Short answer
Yes, often, but only with the load carried properly, and fitting between the rafters comes first. A narrow unit can sit in the gap between two rafters with no cutting at all. A wider unit may need one rafter shortened, with a doubled rafter beside it and a trimmer above and below the opening. A trussed roof is different: its members must not be cut without an engineer's design.
Start by fitting between the rafters
The first question on a survey is whether the roof window can go in without cutting anything. Rafters in older roofs are commonly set at about 600mm centres, which leaves a clear gap of roughly 550mm after allowing for the timber's thickness. That is the width a narrow unit has to fit within.
The VELUX size code tells you the width at a glance: the letter is the width and the digits are the height. A narrow code such as CK04 is designed with exactly this gap in mind. A wider letter, such as the M of an MK04, is wider than the usual gap between rafters, so it asks for more from the roof structure.
We do not trust the nominal spacing. Old roofs wander, so on the survey we measure the real gaps with a tape in the loft, at the position where the window will sit. Our VELUX size guide sets out the codes if you want to check the width of a unit yourself.
When trimming a rafter is acceptable
Trimming a rafter is acceptable when the load it carried is picked up by something else. The rafter is never simply sawn through. The opening is framed first, so the roof still bears on solid timber all the way round the window, and the cut rafter ends rest on a new member rather than hanging in the air.
In practice this means a wider window, a taller one, or a pair of windows set side by side in the same bay. Each may need one or two rafters shortened. How many depends on the size of the opening against the spacing, and the answer goes in the written job plan before the unit is ordered.
Trimming is a structural alteration, so the regulations apply. They are set out in the rooflight Building Regulations guide.
Cutting rafters for a roof window: how the opening is framed
Three pieces of timber do the work. One trimmer runs across the head of the opening and another along the sill. A doubled rafter, which is a second rafter fixed alongside the original, runs up each side. Together they make a box, and the cut rafter ends are fixed into the trimmers.
The doubled rafters take the extra load that has been moved sideways, so they are bolted or nailed to each other in line with the timber sizes set out on the plan. The sizes and fixings follow the span and the roof covering, which is why a slate roof and a lightweight roof are not treated the same.
This is also where the unit's frame meets the structure. The roof window sits in the framed opening, with the flashing laid over the tile or slate battens, and the lining inside is fixed to the same timbers. A neat opening makes for a straight, tidy finish inside the room.
- Trimmer across the head of the opening
- Trimmer across the sill of the opening
- Doubled rafter on each side
- Cut rafter ends fixed into the trimmers
- Timber sizes set out in the job plan
Trussed roofs: why an engineer comes first
Many houses built from the 1960s onwards have a trussed roof, factory-made triangles of slender timbers joined with metal plates. In a trussed roof, the web members and the chords work together as a single structure. Cutting one weakens the whole truss, and the roof can no longer be relied on to carry its load.
For that reason we never cut trusses on site on our own judgment. If the survey shows a trussed roof, the next step is a structural engineer, who designs the support for the opening, often a steel or timber beam spanning between trusses, and gives sizes for the strengthening. That design is part of the job and is attached to the quote.
It can feel like a bigger job, and sometimes it is. But a unit that fits between trusses at their usual spacing needs none of it, which is another reason the fit-between-first check matters. A narrow unit may avoid the engineer altogether.
| Roof structure | Approach |
|---|---|
| Cut roof, unit fits between rafters | No cutting needed |
| Cut roof, unit wider than the gap | Trim a rafter, double the sides, add trimmers |
| Trussed roof, unit fits between trusses | No cutting; check spacing on the survey |
| Trussed roof, unit wider than the gap | Engineer's design before any cutting |
Recording the structure for Building Control
Opening up an existing roof for a window, including any rafter trimming, is building work. Where it is notifiable we send the Building Control notice ourselves, and the structure is part of what it describes. The framed opening, the timber sizes and any engineer's calculation are what Building Control looks at.
The record starts with the survey. The size row of the written job plan lists the rafter spacing we measured and what the opening will need. If an engineer has been involved, their design is referred to on the same page. When the roof window has been fitted, the certificate from Building Control closes the file and you keep it with the house papers.
What happens before the unit is ordered
The sequence runs in a set order so nothing is found out on the roof. Survey first, measuring the rafter gaps and noting whether the roof is cut or trussed. Then the job plan, with the unit and the structural approach. Then the order, and the fitting date agreed when it is placed.
If you are weighing up a bigger window than your rafters allow, tell us at the survey and we will price both routes. The whole job is described under roof window installation, and fitting a VELUX from inside explains how the opening is made once the framing is settled.