Matching old slates on a repair

1Start with where the slate came from

Slate is natural stone split from a quarry, so no two sources look or behave alike. Seafront Victorian terraces often carry Welsh slate, and a repair matches best when it uses stone of the same kind. Welsh and Spanish slate differ in grain, thickness and the way the edge breaks, which we compare in Welsh against Spanish slate.

The best match is usually already on the building. A rear slope, a redundant outbuilding or a lean-to that is coming down can supply slates of the right age and weathering. Reclaimed slate from elsewhere is the next choice, and each slate is checked before it goes up: a sound one rings when tapped, while a dull note can mean a hidden crack. New slate from a close equivalent comes third, and it suits a slope that nobody sees from the road or the garden.

2Size and thickness decide whether it lies flat

Old slates were cut to traditional sizes, and many differ from a modern slate with a similar name. A slate that is a little shorter than its neighbours changes the gauge, the spacing of the battens, which are the timber laths the slates hang from. The courses then stop lining up and the roof looks patched.

Size also decides how the slate sheds water. The headlap is the overlap of one course over the slate two courses below. Cupa Pizarras gives, for a 500 x 300 mm slate at moderate exposure, 115 mm at 20°, 80 mm at 30° and 55 mm between 45° and 75°. Its table gives no value below 25° for 600 x 300 mm or 500 x 250 mm slates, so the size has to suit the pitch.

Thickness matters as much. A thin new slate beside thick, uneven old ones rocks and leaves a gap for driven rain. Before ordering we measure the length, width and thickness of a sound slate and the batten gauge, then check the result against the pitch.

3Colour is judged weathered, not fresh

Slate colour runs from blue-grey through purple to green, and a slate straight off the pallet looks quite unlike one that has spent a century outdoors. Judging a match on a fresh slate is how a repair ends up as a dark patch on an otherwise soft roof.

Weathering is uneven too. Some slates hold their colour for generations, while others dull, streak or pick up a rust-brown tinge, and a sunny slope ages differently from a shaded one. So we compare against slates from the same slope, not a sample from another roof.

There are practical ways to hide a difference. Reclaimed slates go on the visible slope and new ones on the rear. A spread of small repairs can be blended by mixing old and new across a wider area, instead of leaving one block of new slate. A repaired block settles with the weather, though a poor match at the start does not fully disappear.

4Listed buildings and conservation areas

On a listed building, matching is a legal matter as well as a visual one. In the council's guide to listed building consent, a repair made in different material is named as work that needs it. The South Downs National Park Authority treats minor repairs that replicate the original, done in place, as outside consent, while changes that alter or wholly replace original material and affect its character do.

Conservation areas can differ. Where an Article 4 direction applies, normal permission for changing roof coverings can be withdrawn. Directions covering Old Shoreham and Kingston Buci withdraw it for houses that face the street or a footpath, so swapping the roof covering there needs planning permission. A straight repair in the same material seldom does, but ask the Adur & Worthing planning team before ordering anything unusual. The wider picture is in slate roofs in conservation areas.

5Cost, and when to re-slate the slope instead

Matching adds time more than price, because finding sound reclaimed slates can take longer than ordering new ones. For scale, MyBuilder's slate guide (updated 21 August 2026) gives a typical UK range of £100 to £180 per m² for a slate roof. That is a whole-covering figure; a repair is priced on what the survey finds. Slate against tile cost sets it beside other coverings.

There is a point where patching stops making sense. If suitable slates are scarce, or the old ones are failing at the nails across a slope, re-slating that slope with reclaimed stock can serve better than a scatter of mismatched repairs. Where the roof is torched on the underside, the lime needs protecting as well; see torching on slate roofs. We photograph the slope, point out which slates can be saved, and follow with a written quote that states one price. The wider range of work sits under slate roofing.