One homeowner sends a photograph of the old window in the back bedroom and asks for the same again. One builder sends a loft drawing with four openings marked and the word VELUX against each. Both are asking the same question in different ways: which size, exactly? The answer is a code such as M06 or S08, and once you know how to read it, it stops being jargon and becomes a quick way to check the whole job.
The code matters because the opening in the roof is built around it. Rafters are trimmed, insulation is cut, battens are set, and the flashing is ordered, all to the size of the window. If the code changes after the roof is built, something has to be reworked. We write the code against each opening in the survey and send it to the builder with the sizes, so the carpenter can frame to it before the covering goes on.
How the code is built
A full product reference has several parts. At the front come letters that name the type of window, such as whether it is a centre-pivot or a top-hung model, and what glazing it carries. The last part is the size code: one letter followed by two digits. That is the piece this guide is about, because it is the only part that changes the physical size of the opening.
The rule is simple. Read it as width first, then height. Move along the alphabet and the window gets wider; move up the digits and it gets taller. So a window coded M04 is narrower than one coded S04 but the same height, and M04 is shorter than M08 but the same width. The code does not name a measurement in centimetres, which is why people sometimes find it baffling, but it is consistent across the range and that is what makes it useful.
The maker publishes a chart that converts every code into the outer frame size and the glass size, and that chart is the reference we use for every quote. Treat a forum post or a supplier's shorthand as a prompt to check, because an old window can carry a different system altogether.
If you are still deciding between window types, the VELUX windows hub sets out the range, and top hung or centre pivot explains how the two opening styles differ before you settle a size.
Width letters
The width letters in the standard range run from narrow to wide in this order: C, F, M, P, S and U. A few other letters appear on specialist or older models, but these six are the ones you will meet most on a house roof.
| Letter | Where it tends to be chosen |
|---|---|
| C | Narrow openings, such as between closely spaced rafters or on a small dormer cheek |
| F | A little wider than C; useful in small rooms and under low eaves |
| M | A common middle width for bedrooms and bathrooms |
| P | Wider again; suits larger bedrooms and living space |
| S | A broad window for open lofts and studios |
| U | The widest in the standard range; needs a proper trimmed opening |
Choosing a letter is mostly about where the window sits in relation to the structure. A narrow window slides between a pair of rafters with room to spare. A wide one needs one or more rafters cut and a header fitted above and below, which is structural work and brings in the question of who designs the opening. The between the rafters answer explains where that line falls.
Two narrower windows side by side give more flexibility than one wide one, since one can be open while the other stays shut, and they sit more comfortably in a roof with closely spaced rafters. At survey we sketch both options, so the choice is made with the plan in front of you.
Because the letters sit in alphabetical order of width rather than spelling anything out, it helps to keep the maker's chart beside the drawing when you compare sizes.
Height digits
The two digits that follow the letter give the height. The numbers start at 02 for the shortest window and rise through 04, 06 and 08 to the tallest, and the step between them is the same across every width. If you know the width letter, the digits tell you how far up the slope the window reaches.
Height is the measurement that affects the room most. A taller window lets in more sky and lets you see out from a lower position, which matters in a loft where the sloping ceiling means you are often looking up at an angle. It also puts the top of the window higher on the slope, so the sill height and head height have to be checked against the floor and ceiling line inside.
Sill height and sightlines
Where the window sits on the slope is as important as its size. Roof windows that are fitted too high are awkward to reach and give a view of nothing but sky. Windows that are fitted too low can be knocked and bumped. In a bedroom or a living room the bottom edge of the glass usually looks best a little above waist height, so the view out is available when you are standing, and the window can be opened without leaning over furniture. The right level depends on the pitch, the floor height and what is under the window, so we mark it on site and agree it with you before the opening is cut.
Pitch has an effect too: on a shallow roof a tall window takes up a long run of slope. The low pitch roof page explains the limits for shallow slopes, which can restrict the choice of model as well as size.
Matching a size to the rafters
The cleanest installation is one where the window is chosen to suit the rafters it will sit between. Modern roofs are usually built with rafters at regular centres, and the narrower window sizes are made to fit between a pair of them with clearance for the flashing. In that case there is no structural cutting at all, and the opening is framed with simple timber trimmers at the head and sill.
