Why slate is a different job from tiles
A slate roof takes a roof window perfectly well, but the slates themselves set the pace of the work, because they cannot simply be lifted off and hooked back on.
Concrete and clay tiles hang on battens by nibs, and most of them can be slid out and laid back by hand. Slates are held by two nails each, driven through the slate into the batten, and every slate is covered by the two courses above it. To take one off, a slater cuts the nails with a slate ripper, a flat steel blade slid up under the slate. That is slower, and a slate that has been on a roof for a century can snap as it comes free. On the Victorian and Edwardian terraces of Romsey, Mill Road, Petersfield and Chesterton, some of the roofs are still Welsh slate, often with no underlay beneath, just the battens and a lath and plaster ceiling or bare rafters inside.
The other difference is thickness and shape. Slate is flat and thin, so it needs a flashing kit made for flat roofing materials rather than the deeper, profiled kit used on pantiles and interlocking tiles. Our guide to roof window flashings sets out the kits side by side. On this page the focus is the slating around the kit: what comes off, how the edges are cut, and how the roof is closed up again so it looks as if the window was always there.
How the slates are stripped, cut and holed
The opening is cut from inside where possible, but the slating around it usually needs a wider strip and a little more outside reach than a tiled slope.
Once the lining and insulation are out and the rafters are exposed, the fitter marks the frame position and strips the slates over the opening plus a margin of a few courses on each side and above. The margin matters: the side pieces and head piece of the flashing tuck under the surrounding slates, so those slates have to come off to make room and then go back over the flashing. Sound slates are stacked inside in order so they can be reused. The battens are cut to line, and if the window is wider than the gap between rafters, the rafters are cut and trimmed with new timbers, which our guide to trimming rafters for a roof window covers in detail.
Cutting slate is hand work. Each slate that meets the side of the frame is trimmed to width with a slate cutter or on a slating bench, and a fresh nail hole is punched where it will land on a batten. A narrow sliver of slate will not hold and looks wrong, so the slater works the course out using wider slates, or slate-and-a-half widths, at the frame edge so that no cut piece ends up too thin. The courses are kept to the same gauge as the rest of the roof, so the lines of slate run straight past the window without stepping.
| Slate on the roof | What it means for the install |
|---|---|
| Old Welsh slate | Brittle when stripped; spares are sourced to match |
| New natural slate | Cuts cleanly; extras can be bought to match |
| Fibre cement slate | Easy to cut; older ones are tested before cutting |
| Reconstituted slate | Thicker; check the flashing is rated for it |
Making good around the frame
The finish around a roof window on slate is judged from the street, so the last courses are where the care goes.
The flashing goes on from the bottom up. The apron at the foot lies over the slates below the window. The side pieces, soakers or a one-piece side flashing depending on the kit, run up each side of the frame with the cut slates laid over them. The head piece goes last and the slates above lap onto it. Where the last slate in a course cannot be nailed because the batten is hidden under the course above, it is held with a tingle: a narrow strip of lead or copper nailed to the batten in the gap between slates, then folded up over the tail of the new slate. Tingles are standard slating practice and, once folded, sit almost out of sight.
Where old slates break, matching matters more than anything else. Reclaimed Welsh slate of the right size and colour is the usual choice on a Cambridge terrace, because new slate from a different quarry can read as a different grey, especially once it has weathered for a year. If the roof has no underlay, a collar of breathable membrane is fitted round the frame and dressed down onto the slates below, so any wind-driven rain that gets past the slating is led back outside. Inside, the lining is built back with insulation to current Building Regulations.
Pitch, access and the light on a slate slope
Slate is often laid at lower pitches than tile, and a slate roof often faces the street, so pitch, access and planning all get checked before the light plan is settled.
Welsh slate can be laid down to shallow pitches with large slates, and some rear outriggers and lean-to roofs on Cambridge terraces are close to the lower limit that roof windows are rated for. Our answer on the minimum roof pitch for a roof window explains what happens below it. Because the strip of slating is wider, a slate job more often needs someone outside for part of the day, from a tower, a roof ladder or scaffolding, which is covered in do you need scaffolding to fit a roof window. Some slate installs can still be done largely from within the room, as explained in can a roof window be fitted from inside.
On a front slope in a conservation area, a flush conservation roof window with a recessed slate flashing sits lower in the slating and suits the street better than a standard frame. Permitted development usually covers a roof window that does not project more than 150 mm, but an Article 4 direction, a listed building or a flat changes that, and the work is notifiable to Building Control either way, as set out in do roof windows need building regulations. Then comes the part people remember: which way the slope faces. A south-facing slate slope in the open, flat country around Cambridge takes the full summer sun, so the glass and an external blind get chosen with that in mind, as described in how we plan the light.
Questions people also ask
Can you reuse the old slates around a new roof window?
Yes, wherever they come off whole. Sound slates are stacked inside, trimmed where needed and relaid around the flashing. Any that crack are replaced with reclaimed or new slates chosen to match the size, thickness and colour of the rest of the roof.
Does a slate roof cost more to fit a roof window into?
It usually takes longer than a tiled slope, because of the wider strip, hand cutting and matching slates, and outside access may be needed. Our skylight and rooflight costs page explains what drives the price, and a survey sets the real figure.
Which roof window flashing is used on slate?
A kit made for flat roofing materials, either a standard slate kit or a recessed one that sets the window deeper for a flush look. The kit has to be rated for the thickness of the slate, which matters with thicker reconstituted slates.
Can a pitched skylight go in a slate roof on a listed house?
It can be possible, but listed building consent is always needed first, and the design is usually a slim conservation roof window set in the plane of the slates. See our roof window installation in Cambridge page, or book a light survey through our quote form.
