Roof windows come in six common width families, from about 550mm to 1340mm, and five common height families, from about 780mm to 1600mm. Width is set by a letter and height by a number, so the range runs from a small CK02 at 550 by 780mm up to a large UK08 at 1340 by 1400mm. Rafter spacing decides which of these fits your roof cleanly.
The sizes themselves are only half the answer. The other half is the roof they have to go into, because the gap between your rafters constrains the practical width far more than the catalogue does. A size that exists is not the same as a size that fits your roof without cutting structure.
The common size families
Widths are given by a letter and heights by a number, and all the figures below are structural opening dimensions, the gap between the trimmed rafters rather than the size of the glass.
| Width letter | Width | Height number | Height |
|---|---|---|---|
| C | 550mm | 02 | 780mm |
| F | 660mm | 04 | 980mm |
| M | 780mm | 06 | 1180mm |
| P | 940mm | 08 | 1400mm |
| S | 1140mm | 10 | 1600mm |
| U | 1340mm |
Any width pairs with any height, so a code such as MK06 is 780mm wide by 1180mm high. Your survey confirms the actual opening, which on older work is frequently 10mm to 20mm off the nominal.
The smaller families, C and F widths at short heights, suit landings, bathrooms and stairwells in terraces. The middle M and P widths are the everyday loft-bedroom sizes and the ones we fit most around Ipswich. The wide S and U families are for main light sources in larger conversions and new-build plots, and they are where rafter spacing starts to dictate the answer.
Why rafter centres decide the practical choice
Rafters in most homes sit at either 400mm or 600mm centres, meaning the spacing between rafter centrelines. That spacing, and whether your roof is a cut roof or a trussed one, sets how wide a window you can fit without a structural design.
On a traditional cut roof, single-length rafters running wall plate to ridge, one rafter can normally be trimmed out to take a window up to about 940mm wide, a P family unit, using doubled trimmers and no engineer. That covers the C, F, M and P widths on most cut roofs at 400mm centres. Go wider, to an S or U family window, and you are interrupting more structure, which usually needs a structural engineer’s calculation.
At 600mm centres you have more clear width in each bay to begin with, so a mid-size unit can sometimes sit with less intervention. But the covering, the pitch and the load path still have to be checked, so wider centres are not a free pass to a bigger window.
Height is the freer choice, because a taller window interrupts no extra rafters, it just runs further up and down the slope. That is why shallow-pitch roofs so often end up with tall PK10 or UK08 units: on a low pitch you need the extra length to bring the bottom of the frame down to a height where you can see out from a chair, roughly 900mm to 1000mm above the floor. A short window set high in a shallow slope gives you sky and nothing else.
Trussed roofs change the rule completely. Post-1970 estates, including most of the housing around Kesgrave, Martlesham and Grange Farm, use engineered trussed rafters. You cannot cut a truss without a designed solution, whatever the centres are. Any opening over roughly 1.2m wide, or a roof with irregular rafters, needs a structural engineer. We work to the engineer’s design and will not claim a truss can simply be cut.
What this means for your roof
The size that suits the room and the size that suits the roof are not always the same, and the survey is where they get reconciled. A homeowner who wants a wide S-family window for light may have a trussed roof at 600mm centres that makes it a structural job, or a cut roof at 400mm centres where a single trimmed rafter does it easily. Around Ipswich the older terraces off Norwich Road tend to have tighter rafter spacing, which pushes them towards the narrower F and M families. The new-build estates at Kesgrave and Martlesham are trussed, so width is an engineering question there, not a catalogue one. Our rooflight rafter spacing page goes into how the centres and the roof type interact.
Related reading
To match a size to a code, see roof window size codes and the segment-by-segment decode at what roof window codes mean. To read the code off your own window, see how to find your roof window size code. All of it sits under our main VELUX window installation guide.
Get a fixed price
The right size for your roof depends on the room, the rafters and the pitch together, and that is a survey question. Tell us what the room needs and send a photograph of the rafters from inside the loft, and we can tell you which size families are realistic before anyone visits. Then we survey, confirm the opening, and give you one written price, fixed, backed by a 10-year workmanship guarantee from installers with 25 years fitting roof windows.
Request a fixed quote for roof window installation in Ipswich, Suffolk or north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.