The short answer
Not always. A roof window that fits between two rafters needs only short trimmers across the top and bottom, and the rafters stay whole. A wider window means cutting one or more rafters and carrying their load round the opening on doubled timbers. That is a structural alteration, so it needs advice from a structural engineer or surveyor and a word with your building control body.
When the rafters stay whole
Rafters are the sloping timbers of the roof frame, and they rest on the wall plate, the horizontal timber along the top of the wall. They carry the weight of the battens, covering and snow, and pass it to the wall. When a roof window is narrower than the gap between two rafters, the opening is framed without cutting either.
Short horizontal timbers called trimmers go in across the gap, one above the window and one below, and they are fixed to the rafters either side. The rafters themselves are untouched, so the load path stays as it was. Only the battens over the opening are cut, and the window's frame is fixed to the new timbers.
This is the simplest and least costly installation, and it is the reason roof windows are sold in widths made to suit common rafter gaps. On an older roof the gaps may be uneven, or a rafter may sit where you want the glass. So the first step is to measure the real spacing from inside the loft, rather than assume a standard one. Our answer on roof window sizes explains how the width is chosen.
When a rafter has to be cut
Should the window be broader than the space, or the best spot land across a timber, then at least one rafter has to be cut. The cut ends no longer reach the ridge or wall plate, so the load they carried has to go somewhere else. The usual answer is a pair of doubled rafters at either side of the opening, with the trimmers fixed between them, and the cut rafter ends are hung from those trimmers.
The size of those new timbers, their fixings and the span they cover are matters of design, not guesswork. A roof that carries heavy tiles, a long span or a purlin (a horizontal timber that supports the rafters mid-slope) may need larger sections than a light one.
The weight of the covering matters in the other direction too. The Planning Portal suggests consulting a structural engineer or surveyor before anything alters a roof's weight, and cutting into the frame is the same kind of question. We photograph the timbers in the loft and send them to you, so you can see what the plan involves.
Moving the window sideways can avoid the cut altogether.
Structural advice and the people who sign it off
A cut rafter changes how the roof carries load, so the sensible step is advice from a structural engineer or surveyor. They can calculate the doubled timbers, the fixings and any support needed, and give a drawing that building control can read. For a plain single window in a standard roof, the advice can be short. For a wide opening, a pair of windows or a roof with a purlin, it is worth more.
Part A of the Building Regulations deals with structure, and your building control body decides what it needs to see for your address. We ask it, rather than assuming, and pass its answer on to you before the work begins. Our answer on roof windows and Building Regulations covers the wider picture, including energy and escape.
The Party Wall etc. Act 1996 has a right to cut into a party wall for a chimney breast or flue, but it does not make rafters a party wall matter. If your timbers rest on a wall you share with next door, ask a party wall surveyor whether notice applies. Do not rely on building control to answer it, because it is a separate regime.
What happens on the day
Work starts from the outside, with the covering stripped back around the opening and the battens cut. If rafters are to be cut, temporary props or a strongback go in from inside first, so the roof stays supported while the new timbers go in. The doubled rafters and trimmers are fixed, the cut ends are connected, and only then is the opening finished and the frame fitted.
The underlay is dressed into a collar around the frame, the flashing set goes on in the order the maker specifies, and the tiles or slates are brought back up to it. Our answer on the roof window flashing kit shows what comes in the box.
The opening should be weathertight at the end of each day, and we tell you what to expect on each visit. A fixed lining inside the loft can be left to a plasterer or carpenter. Our roof windows and skylights page covers the whole job, and the survey tells you whether your window needs a rafter cut or fits between.
