The short answer
Only if the opening is wider than the gap between rafters, or sits across one. A window narrow enough to sit between two rafters needs nothing cut. A wider one means a rafter is cut and its load carried by new timbers around the opening, a structural alteration that needs approval under the Building Regulations. The work is planned before any slate or tile is lifted.
What a rafter does and why cutting one is not trivial
Rafters are the inclined timbers between the ridge and the wall plate, the timber bedded along the top of the wall at the eaves. They carry battens, covering, snow and wind load into the walls, each sharing the burden with its neighbours.
Cutting through a rafter removes one of those load paths. Its share of the weight lands on the rafters either side, and a window frame cannot carry it, so the structure is rebuilt around the opening before the frame goes in. Before we agree a position with you, we check spacing, timber size, age and span from the loft.
The opposite mistake is just as common: assuming the existing hole can be enlarged. Where an old roof light, a hatch or a chimney opening is already framed, the timbers around it were built for that size only. Widening it by even a modest amount puts a different load on the same members, so the frame has to be assessed again as if it were a fresh opening.
Older roofs add complications: uneven spacing, purlins and struts carrying part of the load, timbers that have moved. Stone slate roofs are heavy, about 1,000 kg per 10 sq m in Cotswold District Council's 2000 guidance, and heavier loads call for more care.
When no cutting is needed
Roof windows come in widths that suit common rafter spacings, and the usual way to avoid cutting is to choose one that sits between two rafters with room for the frame and flashing. It is fixed to the timbers either side, or to short cross timbers called noggins, and the rafters stay whole.
That is a lighter, quicker job, and awkward spacing may favour a narrower model over a wider one needing a trimmer. Our answer on the difference between a roof window and a skylight sets out the products, and the sizes are on the maker's datasheet. Even with nothing structural touched, no batten should be left unsupported once the covering is cut back.
Measure before you buy. Rafter spacing in a loft can be checked with a tape from one timber face to the next, and the clear gap, not the centre to centre distance, is what the window frame has to fit within. A few centimetres of difference decides whether the job is simple or structural, so we confirm it on site rather than from a drawing.
How a trimmed opening is built
Where a wide opening cuts across a rafter, two new cross timbers called trimmers are fixed above and below it, spanning between full-length rafters on either side, usually doubled up to take the extra load. The severed rafter is hung off the trimmers, which leaves a framed opening that carries its own weight.
Timber sizes and fixings
The new timbers must be at least as strong as the rafters they replace, joined with proper hangers. An undersized trimmer can sag and pull the frame out of square, and a damp stain beside the flashing is usually the first symptom.
Why the order matters
The structure is formed and the window and flashings fitted before the opening is left exposed, with a temporary sheet over any gap. Dry spells make the sequence easier to manage; our guide on fitting roof windows in a Stroud winter covers the timing.
Building control, structure and the 25 per cent rule
The Planning Portal lists structural alterations among the cases that need Building Regulations approval, and cutting a rafter is one. That sits apart from the re-roofing thresholds of 25 per cent of the building envelope and 50 per cent of the roof: a window needing a trimmer is caught however small its area.
Heat loss is covered separately. Part L sets roof windows at 1.4 W/m²K, with WER Band B as the alternative, and our guide on Building Regulations for roof windows, Part L and structure shows how the rules fit together, and our skylights page lists the work we do.
Your council's building control (in Stroud District, the Gloucestershire Building Control Partnership) or a registered building control approver can check the work. They will want to know what is cut and what replaces it. On our side, we tell you which consents apply and send the Building Regulations notification when the job needs it. A listed home is different again, so speak to the planning team before a single timber is cut.
What we check on the survey and what it costs
On the survey we start in the loft: rafter size, spacing and condition, purlins or collar ties in the way, earlier alterations, and any rot or beetle near the proposed opening. If a rafter is damaged, moving the window may beat rebuilding around poor timber. You get our photographs, so you can see the same timbers we are looking at, and any concern is explained in plain terms before it appears in a quote.
MyBuilder's 2026 guide puts labour and fitting for a standard roof window at about £350 to £600 with scaffold near £500 where needed; structural work around an opening comes on top, and those figures are national ranges and do not set our own prices. Our written quote after the survey covers scaffold, strip, structure, window, flashings, making good and waste.
Flat roofs raise the equivalent question in rooflight upstands and kerbs. Once the opening is framed, glazed and flashed, the work is covered by a written 10-year workmanship guarantee. To have the structure looked at, ring 01453 933613 or send loft photographs through the enquiry form.