Can You Fit a Roof Window in a Slate Roof?

Why slate is different from tile

Plain and interlocking tiles are thick and heavy, and a flashing kit for them has deep profiled sections that sit over the tile shape. Slate is thin, flat and brittle, and the covering is built up in overlapping courses, each slate held by nails or hooks. The flashing for a slate roof therefore has to be a flat, low-profile design that slides under the slate courses on the sides and above, and laps over them at the bottom. A kit made for a different covering will not seal properly, so the correct slate kit for your make of window is the first thing to get right.

Natural slate also cracks if it is walked on carelessly or if a nail is driven badly. When the slates around an opening are lifted, a proportion of old ones will not survive, so a few spares are needed. Reclaimed or matching slates keep the finish uniform, and our page on mixing old and new slates explains how that is handled.

How the work is done

The first step is marking out the opening between the rafters, then stripping the slates in the area and storing them in order. The battens (the thin timber strips the slates hang from) are cut back, the rafters are trimmed to form the opening and a timber header is fixed across the top and bottom. Often this is done from the loft side while the roof covering is protected from rain.

The window frame is then fixed to the rafters, and the underlay is cut and dressed up to the frame so water that gets under the slates still has a path out. The flashing kit goes on next, built from the bottom upwards so each piece overlaps the one below. The side gutters and the head apron sit under the slates, and the slates are relaid on top, each one cut where needed to meet the kit neatly. Finally the interior is lined and finished.

Planning and building rules to check

Fitting a roof window to the back or side slope of a house is often permitted development, but there are limits on how far it projects, and the rules are stricter for flats, listed buildings, conservation areas and Article 4 areas. Slate roofs are common on older homes, so these restrictions come up often. Read our guide on planning permission for a skylight before you order anything. If the house is listed, raise it with the conservation team at the council before ordering, since changes to the roof's look may need consent.

Building Regulations also apply to the window itself, including its energy performance and, if it serves a loft room, escape and structure. The page on roof window building regulations covers it.

Choosing a window for a period roof

On an older house, the look of the roof matters as much as the technical fit. A conservation-style rooflight, which is a slim unit that sits flush with the slates and has a central glazing bar, blends in better than a standard unit with a thick frame. Our page on conservation rooflights compares them. Position also matters: aligning the window with the rhythm of the courses and with the rafter spacing gives a neater result and means less cutting of slates.

What can go wrong on a slate roof

The common faults are all to do with the junction. A kit pushed in too far under the slates can leave the head apron too short to carry water over the top course. Side gutters that are not dressed down fully allow water to creep sideways beneath the slate edges. Nails driven through the flashing instead of beside it make slow leaks that show up as stains months later. Cut slates left with a thin, sharp edge can chip over winter and let wind-driven rain through. Good slate work avoids these by taking time over each course round the opening, and by testing the finished junction with a hose before the scaffold or ladders come down.

We survey the roof first, photograph the slates, rafters and any weak spots, and tell you plainly if the slates are tired enough that fitting a window would be a good moment to carry out repairs. You then receive a fixed written quote. For more on the full range of window work, see roof windows in Basingstoke.