A roof window code has two parts. The model prefix, three letters such as GGL or GPU, tells you the window type and finish. The size code, a letter and a number such as MK06, tells you the dimensions: the letter sets the width, the number sets the height. Read both and you can identify your unit before you ring a single installer.
People find the code on the data plate and then have no idea what it says. It is not random. Each part carries a specific piece of information, and once you can decode it you know your window’s type, finish and opening size without guessing. Here is what each segment means.
The two halves of a code
Take a full code like GGL MK06. The first block, GGL, is the model. The second block, MK06, is the size. They answer different questions, and installers quote against both.
The model prefix: type and finish
The three-letter model prefix tells you how the window opens and what it is finished in. The two most common families are the ones you will almost certainly have.
- GGL and GGU are centre-pivot units, where the sash rotates about a hinge roughly halfway up. GGL is a pine unit with a lacquered timber finish inside. GGU is finished in white polyurethane, the right choice for a bathroom or any humid room because it shrugs off moisture.
- GPL and GPU are top-hung units, where the sash swings outward from a hinge at the top so you can sit a desk or bed directly beneath it. GPL is pine, GPU is white polyurethane. Top-hung escape versions are what meet Part B in a habitable loft.
So the final letter of the prefix, L or U, is your finish: L for lacquered pine, U for white polyurethane. The middle letters tell you the operating type. That one detail decides whether the unit can serve as a fire escape and how the room below can be furnished.
The size code: width, range, height
The size code is where the dimensions live. It reads as a width letter, a range letter, then a height number. In MK06, M is the width, K is the size-range series that most current units use, and 06 is the height. The dimensions are the structural opening, the gap between the trimmed rafters, not the size of the glass.
| Segment | Example | What it tells you |
|---|---|---|
| Model, letters 1 to 2 | GG (centre-pivot), GP (top-hung) | How the sash opens |
| Model, final letter | L (pine), U (white polyurethane) | The internal finish |
| Size, width letter | C, F, M, P, S, U | The opening width |
| Size, range letter | K | The size-code series most current units use |
| Size, height number | 02, 04, 06, 08, 10 | The opening height |
Reading the width and height
The width letter and height number are the two figures that decide whether a replacement drops straight into your opening. Here is what each stands for.
- Width letters: C is 550mm, F is 660mm, M is 780mm, P is 940mm, S is 1140mm, U is 1340mm.
- Height numbers: 02 is 780mm, 04 is 980mm, 06 is 1180mm, 08 is 1400mm, 10 is 1600mm.
Put them together and MK06 is 780mm by 1180mm, CK02 is 550mm by 780mm, and UK08 is 1340mm by 1400mm. Because these codes have stayed broadly stable for decades, a unit fitted in 2004 can usually be matched to a current model on the same code without touching the rafters. That stability is what makes a like-for-like replacement quick.
What this means before you ring round
Decoding your own code puts you in a stronger position on the phone. You can say you have a GGL MK06, a pine centre-pivot at 780 by 1180mm, rather than describing a window and hoping. It also flags the things that change a quote: a top-hung GPL or GPU costs more than a centre-pivot for the same size, and a wide S or U code may have needed a rafter trimmed out originally, which matters if it is ever moved or enlarged.
One caution. Blinds and accessories are sized to the window generation, not just the size code, so a matching code does not guarantee your old blinds transfer. The frame profile changes between generations even when the code holds. Our blind compatibility page explains why.
Older codes can look slightly different, and that is worth knowing if your window predates 2000. The width letters and height numbers have stayed broadly constant, which is the part that matters for matching an opening, but the range letter and some model prefixes have shifted across generations as frame profiles changed. So a very old unit may carry a code that is not quite a current one letter for letter. The width and height still tell you the opening, and that is usually enough to find a modern equivalent. Where the code is genuinely obsolete, we fall back on measuring the opening directly at survey.
Related reading
This is the decode. For where the plate sits and how to read it in the first place, see how to find your roof window size code. For the fuller code-and-size matrix, read roof window size codes. And for the common size families laid out, see what sizes roof windows come in. All of it sits under our main VELUX window installation guide.
Get a fixed price
If you have decoded your window and want a replacement matched to it, send us the code and a photograph of the plate. We will tell you whether a like-for-like swap is possible and roughly what it costs, then survey to confirm the opening and give you one written price. That price is fixed and every job carries a 10-year workmanship guarantee from installers with 25 years fitting roof windows.
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