Yes. Every major manufacturer works to a short list of standard frame sizes, and the list is far shorter than people expect: roughly six widths and five or six heights, which gives you around thirty combinations in the mainstream range. Those sizes are not arbitrary. They are set by the spacing of the rafters they have to fit between, which is why the widths are almost identical from brand to brand even though the code printed on each one is different. Bespoke sizes exist. They are slow, dear, and needed far less often than they are asked for.
A short grid, and why the whole trade uses it
The mainstream widths run at 550, 660, 780, 940, 1140 and 1340mm. The heights run at 780, 980, 1180, 1400 and 1600mm. That is the working range for a pitched roof window in this country, and the sizes are outer frame dimensions, not glass and not the hole in the ceiling.
Standardisation is not laziness. It means the flashing kits, the insulation collars, the underfelt collars, the internal linings, the blinds and the shutters are all made to the same grid, so every accessory has a defined counterpart. The moment a frame leaves that grid, every one of those items has to be made specially too, and that is where the cost actually accumulates.
Why the widths sit where they do
Rafters in a British pitched roof are usually at 400mm or 600mm centres, and a roof window fits between them, with the trimmers taking the interrupted load. A 550mm frame slips between rafters at 600 centres with a workable gap either side and no rafter cut at all. A 780mm frame needs one rafter removed and trimmed. A 1140 or 1340 frame needs two.
That is the entire logic. Each width step corresponds to a different amount of structural work, and the price of the installation steps with it rather than with the glass area. It is also why the jump from 780 to 940 costs more than the extra 160mm of frame suggests.
Heights answer a different question entirely
The height is not constrained by structure in the same way, because the frame runs with the rafters rather than across them. What limits it is the roof itself: the distance from the wall plate to the ridge along the slope, less the space you need above the plate and below the ridge for the flashing and the covering to work.
The other thing height buys is view. A taller unit set with its bottom edge low enough gives a seated view out; a short unit high up gives sky only. On a shallow pitch you need more height to get the same view, because the glass is closer to horizontal. That trade is set out on roof window cill height and the view out.
The millimetres agree, the codes do not
A 780 by 1180mm frame is a 780 by 1180mm frame whether it comes from VELUX, Fakro, Keylite or Roto. What differs is the reference each brand prints on it, and the sizes are close but not always identical to the last millimetre, which matters when you are dropping a new unit into an existing opening.
The codes themselves, what the letter means and what the number means, are covered on roof window sizes and how they are coded. For the purpose of this question, the useful point is that a size code is not a universal language. Quoting one brand’s code at another brand’s supplier gets you a shrug, so give millimetres.
Standard against made to order
| Standard size | Made to order | |
|---|---|---|
| Lead time | Days, often from stock | Weeks, sometimes considerably more |
| Flashing kit | Off the shelf for your covering | Made with the unit or adapted |
| Blinds and shutters | Full range, stocked | Restricted, made to size, dearer |
| Replacement glass later | A stocked spare part | Ordered specially each time |
| Resale of a spare unit | Easy | Practically nil |
The last row is the one people never think about. A standard unit ordered in error can go back or go on to the next job. A bespoke unit in the wrong size is firewood.
What a special size really costs you
The premium on the unit itself is the smallest part of it. The real cost lands later, in three places. Blinds, because a made-to-size blackout blind for a non-standard frame is a fraction of the unit price and has to be ordered from the same maker. Glass, because in ten years when a sealed unit mists, a stocked pane arrives in a week and a special one does not. And time, because a bespoke frame that arrives damaged puts the whole job back to the start of the queue rather than to next Tuesday.
There are genuine cases for a special size. A conservation-area roof with a fixed sight line, a listed building where the opening dimension has been agreed with the conservation officer, a light well of a set width. Outside those, the honest advice is usually to move to the nearest standard size and adjust the position instead.
When the rafters suit no standard width
Older Oxford roofs frequently have rafters at odd centres, particularly in Victorian terraces where the spacing was set by what timber the builder had. You may find 430mm in one bay and 520mm in the next.
That does not force a bespoke unit. It forces a trimmed opening. The rafters either side of the window are doubled or a trimmer is fitted top and bottom, the interrupted rafter is cut and supported, and the standard frame is set into the framed hole with the tolerances the manufacturer asks for. That is normal work and it is priced into any proper quotation. The size of the window and the spacing of the rafters are two separate problems, and only one of them needs the joinery.
Old windows that match no current grid
If you are replacing a window fitted in the eighties or nineties, expect the frame to be close to a current size but not equal to it. Manufacturers have adjusted their sizes over the years, and some ranges have gone entirely.
The usual outcome is that the nearest current standard is within twenty or thirty millimetres, and the difference is taken up in the framing and the internal lining rather than by ordering a special. Occasionally the nearest standard is larger, which means enlarging the opening, and that has its own structural and Building Control consequences. We cover that on replacing an old roof window with a modern unit.
How we settle the size on a survey
We measure the rafter centres, the depth of the rafters, the slope length available from plate to ridge, the pitch and the covering. Then we look at what the room is for and where you will be standing or sitting when you use it. Those two sets of numbers usually leave two candidate sizes, and we tell you what each one gives you and what each one costs to install.
If you want the reasoning behind choosing between them, what size roof window should I get works through it. We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road, fitting VELUX, Fakro, Keylite and Roto units. Fifteen years in the trade, Building Control notification handled, the roof never left open overnight, and ten years on the workmanship. A range by email, then a fixed price after a survey. 01865 704245.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
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