Can a roof window be fitted in a slate roof?

A roof window lies level with the slope, between two rafters, and the roofer works from the top. The slates over the opening come off first, and a rafter is trimmed if the window is wider than the gap. A flashing kit made for the window and the slate profile then goes in: a lower apron, side pieces and a top gutter, each layered with the slates so water runs over every joint, never into it.

The slates round the opening have to be cut to the flashing, and any slate that is cracked or brittle can snap as its nails are drawn. Spare slates are therefore worth having to hand, and matched ones are better still. Our page on Velux flashing kits describes the pieces in more detail, and the skylights and roof windows hub lists everything else we do with them.

A slate roof can be very long-lived, but the slates next to the opening have to be lifted and hung again, and age is the problem. A slate that has begun to flake or split will not survive being handled. If many are failing, the straight advice is that cutting a window into that area is the wrong moment, and that the slating wants repair or renewal first. We photograph the slates around the proposed position so you can see exactly what we saw from the ladder. A position where the slates underneath are sound saves cutting and gives the flashing a firmer bed.

It is also the time to look at fixings and underlay, because a window changes where water goes in a storm. Slipped slates around an opening are a common weak point. If the roof shows the symptoms in our guide to signs a slate roof is failing, we recommend dealing with those first, because a new window on a tired roof leaves you paying for access twice.

Pitch matters to slate before any window arrives. Cupa Pizarras takes BS 5534:2014+A2:2018 to mean that a 500 mm natural slate on a moderately exposed site wants at least 20 degrees, with hook fixings kept to slopes of 25 degrees or more. A roof window does not alter those limits, but a very shallow slate roof is a poor candidate for any opening, because the flashing relies on the lap of the slates to shed water.

The headlap, which is the cover a slate gets from the course two rows higher up, also governs how far slates can be raised at the window sides. A roof lapped generously gives more room to work. Check the data sheet for the window you choose, because manufacturers publish the range of pitches and coverings each kit suits, and confirm it against a measured pitch, never a guess made from the pavement. Dormer cheeks and hips need extra thought, since the slope there is not a simple plane.

Many houses can take roof windows without a planning application, as permitted development, but the conditions are tight. The window must not stand more than 0.15 metres proud of the original roof plane, and it must not climb higher than the top of the existing roof. In a side slope the glass must be obscure, and fixed shut unless the opening part is more than 1.7 metres above the floor. Flats do not have these rights.

A listed building needs listed building consent. Many slate roofs sit in conservation areas, some with local controls, so ask the council before ordering. Our guide to roof windows in a conservation area covers the extra care, and our skylight planning permission guide goes through the rules in order.

On the Building Regulations side, a new roof window must achieve 1.4 on U-value, or Band B under the Window Energy Rating scheme, according to Approved Document L 2021, Table 4.2, until 24 March 2027, and Table 3.2 of the 2026 edition after that date. Your building control body confirms how this applies.

A single written price follows the survey. It depends on the size and number of windows, the flashing kit for slate, how many slates need replacing, whether rafters must be trimmed, whether the room below is ready for the window, and the way in. A higher roof may need a scaffold tower or full scaffold, and scaffold over a public footway needs the highway authority's licence. Ask whether VAT and scaffolding sit inside any estimate before you weigh one against another.

For a rough sense of cost, see our Velux installation cost guide. Afterwards we leave the slating tidy round the window, and the old slate offcuts and packaging go away with us. Where the old slates are reused, we keep the best ones back for the cuts around the frame, so the finished edge looks square from the street.