What size roof window do I need?

Rafters are the angled timbers under the slates or tiles, spanning between ridge and gutter line. A roof window is made to drop into the gap between two of them, so the first measurement is the clear distance between the rafters in the slope you have in mind. If the unit is narrower than that gap, the frame can be fixed to short timber pieces called trimmers without touching the main structure. If it is wider, one or more rafters must be cut and the load carried round the opening by new, doubled timbers.

That second case is perfectly normal but changes the job. It means a structural check, more labour and often a different quote. Cutting a rafter without a plan for the load can leave the slope sagging, with the covering opening up above it. Our survey measures the spacing from the loft side, and the quote spells out whether the window is a straight fit or involves trimming.

Reading the room from inside

Stand under the proposed spot. A window set high in the slope lets in more sky and less view; one set lower gives a view out when you are standing. A good compromise is to think about where your eyes will be in that room most of the day, whether sitting up in bed, at a desk or on the stairs.

Manufacturers build each model in a set of standard sizes, usually named by a code for width and height. You pick from the list, so the question becomes which of those sizes suits the slope. A tall, narrow unit fits between rafters more easily and brings light deep into a long room. A wider unit spreads light across the floor but needs more of the slope clear of obstruction.

Opening style matters as much as size. A centre-pivot model rotates about a bar across its middle, so you can clean the outer face from inside while the lower half swings out. A top-hung model opens like a skylight lid, giving a clearer view if the sill is lower. In a steep slope a centre-pivot is comfortable to reach; in a shallow one a top-hung opening may be easier on the head. MyBuilder's 2026 guide prices a basic centre-pivot unit at £305 and fitting at £350 to £600 on standard jobs, so the size you choose affects the quote less than the access or the trimming does.

Bigger is not always better. A large pane in a south-facing slope can overheat the room, a point we take up where we discuss overheating under glass. Blinds and glazing options help, but sometimes a smaller unit or a different position is the neater answer.

A window in a bedroom or loft room may have to double as the way out in a fire. For a home with only one stair, Approved Document B asks that the openable area is at least 0.33 m², with a clear height of at least 450 mm and a clear width of at least 450 mm. The lowest edge of that opening may sit no higher than 1100 mm off the floor, and the sash has to stay open unaided. Those are minimum figures, and one dimension can be larger than the other as long as the area is met.

These numbers describe the opening, not the glass. A roof window with a thick frame or a restricted hinge may offer less clear space than its catalogue size suggests, so check the opening dimensions in the data sheet. The 1100 mm rule also means a sill higher on the slope may fail even if the window itself is big enough.

Escape is only one use for a loft. The rules treat a roof window as the way out only where the floor sits within 4.5 m of the ground. Our guide to roof windows for Stroud loft conversions covers that side in full, so we leave the stair and protected route rules there.

Under the General Permitted Development Order, Class C covers roof windows. The order rules out a unit that sticks out more than 0.15 metres from the slope's plane, or that rises above the top of the original roof. A tall frame set near the ridge can therefore run into the second limit. Class C does not carry the same restriction as dormers on conservation areas or the Cotswolds National Landscape, although an Article 4 direction, a planning condition or a listed building can remove the right. If your house is in any of those categories, check with Stroud District Council's planning team first.

Slopes on a side elevation bring a further condition, covered in our answer on obscure glass for side-facing windows, and it can push the opening higher up the slope.

A tidy way to decide is to note down four things in order.

1. The clear rafter gap at the spot you want, measured from the loft side.
2. Whether the window needs to be an escape window.
3. How much of the roof slope is free of chimneys, valleys and other windows.
4. The direction the slope faces, which governs sun and glare.

With those four, the shortlist usually narrows to two or three catalogue sizes. Allow for the flashing kit, the metal surround that seals the frame to the covering. It extends beyond the window itself and needs clear space around it, so the roof area you reserve must be larger than the glass. See our guide to flat roof skylights that leak for what goes wrong where that detail is rushed.

A flashing kit is made for the covering, whether plain tile, slate or stone, so say which covering you have. Like for like applies: stone roofs get stone-compatible detailing. The quote that follows our survey is backed by photographs of your own roof, and finished work carries our 10-year workmanship guarantee in writing. A cheaper option such as a smaller unit is worth asking about, and our answer on whether a leaking skylight is covered explains why a well-fitted unit matters to your insurer. The full range of options is on our roof windows and skylights page.