Roof window & skylight installation

A roof window is a hole in a working roof, so all of the risk lives in the opening and the weathering rather than in the unit. A decent window from any of the major manufacturers, fitted into a properly trimmed opening with the correct flashing kit, is as watertight as the roof around it and will stay that way for decades.

The structural part comes first. A new opening almost always means cutting at least one rafter, and a cut rafter has to be trimmed — new timbers across the head and the sill of the opening, and the cut rafter's load transferred into the rafters either side. For a single standard-width window that is routine carpentry. For a wider opening, or a larger unit, or several units in a row, it can need proper structural design and Building Control involvement.

Then the flashing kit, which is the part people economise on and should not. Every manufacturer supplies kits matched to the covering — one for slate and thin flat coverings, another for profiled tiles, another for very deep profiles, and a separate one for combining units side by side. Using the wrong kit, or dressing lead over the top because the right kit was not ordered, is the single commonest reason a roof window leaks. It usually shows at the top corners, usually within two winters.

Position matters more than people expect. Too low and you get a window you look at rather than out of; too high and the room does not get the light on the floor. There are also practical constraints — the opening has to sit between structural members without cutting more than it should, it needs to clear the ridge and the eaves by sensible distances, and on a room-in-roof the head height decides where it can sensibly go.

Ventilation and condensation get overlooked and cause a lot of the calls that follow. A roof window is a cold surface, so warm humid air in the room condenses on the glass and runs onto the sill — which looks exactly like a leak. Most units have a background ventilation flap; using it, and having reasonable ventilation in the room, prevents the problem. On a bathroom or a kitchen it is not optional.

Planning is usually straightforward but not always. Roof windows on a rear or side slope are typically permitted development, subject to limits on how far they project and on obscure glazing where they overlook a boundary. In a conservation area, or on a listed building, or where an Article 4 direction applies — and Macclesfield has those at Christ Church and High Street — a window on a front elevation may need permission. Worth checking before ordering.