Question answered
What size roof window do I need?
The short answer
Choose a roof window by the clear gap between two rafters, the angled beams that hold up the tiles or slates. Makers sell a range of standard widths and heights, and the right size is the biggest one that fits that gap with the clearance the fitting instructions ask for, at a sill height that suits the room. Check the maker's current size chart, then let a survey confirm it against your actual timbers.
A roof window is chosen by the gap it has to sit in, not by the room beneath it. The rafters decide the width, the room decides the height, and a survey checks both against the real timbers.
The rafters decide how wide the window can be
Behind the tiles or slates, a roof is carried by rafters, usually set at regular spacing along the slope. A roof window is framed into the gap between them. The neat job is a window that fits that gap with a little room either side, because nothing structural then has to be cut.
Spacing is not always tidy. Older roofs can have rafters that wander, and a house that has been re-roofed or altered may have gaps of several different widths. We measure the clear gap inside the loft, between the faces of the timbers, rather than trusting a figure from a plan or from the house next door.
When the best window is wider than the gap, a rafter has to be cut and the load carried round the opening by trimmers, which are short cross timbers fixed either side. Cutting a rafter counts as structural work, which the Planning Portal puts on its list of changes needing a building control application. It is quite doable, but it changes the job, so size is settled at survey stage.
Height, sill and the view from the room
The height of a roof window matters as much as its width, and it is set by where you want the bottom edge. A sill that is too low becomes a place to bang a knee, while one that is too high gives a view of nothing but sky when you are sitting down.
In a loft bedroom, the window may double as the escape route if there is a fire. To qualify, Approved Document B wants an opening of at least 0.33 m2, no narrower or shorter than 450 mm, with its lower edge at most 1100 mm off the floor. Plenty of standard sizes manage that, but not at every position on the slope, which is why the sill height and the size are chosen together. There is more in our answer on escape roof windows.
Roof slope matters here too. On a shallow pitch, a tall window needs a longer cut through the covering, and the head of the opening can end up near the ridge.
Reading a maker's size chart
Makers publish their sizes as a grid of widths and heights in millimetres, often shortened into a code. Those figures describe the outer frame. The glass you see from inside is smaller, because the frame, the sash and the lining all take a share of the opening. A window that looks generous on paper can feel modest once it is in.
Charts also change. Ranges are updated, some sizes are dropped and new ones are added, so a chart you saved a few years ago may not match what a supplier can order now. Always check the current chart for the model you want, and check whether the size is offered in that model, since not every size comes in every type.
It also pays to ask whether the flashing kit (the metal surround that weatherproofs the frame against the covering) is sold for your size and your roof covering. A tile kit and a slate kit are different products, and our page on roof window flashing kits explains why.
Matching an old window or choosing a new size
If you are swapping a window for the same position, the simplest route is a maker's size that matches the old frame. The existing opening is already framed, the covering is already cut, and the work stays small. Details of the swap are on our page about replacing an old roof window.
Old windows do not always have a modern equivalent in the same dimensions, and the nearest listed size may be a little smaller or a little larger. A smaller one leaves a gap to be filled, and the lining and flashing can be made to cover it. A larger one may need the opening enlarging, with the rafter and trimmer question above.
If the window is to be electric, the size can also affect which motor and which supply you need. We cover that in our answer on electric roof windows.
What we measure on the survey
Our survey records the gaps between rafters, the pitch, the covering, the insulation and the room below, then works out what truly fits. Pitch is worth checking early, because every window has a lowest angle it can be used at; our answer on the minimum pitch for a roof window covers that.
Photographs of the timbers and slope you actually have sit alongside the written quote. The size named there is the one we measured for, and the length of the job follows from it; see how long it takes to fit a roof window. Our wider skylights and roof windows page covers the whole range of work.
Sizing a roof window: two more questions
Is a wider roof window always better than a taller one?
Not always. A wide window spreads light across a room, while a tall one gives a longer view up the slope and suits a steeper roof. Width is limited by the rafters, so extra height is often the cheaper way to gain glass without cutting timber.
Can I fit two small windows instead of one large one?
Yes, and it can suit a long room. Two windows side by side each need their own gap or a shared trimmed opening, so the saving is not automatic. Compare the layouts at survey and judge them by light, view and what the timbers allow.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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