A single patch of fresh slates on a hundred-year-old roof can be spotted from the pavement. The colour is too flat, the edges sit a fraction proud, and the whole repair reads as a rectangle of newness rather than part of the roof. Getting salvaged slate to disappear into an existing slope comes down to three things: finding the right slate, matching its size and thickness, and handling colour with a bit of restraint.

None of it is difficult. It just rewards anyone willing to measure properly, sort carefully and resist the temptation to fill a damaged area with whatever the yard had in the pile that week.

Start by measuring what you already have

Before you buy anything, get up close to the roof and take notes. Slate sizes are usually quoted in imperial figures — 20×10, 18×12, 16×8 — but the actual slates on an old roof rarely measure exactly what the name suggests. A nominal 20×10 might be 495mm by 250mm on the day, and half a centimetre of drift across a slope is enough to throw your courses out.

Measure several slates in different courses rather than one, and measure them across the width as well as the length. Then check the gauge: the exposed height of a single course. On older roofs the battens are already fixed, so your replacement slates have to work with that spacing.

Two further checks are worth the effort. First, look at the underside from the loft if you can get in, and note the batten spacing and whether the nails are copper, stainless or something older and rusted. Second, photograph the roof in dry weather and again after rain. Slate changes colour dramatically when wet, and a slate that looks a good match at noon in July may look quite wrong by February.

On some older buildings the courses diminish — larger slates at the eaves, smaller ones towards the ridge. If that is what you are dealing with, a single size will not do.

Where to find reclaimed slate

Salvaged slate is not a commodity you can order to spec. It circulates through a handful of channels, and each has its own trade-offs:

• Architectural salvage yards. The best option if you can visit in person. Ask to sort through the pallets and check for yourself rather than accepting a graded pile sight unseen.
• Roofing contractors. Anyone stripping a roof of the right slate is a potential supplier. Ask to be told when a suitable job comes up, and be ready to collect quickly — stripped slate often goes straight to the tip.
• Online marketplaces and local selling groups. Cheap and plentiful, but you rarely know the condition, the thickness or where it came from. Fine for a shed, risky for a front elevation.
• Your own roof. If you are stripping a rear slope, keep every sound slate for the front. Matching is never easier than when the slate has come off the same building.

Buy more than you think you need. Slates break during handling, and a few spares in the garage will save a lot of hunting later. Do not trust a seller's grade label either — grading means different things at different yards.

Size, gauge and headlap

If you are re-slating a whole slope, you have some freedom. If you are patching, you do not: the new slates must be the same length and width as the old ones, or the bond will not line up and the laps will not work.

Gauge is calculated from the slate length and the headlap. A slate 500mm long with a 75mm headlap sits on a gauge of 212.5mm, because the headlap is deducted once and the remainder halved. Headlap itself increases as the pitch falls and as the roof's exposure to wind-driven rain rises — commonly somewhere between 65mm and 90mm on UK roofs, so check against current standards and the slate supplier's guidance rather than guessing.

What to check before you buy

1. Length and width of several slates, measured, not assumed.
2. Gauge of the existing courses, so battens and laps line up.
3. Thickness, ideally with a caliper.
4. Number of slates needed, worked out from the gauge and the slate width, plus an allowance for breakages.
5. Colour and texture in both dry and wet conditions.

Thickness, soundness and re-holing

Reclaimed slates vary far more in thickness than new ones. Sorting them into piles before you start is not fussiness — a thick slate sitting between two thin ones will crack the moment anyone walks the roof. Keep each course roughly consistent, and put the heavier slates where they will do the least harm.

A simple ring test tells you a lot. Hold a slate between finger and thumb and tap it with a nail or the handle of a scraper. A clear ring and a faint flex suggest sound slate. A dull thud usually means a hairline crack, even if you cannot see one. Check the edges too, particularly on imported slates, for laminations that will peel apart after a winter of frost.

Clean off old mortar, bitumen and moss with a scraper and a stiff brush. Avoid hammering, which is a good way to create the cracks you were trying to avoid. When it comes to fixing, do not reuse the old nail holes. Punch a new one from the underside of the slate, keeping it at least 25mm from the old hole and from the edges, and use copper or stainless steel nails. Galvanised nails have no place on a slate roof.

Colour, weathering and blending

Slate colour comes largely from the quarry, not from age, so the first question is where your slate originated. Welsh slates tend towards blue-grey and heather purple, Cumbrian slates to green and silver-green, Cornish slates to a cooler grey. Imported slates are often more uniform, and that uniformity is exactly what gives a repair away.

New slate also looks different from old for years, because the surface has not been weathered. Rain, frost and sunlight dull and mottle it gradually. That means a patch of genuinely new slate will stand out at first no matter how well you match the quarry.

The answer is to blend rather than patch:

• Sort your salvaged slates into roughly three groups — lighter, mid and darker — and alternate between them as you work across the slope.
• Spread any new slates thinly across the whole area, never in a block, and never more than a few in a single course.
• Keep the cleanest, flattest slates for the most visible places: the front elevation, around dormers, above the entrance.
• Save the stained or slightly chipped ones for a hidden slope, a valley, or somewhere under a ridge.

If a repair still looks too sharp, swapping a few sound existing slates out of the surrounding courses and re-laying them among the replacements is an old trick that works surprisingly well.

A sensible order of work

Working through it in this sequence saves a lot of wasted trips up the ladder:

1. Measure the existing slates and gauge, and photograph the roof dry and wet.
2. Source a small sample first and hold it against the roof before committing to a batch.
3. Sort the full batch by size, thickness and colour while it is still on the ground.
4. Strip carefully and keep every sound slate from the same roof.
5. Set out battens to the original gauge.
6. Work across the slope, mixing your colour piles as you go.
7. Store the leftovers somewhere dry.

Two last points. Roofing is genuinely dangerous work and fragile old slate gives no warning, so use a competent roofer with proper edge protection. And if the building is listed or sits in a conservation area, speak to the local conservation officer before changing the roof covering — consent requirements can catch people out, and it is worth taking professional advice on that point rather than assuming.

Photo: MrsBrown / Pixabay