling below and the loft for damp, and put a bowl under any drip. A cracked slate over a valley is worth urgent attention, since valleys carry the most water; our guide to slate valleys explains why.
Where rain is getting in, we secure the roof before anything else. Stay off the roof yourself, and take pictures from the ground. A clear photograph of the broken slate, with the neighbouring ones in view, helps us plan the visit and bring the right slate to match.
Broken slate: two more questions
Can a cracked slate be glued back together?
Adhesive may hold the two halves for a while, but the joint will not stay watertight as the slate moves. The piece also adds weight and thickness to the course, so a glued slate is not a repair we would recommend.
How do I know whether the crack has let water in?
Look in the loft for a damp patch, a dark stain on the underlay or a wet batten beneath the slate. Stains on the ceiling often appear lower down the slope than the break itself.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.