Can You Fit a Roof Window in a Slate Roof?

Slates are thin, flat and laid in overlapping rows, so water is shed by the lap rather than by the shape of the covering. A roof window cut into that pattern breaks the rows, and the gap around the frame has to be closed with a flashing kit made for the thickness of slate. The kit is made up of metal sections that slot under the slates at the top and sides and lap over them at the bottom. A kit meant for profiled tiles does not suit slate, which is why the covering must be named when the window is ordered. Our page on Velux flashing kit types explains the main families.

The slates beside the frame are cut to follow its edge, and each cut slate is held with a tingle or a fixing placed where a nail would not split it. A neat job leaves a narrow, even margin of slate around the window. A roofer who rushes this stage ends up smearing sealant into gaps, and sealant is the first thing to fail when sun and frost move the slates. Ask who will cut the slates and how each one will be held.

On a tile roof the window often stands a little proud of the surface. On slate, many owners prefer it recessed, so that the glass sits lower and the frame lines up with the slate face. A recessed fitting looks calmer from the street and suits period houses, but it means lowering the frame into the rafter space, and the flashing has to be a recessed type with a deeper apron. The trade-off is that a deeper window sits closer to the water running down the slope, so the pieces around the head of the frame need careful lapping.

Whatever the depth, the frame has to be fixed to the rafters and not to the battens alone. Slates are heavy and brittle, so the roofer supports them from inside the opening and from the ridge side while the old ones are lifted. Alongside, the lining inside the loft needs insulating and sealing so that warm air does not reach the cold frame. If you are thinking of a larger opening later, our answer on replacing a Velux with a bigger window covers the structural side.

Natural slate can last well over a century when it is looked after. CoRC, the roofing trade body, says slate which is well maintained can last 100+ years. That is a reason to be gentle with an old roof. Brittle slates crack when disturbed, nail sickness can leave them loose, and a salvaged slate of the same size and colour is worth keeping for the cut edges around a new opening. Ask your roofer how many spares are likely to be needed.

On planning, Class C of the permitted development order covers roof alterations, and a window that sits flat with the slope is normally within it, but it must not stand more than 0.15 metres proud of the original slope. On a side slope the glass has to be obscure. An Article 4 direction, which some conservation areas carry, can change what is allowed, so check with your local planning team. A listed house keeps those rights too, but listed building consent is needed where the work touches its character. Our page on conservation rooflights goes into the slimmer, flatter designs often accepted on these roofs.

Before pricing, we look at the slate itself: its size, its condition, and whether the battens and rafters can carry a new opening. Photos of your own roof come with the price, showing you the same slates we looked at. Where the fixings are corroding across the whole slope, we tell you, because a new window in tired slate may be better paired with a stretch of re-slating around it.

As a national yardstick, Checkatrade's October 2026 window fitting guide gives an average of £1,250 for a new roof window and £1,025 to replace one. Slate, a recessed fit, scaffold and any rafter work push a job above a simple like-for-like swap, and the guide's figures cannot see your roof, so use them only to get your bearings. Check that VAT and scaffolding are in any quote you receive. Our written price is one total, given after the survey.

For a brand new opening in Crawley, Crawley Building Control asks to approve altered roof openings, and we check with that team first. In other areas, your own building control body can tell you. For the wider picture, see our roof windows and skylights page.