Older houses are less regular. Rafters in a Victorian or Edwardian roof may be unevenly spaced, set at different depths, or carry a purlin that crosses the planned opening. A size that looked right on the drawing may land on a rafter, a hip or a chimney. We check on the roof and from the loft side, and we tell the builder which rafters are affected and what size avoids them. Where cutting is needed, the structural alteration is a matter for whoever is designing the roof, and we ask for the details in writing before we order.
- Measure the clear distance between rafters, not the centre-to-centre spacing.
- Allow room on each side for the flashing and the insulation surround.
- Check the position of any purlin, wall plate or collar tie that crosses the opening.
- Check for water tanks, pipework and cables on the loft side of the planned opening.
A flashing kit is made to the same code as the window, so the surround adds to the opening. A kit takes up room on both sides of the frame, and the width available between rafters has to cover both. That is why a window that fits comfortably on paper can end up tight on site. The roof window flashing kits guide covers how the surround is matched to the covering.
Either the rafter layout is fixed first and the sizes chosen to suit it, or the window is chosen first and the rafters trimmed to it. Both routes work, as long as the decision is written down before the carpenter starts.
Finding the code on an old window
If you are replacing a window, you probably want the same size again, and the quickest way to find it is the label. Most VELUX windows carry an identification label on the frame, usually on the inside edge of the sash or the frame, and it is generally visible when the sash is open. The label gives the full product reference, and the size code is the last part of it.
Labels fade, get painted over and sometimes disappear, particularly on windows that are decades old. Older windows can also be marked with a number-only code that pre-dates the letter and digit system, and there is no simple formula for turning those into a current code. When the label has gone, the reliable method is to measure the outer frame, width and height, and compare it to the maker's chart. We do that on the survey, taking the measurement from the outside as well as the inside, and we confirm the code in writing.
What to photograph for a quote
- The label, close up, with the sash open.
- The whole window from inside, showing the lining and any blind.
- The window from the garden or street, showing the covering around it.
- The loft side of the opening, if you can reach it safely.
A photograph is a good starting point, but it is not a measurement, so we still visit before the quote is final. Replacing a whole window changes more than the glass. The frame, the flashing, the collar and the lining may all change, and the new window may not be a straight swap. The replacing an old VELUX guide covers what to expect, and the cost of fitting explains what goes into the figure on the quote.
If the old window was another make, measure and photograph it and we will find the nearest current size.
Sizes for lofts and landings
There is no single right size for a type of room, but a few patterns come up often enough to be useful. In a loft bedroom the window is usually chosen for light and for a view from the bed, and the room can often take a wider code than a landing or a bathroom can. In a loft bathroom the position over the bath or basin is as important as the size. On a landing or stairwell, the window may be as much about lighting a dark space as providing a view.
| Room | What to weigh up |
|---|---|
| Loft bedroom | Wider codes spread light; a taller code gives a view from the bed |
| Loft bathroom | Position over bath or basin; ventilation; privacy |
| Landing or stairwell | A narrow, tall code lights the stairs without losing wall space |
| Home office | Even light across the desk; avoid glare on the screen |
| Small box room | A narrow code between rafters often fits with no structural work |
Escape is a separate question. If the loft room is a habitable room on a house with an upper storey, a window may need to be big enough and low enough to serve as a means of escape, or the stair has to be protected. Under Approved Document B an escape window serves only storeys within 4.5m of ground level, so a loft above two storeys relies on a protected stair. The fire escape answer sets out how this works, and your building control body will confirm the requirement for your plans.
There is no Building Regulations daylight requirement for existing houses in England, so the amount of glass is a design choice. That leaves the decision to you and your designer, which is helpful, since the room can be lit to suit how it will be used.
Once the codes are fixed, they go on the order and into the written sizes for the builder, who can then frame each opening to its code. Blinds are best chosen at the same moment; see choosing blinds with the window.
Sizing questions we are asked
- Can I swap to a different size when I replace a window?
- Often, though a larger size means altering the opening and the rafters, and a smaller one means making good the roof and lining around it. We discuss both at survey and put the chosen code in the quote.
- Do all VELUX windows in a roof need to be the same size?
- No. Mixing sizes is common, for instance a wider window in a bedroom and a narrow one on the landing. What matters is that each opening is framed to the code written against it.
- Is the code the same for top-hung and centre-pivot models?
- The size code system is shared, but the type letters that come before it differ. The size does not tell you how the window opens, so confirm the type separately when you order.
