Slate roofing in Bath, matched to the roof you have

A slate roof has no glue, no mortar across the slope and nothing sealing one slate to its neighbour. It stays dry because of the way the slates overlap. They are laid in horizontal rows called courses, starting at the eaves and working up to the ridge. Each slate is nailed to a batten, a sawn timber strip fixed across the rafters, and each course hides the nails of the course below.

Two overlaps do the work. The head lap is the distance by which a slate covers the slate two courses beneath it, so that at the lap there are three thicknesses of slate. The side lap is how far the slates in one course cover the vertical joint between the slates under them. Joints are staggered by half a slate, so no joint sits above another. Rain running down a joint lands on the middle of the slate below and is carried on down the roof.

Why pitch and lap go together

On a steep slope rain runs off fast, and a modest head lap is enough. On a shallower slope water moves slowly and wind can drive it up the face of the slate, so the lap has to grow. Slate size follows from that. Long slates on a low-pitched roof give a generous lap without crowding the courses, while shorter slates suit steeper work. When we match an existing roof we measure the gauge, the spacing between batten centres, because it tells us the size and lap the original slater chose and what any replacement has to fit.

Slates are made in traditional named sizes, and older roofs around Bath were sometimes laid in sizes that are harder to find now. Matching is therefore about more than colour. A replacement has to be the right length for the gauge, close in thickness so it lies flat in its course, and similar in surface and edge so it does not stand out on the slope.

Welsh, Spanish and reclaimed slate

Natural slate is quarried, split by hand or machine and dressed to size. Welsh slate is a familiar sight on the city's period roofs. When the Bath Preservation Trust commented on proposals for the Grade I listed 33 St James's Square, it described the roofs as predominantly Welsh natural slate across the roof pitch. At Lambridge Place the Trust supported putting natural Welsh slate back in place of artificial slate on a terrace mansard, the double-pitched roof that rises steeply from the parapet before easing off near the top.

Spanish slate is widely used today and costs less per square metre, though quality varies from quarry to quarry, and poorer slate can contain minerals that rust and stain the slope. Reclaimed slate, taken from other roofs, can be the closest match to a weathered roof, but it needs checking piece by piece for thickness, soft edges and holes that have split. Man-made fibre-cement slates are lighter and uniform; they suit some modern houses but look flat beside natural slate. We tell you which source suits your roof, and why, before you commit.

If you want the detail on sizes, laps and nailing positions, our guide to slate sizes, laps and fixing methods sets them out course by course. For the choice of material, the comparison of Welsh and Spanish slate looks at grading, colour and how each weathers over time. Both pages are worth reading before you compare quotes, because two prices for the same roof can hide very different slate.

Battens are the rows of sawn timber fixed across the rafters at the gauge, and every slate hangs from them. They are slim sections of wood sitting directly under the covering, so any water that gets past the slates reaches them first. At the eaves, round chimneys and beside valleys, old battens often turn soft or split along the line of nails. A batten that cannot grip a nail cannot hold a slate, however good the slate.

Below the battens sits the underlay, a membrane laid over the rafters that catches wind-blown rain and fine snow and runs it out to the gutter. The oldest roofs have none. Instead the underside of the slates was torched, pointed with lime mortar between the laps to stop draughts. Other roofs have sarking: close-boarded timber across the rafters, carrying the battens. Old bitumen felt underlay turns brittle and tears where it sags between rafters, which you can usually see from the loft.

What a re-slate involves

When the nails and the battens have both gone, the right course is to strip back to the rafters and start again. The slates come off by hand, a slope at a time, and are sorted on the scaffold. Sound ones are stacked for reuse. Cracked, soft or badly holed ones are put aside. Old battens and underlay are removed, the rafters are checked and repaired where needed, and a breathable underlay goes on, followed by new treated battens set out to the gauge. The salvaged slates are refixed with copper or stainless nails, and matching new or reclaimed slate fills the gap, kept to the slopes that show least.

Each slope is made weathertight before the next one is opened, and the roof is covered at the end of every working day.

When Building Regulations come in

Stripping and relaying a whole roof counts as re-covering, so the Planning Portal thresholds apply. Below 25 per cent of the whole building envelope and 50 per cent of the roof, re-covering does not normally need building control approval. Past either line it does, and you may be asked to bring the roof up to current thermal standards, which affects where insulation goes and how the void is ventilated. A covering at least 15 per cent heavier than the one it replaces also needs approval, and the rafters must be checked to carry it. Swapping lightweight fibre-cement slates for natural slate is one case where that question arises. Where approval is needed we make the arrangements with B&NES or a private approver and keep you told at each stage.

Figures: Planning Portal, roofs and Building Regulations guidance (2026)

Our guide to re-slating a roof walks through the order of work, how long each stage takes and what happens to the slates while the roof is open. It also covers how salvaged slate is graded, and why the best of it goes back on the front slope where it will be seen. If you are unsure whether your roof has reached that point, the survey will tell you.

Not every older roof near Bath is slate. In some of the villages to the south and south-east, older houses still carry stone tiles: thick, split pieces of limestone, often called Cotswold stone slates, laid in a pattern quite different from Welsh slate. The B&NES conservation area appraisal for Wellow says original Cotswold stone tiles survive in places among its red clay tiles, and describes them as rare and to be preserved. The appraisal for Freshford and Sharpstone notes that a significant number of buildings there retain their Cotswold stone tiles.

How a stone tile roof is laid

Stone tiles come in random lengths, so they are sorted on the ground and laid in diminishing courses: the largest, heaviest tiles at the eaves and steadily smaller ones towards the ridge. Because the tiles shrink up the slope, the gauge closes up too, and the battens or laths are set out course by course rather than at one spacing. Traditionally each tile hung on its batten from a peg, often of oak, driven through a hole near its head, and the underside was torched with lime. The result is heavy, steep and very thick at the laps, and it reads quite differently from slate when seen from the lane.

Repair before replacement

Stone tile roofs tend to fail at the fixings and timbers rather than the stone. Pegs rot, laths break, torching drops out and tiles slip. A slipped tile can often be rehung on a new peg, or fixed with a stainless nail where a peg will not hold. Where a whole slope has gone, the tiles are stripped, sorted by length and relaid in their courses on new battens, with the shortfall made up in reclaimed stone of the same kind and size. Concrete look-alikes are no match. The Freshford appraisal says re-covering in concrete tiles detracts markedly from the village's character.

Finding matching stone is the slow part of these jobs. Reclaimed tiles vary in colour, thickness and weathering, so we look at what a supplier actually holds before settling a size range. Newly split stone can be made to suit, though it takes years to tone down. On a listed building or in a conservation area, check with the council before changing anything about the covering, and keep the old tiles until that conversation has happened.

Weight matters as well. A stone roof is far heavier than slate, so any change to battens, rafters or the covering is planned with the load in mind, and a roof that has sagged under its stone gets its structure looked at before any tiles go back.

Our guide to Cotswold stone roof tiles explains the stone, the sizes and how the courses are set out. If your house is listed or sits within a conservation area, the page on slate and stone roofs on listed homes covers matching, consent and the methods conservation officers tend to expect. Keep any loose tiles you find; they may be the best match there is.