The short answer
A roof window fits between rafters without cutting any of them only if it is narrower than the clear gap between two neighbouring rafters. With rafters at 600mm centres, that gap is a little over 550mm once the timber thickness is taken off, which is why the narrowest standard sizes are made to fit. Anything wider means cutting at least one rafter and framing the opening with trimmers, and that is a structural alteration.
So the useful question is not how wide a window can go, but whether the width you want sits within a rafter bay or spans several. The first is simple. The second is perfectly normal, but it needs proper framing and Building Control.
Reading the gap between rafters
Rafters are described by their centres, the distance from the middle of one to the middle of the next. Modern roofs commonly use 400mm, 450mm or 600mm. The clear gap is the centres minus the width of the timber, which is often between 38mm and 50mm. So a roof at 600mm centres has a clear bay of roughly 550 to 560mm, and one at 400mm centres about 350mm.
Older roofs are less predictable. Victorian and Georgian rafters can be hand-sawn, irregular, wider spaced, or twisted, and in cob cottages across the Culm and Exe valleys the roof may be a mix of timbers added across several centuries. Always measure the actual gap on the day, in the loft, not from a drawing.
How window sizes relate to the bay
Roof windows are sold in a range of widths, usually coded by a letter, with C the narrowest at about 55cm and M at about 78cm. The narrow sizes are designed so that at standard rafter centres the frame sits between rafters and needs no structural cutting. A wider window, such as an M size, is wider than a standard bay, so the rafter beside it has to be cut and the load carried round the opening.
Frame width is not the same as the width of the gap you need. The frame, flashing and a little tolerance take up space, and the manufacturer's instructions give the rough opening required. When a window is chosen to sit in a bay, we confirm that the frame and flashing fit the measured gap.
What happens when you go wider
A window that spans more than one bay needs a framed opening. The rafters that cross it are cut and fixed to a header above and below the opening, and a doubled rafter, or trimmer, runs each side to carry the load. Those timbers are sized to the span and the roof loads, and the work follows the guidance in the Building Regulations for structural alterations.
If the opening interrupts a purlin or a hip or valley, or if the rafters are part of manufactured trusses, the situation changes. Trusses are engineered as a system, and cutting a member without a design can weaken the whole roof. In that case a structural engineer's input is needed before any timber is cut. It is also a reason to be careful about a quote that does not mention how the opening will be framed.
Checking the loft before you choose
A few minutes in the loft saves a lot of guesswork. Look at the rafters from below: are they evenly spaced, or do they bunch near a chimney or a hip? Note any purlin crossing the slope, any wiring or pipework running through the roof space, and how much insulation sits between the timbers. A roof that was insulated years ago may have boarding or foil fixed across the rafters, which has to be removed and replaced around the opening.
Photographs taken from the loft hatch, with a tape measure laid across one bay for scale, let us give a better first view before the survey. They do not replace it, because the survey is where the real measurements are taken.
Several windows side by side
Two or more windows in a row are often chosen for a loft room. If they sit in adjacent bays, the rafter between them can stay, which keeps the structure simple and leaves a narrow strip of roof covering between the frames. If they are set close together, the gap can be too narrow to weather properly, so the maker specifies a minimum distance, or a combination kit, to join them. Our guide to roof windows for loft conversions in Devon homes looks at the layouts that work.
What it means for the finish inside
The framing is hidden once the job is done, but it decides what the room looks like. A window cut in a single bay has a narrow lining with little room to splay, and the sides are often just straight returns to the rafters. A wider framed opening gives a larger ceiling opening, and a splay becomes possible if the structure beside it has room.
The lining and its angles depend on the rafter positions, so we set them out together, and describe them in the finish schedule, with the ceiling build and the way the lining is formed.
Planning and Building Regulations
Under the Class C permitted development right in the GPDO, a house roof window is normally allowed if it stands out no more than 150mm and stays below the ridge line. Listed buildings, conservation areas with an Article 4 direction, and flats all follow different rules, so ask your council's planning department first. Trimming a rafter alters the structure, and the new window has to satisfy Part L; that makes it building work, and Building Control is notified by us.
What to do next
When you ask for a quote, tell us the width of window you have in mind and, if you can, photograph the underside of the roof, showing the rafters. We will measure the real gap at survey and say whether the window fits a bay or needs framing. For the products, see top-hung or centre-pivot roof windows, and for the whole job, roof window installation.
The option that looks wide on paper is often easy on site, and a small one is not always as simple as it sounds. Ask for a quote and we will check the rafters before we promise either.