Matching slates on a Stroud extension or repair

The English Heritage policy printed in Cotswold District Council's 2000 stone slate guidance lists what a new slate has to share with an old one: the rock it is cut from, its colour and texture, and its size and thickness. It was written for stone, yet a slater runs the same checklist on natural slate.

Taking them one at a time. The rock matters because slates from different quarries weather differently: one stays blue-grey, another goes green or rusty brown. Colour is what you see from the ground, but texture shows up in raking light and decides whether the new slope looks smooth beside a rough old one. Size sets the pattern of the courses. Thickness sets the shadow line at the tail of each slate, which is the thin dark edge you can see from the street.

A mismatch in any one of these can show. Sawn slate where the original was split can look wrong, and English Heritage policy calls such substitutes technically and aesthetically unacceptable in some settings.

A fresh slate has a crisp, even surface. Decades of weather soften the colour, dull the sheen and sometimes bring out streaks, and moss and lichen alter it further. So a new extension slope beside a weathered main roof is likely to look darker or more uniform for a few years, even if the slate is the right type.

There are practical ways to reduce the contrast. The first is to choose a slate type that weathers in the same direction as the original. The second is to lay the new work in a plane of its own, where the join is a clear line such as a ridge, valley or wall abutment, instead of mixing new and old in a single slope. The third is to blend the slates from several packs, so one batch does not set the tone for an entire slope.

Where a roof has a few slipped slates in the middle of a slope, mixing is unavoidable and a close match matters more. Our answer on reusing old slates covers slates taken from your own roof, which are often the best source for a small patch.

The survey records the size by measuring a slate's length and width and the exposed part of each course. Sizes are often quoted in inches on older roofs and millimetres on new ones, so the supplier's list may need converting. Thickness is harder to see, because a slate lies flat on its neighbour, but a slipped or spare slate shows it, and so does the cut edge at a valley or verge.

Size and headlap are linked. The headlap, meaning how far each course overlaps the course two rows down, depends on the slate, the pitch and the exposure. Cupa Pizarras, quoting the standard BS 5534, gives a 115 mm headlap for a 500 mm slate at 20 degrees in moderate exposure. Matching a size therefore means keeping the headlap and gauge compatible with the roof's pitch, and our guide to slate sizes and headlaps sets out how that works.

A sample sent to a supplier for matching is more reliable than a photograph. Keep a slipped slate if you have one.

Some old slate has no modern equivalent. The quarry may have closed, or the colour may be one no current supplier offers. In that case reclaimed slate, taken from another building, can be the only way to match.

Reclaimed slate comes with cautions. The same 2000 guidance observes that a market in second-hand stone slate has seen old buildings stripped, now and then by thieves. Its quoted policy would keep reclaimed slate on the building it came off. That is a stone rule, though it shows how a conservation officer thinks. Ask a dealer where the slates came from, and keep the paperwork.

Quality varies too. Reclaimed slates may have lost their nail holes' strength, carry old cracks or show delamination at the tails. We sort them by sound, ring and thickness, and discard the doubtful ones. Expect to pay for sorting and for extra stock to allow for wastage.

Should the house be listed or inside a conservation area, raise the choice of slate with Stroud District Council before ordering, since consent can hinge on the material.

An extension joining a slate roof raises its own questions. The new slope must meet the old one cleanly, at a valley, along a hip or against a flashing. If the extension adds more than a small part of the building, the Building Regulations may apply to the work, so ask your registered building control approver or the council's building control.

Airflow also needs care when two roof spaces meet; see our answer on how a slate roof is ventilated. And if you are weighing up the age of the original slate before you choose, our note on how long a slate roof lasts gives sourced ranges.

Our survey photographs your existing slates and records their size, then the written quote covers matching slate, battens, labour and scaffold. A written 10-year guarantee covers our workmanship. More on the trade is on our slate roofing page.