If you want to know about VELUX window sizes, the first thing to understand is that the maker does not sell windows by a single dimension. Every roof window carries a code, a letter or letters followed by digits, and that code stands for the width and height of the frame that sits in the roof. This guide explains how to read the code, why the same code can mean two slightly different things depending on the age of the window, and how we turn a code on a data plate into a window that fits first time. It sits within our wider work on VELUX window installation and replacement across Coventry and the 30 road miles around it.
What a VELUX size code actually tells you
A code such as MK04 packs the dimensions of the roof opening into four characters. In MK04 the letter is the width, the digits the height. The letter belongs to a published ladder of widths, running from the narrow frames used between closely spaced rafters up to the broad ones that span a whole bay of a loft room. The digits belong to a matching ladder of heights, from short windows that suit a low eaves slope to tall ones that reach most of the way up a rafter run.
The point of the code is that it describes the frame, not the glass. The visible glass is smaller than the frame, because the sash, the seals and the flashing all take up room. So if you measure the daylight you can see from inside and try to match it to a chart, you will land on the wrong size. The code is the reliable route, and the maker publishes the exact dimensions behind every code in its own size chart, which we check against for each quote.
Older windows can use a single letter or a number-only code instead. That is why a code read off a window that has been in the roof for a few decades does not always line up neatly with a current chart. We treat an old code as a strong clue and then confirm it with a tape measure on the day of the survey.
Why size matters more on a roof than on a wall
A window in a wall sits in a masonry opening that a builder can adjust. A roof window sits between rafters that were placed when the house was built, and those rafters are structure. Cutting one is a job for a trimmer and a carpenter, and it is the reason a change of size is a bigger decision than a change of style.
This leads to the central rule of replacement work. If the new window has the same code as the old one, it drops into the same opening and the rafters stay as they are. If the new window is wider or taller, the roof structure around it changes, the insulation changes with it and the plasterwork inside has to follow. A wider window is a building project. A like-for-like swap is a replacement, which is a far shorter job.
The code describes the hole in the roof. Match the code and you match the hole.
Reading the letters and the digits
Think of the code in two parts and work through them in this order.
- The letter or letters give the width of the frame, measured across the roof slope from one side to the other.
- The digits give the height of the frame, measured up the slope from the lower edge to the upper edge.
- The type code before the size, and the glazing number after it, tell you the opening type and glass, which matter for the order but not for the size.
Width and height are both measured on the slope of the roof, not as a vertical drop. A tall frame on a shallow pitch can look short from inside the room, because the slope foreshortens it. That surprises people when they compare a code with what they see from the floor.
The table below sets out where each part of a code comes from and what it means for the job.
| Part of the code | What it describes | Why it matters |
|---|---|---|
| The letter | Frame width across the slope | Decides whether the rafters can stay as they are |
| The digits | Frame height up the slope | Decides how the flashing sits against the tiles above and below |
| Type and glazing codes | Glazing, opening type or finish | Decides the specification, not the opening |
| The production date | Age of the unit | Tells us which generation of window you have |
Where the code lives and how to confirm it
The code is printed on a data plate fixed to the window. On VELUX windows the plate usually sits at the top of the frame, and you see it when you open the window. Our separate page on how to find your VELUX code walks through the usual places, including what to do when the plate has been painted over or has worn away.
If you cannot find a plate, there is still a route. We measure the frame at the survey, from the inside where the lining meets the frame and from the roof where the flashing shows its outer edge. Between the two readings we identify the closest published size, and we say so on the written quote so there is nothing left to interpret later.
Choosing a size for a new window rather than a replacement
When the window is new, there is no old code to match, so the choice follows the room. Four things drive it.
- The rafter spacing. A frame that fits between two rafters needs no structural change, so the spacing sets the widest easy option.
- The height of the slope. A window on a short slope has to leave room for the flashing above and below it, and the tile courses either side have to be made good.
- Head height inside. A window set too low gives a view of the floor and nothing else. A window set too high leaves the lower part of the room dark.
- The reveal inside. The deeper the roof build-up, the more light the splayed opening can throw into the room, and the more finish there is to plaster.
Roof pitch changes the picture as well. On a steep roof a taller window looks proportionate, on a shallow one a wide, short window tends to sit better. We place a straight edge and a tape on the slope at the survey so the proportions are judged against your actual roof and not a catalogue photograph.
Matching a size when you replace an old window
Replacing a window with the same code is the simplest route, and it keeps the existing rafters, flashing layout and reveal. It is still building work, and our page on replacing an old VELUX window covers how the job runs from strip-out to plaster. The size question comes down to three possible answers.
- The same size as before. This is the usual choice, because the opening is already formed and finished.
- A taller window in the same width. This can often be done by lowering the lower edge or raising the upper edge slightly, with tile adjustment and some plaster work.
- A wider window. This changes the rafters and needs a carpenter, so it is priced and planned as a structural alteration.
Whatever you choose, swapping the whole unit counts as notifiable building work, and the replacement has to satisfy Part L. The Building Control notification is ours to handle when the work is notifiable, so the size decision and the paperwork are settled together.
Energy figures and how they link to size
Larger windows lose and gain more heat in absolute terms, so the energy standard matters more as the frame grows. For an existing home, Part L 2021 Table 4.2 sets roof windows at 1.4 W/m²K, with Window Energy Rating band B as the alternative route. A rooflight in a flat roof or a lantern has its own limit of 2.2 W/m²K. A unit set into a pitched roof counts as a roof window, which is why a conservation rooflight in a slope is held to the roof window figure.
Neither figure changes with the size code. What changes is the glazing specification the maker offers in each size, so we confirm the exact figure with the maker for the size on your quote and write it on the paperwork.
Common size mistakes to avoid
Most size problems come from a handful of habits, and all of them are easy to avoid once you know them.
- Measuring the visible glass and treating it as the frame. The frame is always larger, so the answer comes out too small.
- Reading the date of manufacture as part of the size. The date on the plate is separate from the code and tells you nothing about the dimensions.
- Assuming two windows in the same roof are the same size because they look alike. Neighbouring windows are sometimes fitted years apart in different frames.
- Copying the size of a window from next door. Rafter spacing differs even between houses built to the same design, and a size that suits one roof can clash with another.
Older Coventry housing adds a further wrinkle. Where a loft was converted a long time ago, the rafters may have been trimmed to suit a window that is no longer made in that exact size. In that case we look at the nearest current frame and see whether the opening can take it as it stands or needs a small adjustment, then we explain the options on the quote before any order is placed.
Getting a size confirmed before anything is ordered
The most expensive mistakes in roof window work come from ordering a unit to a guessed size. A wrong frame arrives, the order cannot be changed, and the job stalls while a new one is made. We remove that risk with the survey.
At the free survey we read the data plate, measure the frame, check the rafter spacing from the loft where it is accessible and look at how the roof covering meets the frame. You then receive a fixed written quote that states the code, the specification and the finish inside the room. Nothing is ordered until you accept it. Once the work begins, the window goes in with its flashing and seals completed on the roof, and the reveal is opened out where the roof depth allows, and the plaster is finished, so the finished window looks as though it was always part of the room.
If your sums point to a larger window than the one you have, mention it when you book. It changes the survey from a check of dimensions into a conversation about structure, and we plan the visit accordingly.
