The short answer
A rafter can be cut for a roof window only if the load it carried is picked up by new timbers around the opening. Those timbers are called trimmers and a header. A narrow window between two rafters needs little cutting, while a wide one needs a structural design. Ask building control, and an engineer where the span is large, before cutting.
How an opening is trimmed
Rafters are the sloping timbers that run from the ridge to the wall and carry the battens, the covering and the weather loads. When a window opening interrupts a rafter, the cut length has nothing to rest on at one end. The answer is to build a frame around the opening that takes the weight and sends it sideways to the rafters either side.
The frame has two parts. A header is a horizontal timber fixed across the top of the opening, and a trimmer is a matching one across the bottom. The cut rafter ends are fixed to these, and the load follows the header and trimmer out to the full-length rafters on each side. The joints are made with metal hangers or screws sized for the load, not with a few nails.
The timbers beside the opening often need doubling, because they now carry what two rafters carried before. That is why a window that sits neatly between two existing rafters is so much easier than one that straddles them, since it needs nothing cut at all.
The rule is simple: no rafter is cut until the replacement path for its load is planned. We measure the rafter size and spacing in the loft at the survey and photograph the timber, so the frame is designed for what is actually there.
Does it need building control
Altering the structure of a roof is building work, and building control can take an interest. The Planning Portal gives a re-covering test of under 25 per cent of the building envelope and under 50 per cent of the roof, but that test is about changing the covering, not about cutting timber. A structural opening is a separate question.
Put the question to your building control body before any cutting starts. For Farnborough, Aldershot and Hart addresses that is the Hart and Rushmoor Building Control Partnership, on 01252 398715. Other councils run their own services, and some, such as Wokingham, use a shared one. Registered building control approvers also operate under the regime in force since 6 April 2024.
Roof windows are not counted as thermal elements under the Building Regulations 2010, but they still have to meet the energy standard for windows, and the structure around them has to be sound. If the window serves a loft conversion, Approved Document B on fire escape and stairs comes into play as well.
We tell you which route we expect, and who is making the application, in the written scope. If an engineer is needed, that is stated before you commit.
Wider windows and what changes
Width is what makes a trimmed opening harder. A window that fits between two rafters, which are commonly spaced at a regular interval, needs only a header and a trimmer fixed to them. A window wider than the gap means cutting at least one full rafter, and a very wide window or a pair side by side cuts several.
Each extra cut rafter increases the load the header has to carry. The header then needs to be a deeper or doubled timber, and the rafters at each side take more weight, which can mean strengthening them. At some point the arrangement stops being a carpenter's detail and needs calculation, and that is where an engineer's input earns its place.
The size of the rafters matters too. Small or old rafters, or ones that are already notched, have less spare capacity. A purlin, the beam that supports the rafters mid-slope, may also sit in the way, and moving or cutting it is major structural work.
Our answer on whether a roof window fits any roof deals with pitch and covering limits, and the one on a dormer or a roof window covers when a bigger structural change is the better route.
Doing it with the roof open to the weather
Cutting a rafter means the roof is open, so the order of work matters. We prepare the frame timbers before the opening is cut, mark the position from inside and out, remove the covering around it, then cut and fix the frame in one visit. The roof is never left with a hole and no cover overnight.
Scaffold or a roof ladder is used for access, and for a window high on a slope or over a pavement a scaffold may be needed. A fit from inside is possible only where the work can be done safely, which our page on fitting a roof window from inside explains.
The covering and flashing go back around the new window, and the lining and insulation are finished inside. A poor finish at this stage causes cold bridges and later leaks, so the quality of the trimming and the insulation collar both matter.
You see photos of the timbers and the frame before the lining goes over them. A single written total follows, with the work named in the scope. See roof windows and skylights for the rest of the service.
