Every VELUX roof window carries a size code, a letter followed by digits. It is the quickest way to say how big a window is, and it is the first thing we look for when you ask about replacing one. Read correctly, it saves a tape measure on the ladder and a wrong order at the merchant.
A skylight is the homeowner's word for these windows, and rooflight is the trade's. Whichever you use, the code works the same way. This guide explains the pattern, shows where to find it, and sets out what a size chart can and cannot tell you before a survey.
It sits within the VELUX windows hub, which covers the whole job from first look to finished room.
The letter is the width, the digits are the height
The rule is short. Take a code such as MK04: the first part names how wide, the numbers after it say how tall. Read it left to right: width first, then height. A change of letter means a different width, and a change in the digits means a different height.
In centimetres, the width letters are C = 55, F = 66, M = 78, P = 94, S = 114 and U = 134. The height digits are 01 = 70, 02 = 78, 04 = 98, 06 = 118, 08 = 140 and 10 = 160. So CK04 is 55 × 98 cm, MK06 is 78 × 118 cm and PK08 is 94 × 140 cm. These are outer frame sizes, width by height, not the glass or the hole in the roof. The code is on the data plate inside the window frame, at the top of the sash or frame.
That is all the code does. It does not say how old the window is, which glass it has or how it opens. Those details come from the type letters that sit before the size, and from the product label itself.
The point of the system is that sizes follow a ladder. Each width letter is a step wider than the one before, and each height number is a step taller. Once you know your code, you know where your window sits on both ladders and which neighbours are available if you want to change.
Where to find the code on your window
The code is printed on a label, and the label lives in a few predictable places. Open the sash and look at the top of the frame or the edge of the sash itself. On many windows you will find a small white sticker with a string of letters and numbers.
If the sticker has gone, look at the paperwork. A receipt, an installation certificate or a manual from the original fitting may name the model. Inside a loft, the code may also be visible on the frame behind the lining, though a survey is a safer way to confirm it than prying at the plaster.
Photographs help. A clear picture of the label, plus one of the whole window from inside and one from outside, lets us give a first view before the visit. We still measure on site, because a printed code gives the product size and the survey gives the opening size.
A size chart, and what it will not tell you
The manufacturer publishes a VELUX size chart that lists each code against its width and height in centimetres. It is a useful reference when you are comparing options or planning a new opening, and we use it at every survey.
What a chart cannot show is your roof. It lists the window, not the rafters beside it, the tiles around it or the room beneath it. A window that exists in the range may still be the wrong choice if the rafter spacing is tight or the pitch is shallow. That is why a chart is a starting point and a survey is the decision.
The table below sets out what each part of a code tells you, and what it leaves out.
| Part | Tells you | Does not tell you |
|---|---|---|
| Width letter | How wide the window is | Which glass it has |
| Height digits | How tall the window is | How it opens |
| Type letters | The model family | The age of the unit |
| Label | Product and batch details | The opening in the roof |
Matching an old code to a new window
Ranges change. A code that was current years ago may have a close cousin today, and the closest match is not always an exact match. If your window is a discontinued size, the nearest current code might be a little narrower or a little shorter, and a flashing kit is chosen to close the difference around it.
For a like for like swap, that small gap is dealt with in the flashing, as described in replacing a VELUX window. The rafters stay where they are and the new frame sits in the same place. Where the nearest size would leave an awkward gap, we may suggest the next code up and trim the opening to suit.
This is another reason not to order from a code alone. We check the opening, the lining and the covering at the survey, and the written quote names the exact window that will be fitted.
Choosing a size for the room
A size code tells you how big a window is. Whether it is the right size is a question for the room. A bedroom wants enough glass to wake you gently without becoming a heat trap. A kitchen wants light over the worktop. A landing or stair wants enough to carry daylight down into the hall.
Height matters as much as width. A tall window in a steep roof lets light reach further across the floor, because it sees more of the sky above the line of the eaves. A wide, short window gives a broad band of light. Both suit different rooms, which is why we ask how you use the space before we suggest a code.
A larger code lets more of that sky into the room, and a smaller one lets in less. The choice is yours, and we help you weigh it.
Size, rafters and the opening in the roof
A roof is built on rafters at fixed spacing, and a window opening has to fit between them or be framed around them. Some sizes sit neatly between two rafters. Larger ones may need a rafter cut and a trimmer fitted, which is structural work and adds time.
That is the practical limit on size. If you wonder whether you can step up a code or two, the answer is in whether a VELUX window can be made bigger. The rafter spacing, the roof covering and the room underneath all shape what is possible.
Pitch also counts. Each window is designed for a range of roof pitches, and a very shallow roof can rule out some types. We measure the pitch at the survey so the window is chosen for the roof you have.
Reading a code in the loft and on the landing
Codes are easiest to read when you know the room they came from. A loft bedroom often has a tall window set low in the slope, so the height digits are generous and the width letter stays modest. A landing may carry a narrow, tall window to pull daylight down a stairwell. A kitchen or bathroom window tends to be wider, because the aim is a band of light over a worktop or a basin.
Looking at the codes already in your roof is a quick way to see what you have lived with. If two windows in the same room carry different codes, that tells us the roof has been worked on before, and it is worth a closer look at the lining and flashing around each.
When a house has several windows, we record every code at the survey and quote them as a set. Matching sizes across a roof keeps the outside tidy, because rows of windows look best when their tops and bottoms line up with the courses of tile or slate.
Where windows sit in a row, a mixture of widths can still work if the heights match. The eye reads the line of the tops and bottoms first, and the widths second.
Glass, blinds and fittings that follow the size
The size code also decides what else fits. Blinds, shutters and insect screens are made to the same codes, so a blind for one size will not suit another. The flashing kit is matched to the code and to the roof covering. Even the glass is ordered by size, and larger panes can be heavier, so access and lifting are planned in advance.
This is useful when you are replacing. If the new window keeps the old code, an existing blind may carry over where it is the same model range. If the code changes, new blinds are priced with the window in the written quote. Either way, nothing is left as a surprise.
Energy performance follows the glass rather than the size, but a larger window has more glass to lose heat through. The current Building Regulations limit applies to the whole unit, so a bigger window must still meet it. The survey confirms that the window chosen does.
If you are weighing a bigger code for a loft room, think about summer as well as winter. A large roof window faces the sky, and the sun is high in July. A blind made for the window, fitted with it, is the simple answer to the glare.
A short glossary for reading a quote
A quote for a replacement uses a few terms that are easy to mix up. They are set out here in plain words so the written quote reads clearly.
Words you will hear
Size code
The letter and digits that name the width and height of the window.
Sash
The moving section of the window, which carries the glass.
Frame
The fixed outer part that sits in the roof and carries the sash.
Flashing
The metal collar that seals the frame to the roof covering.
Rafter
The angled timber under the roof covering, which limits how wide an opening can be.
Roof window
A window lying in the slope of a roof, in Part L's 1.4 W/m²K category.
The survey visit, step by step
The survey turns the code into a decision. We read the label, measure the opening, check the rafters and look at the covering. We note the pitch and the room, and we ask what you want to change.
The survey is free, and because the quote is fixed and written, the figure you agree is the figure you pay. The fitting itself is one or two days of work on a standard rooflight, with the inside finished.
Everything is installed to current Building Regulations, and if the job is notifiable, telling Building Control is part of our service. For Milton Keynes homes, that means one call, one survey and one fixed quote.
Before the survey
- Photograph the label on the frame or sash
- Note the room and which way the roof faces
- Keep any old receipt or manual
- Think about glass, blind and opening style
- Decide whether the size should stay the same
Size codes in plain answers
Which part of a VELUX code is the width?
Letter for width, digits for height. In a code such as MK04, the letter names the width step and the digits name the height step.
Can I measure the window myself instead of reading the code?
You can, and it helps, but the code is faster and more accurate for the product itself. The opening in the roof is measured at the survey, so the quote names the exact window that will be fitted.
Will a different size code fit my existing roof?
Sometimes. Rafter spacing and pitch decide, which is why we check on site. A close size can be fitted with a matching flashing kit, and a larger one may need rafters trimmed and a new lining.
