Reading a VELUX size code

Where the plate is

Open the window and look at the top edge of the sash, or on the frame beside the hinge. It is a small metal or printed label.

On a centre pivot window, rotating the sash brings it into easy view. Photograph it rather than transcribing it; the characters are small and a mistyped code buys the wrong window.

What the code means

The first letters identify the model type: the opening action, the frame material and the glazing generation. Those change over the decades, which is how a code dates a window.

The size is given as a letter and a number, for example MK04 or CK02. The letter group corresponds to a width and the digits to a height. The pairing gives one specific frame dimension.

You do not need to decode it yourself. Send us the photograph and we will tell you what it is, whether it is still made, and what the options are if not.

Why the frame size matters

The code refers to the outer frame, and that is the dimension the opening was built around. A replacement has to match it, or the opening has to be altered.

The visible glass is meaningfully smaller than the frame, typically 120mm to 180mm less in each direction, which matters when you are estimating light rather than fit.

When the plate is gone

Painted over, worn away or removed. Then the opening is measured instead, which means a survey rather than a phone call.

It is also worth measuring when a plate is present but the window looks like it has been altered, because a previous installer may have adapted an opening rather than matching it.

Obsolete sizes

Ranges change. A window from the 1980s or 1990s may be a size no longer made, which comes up constantly on Colchester’s older loft conversions. Options are on the VELUX page.

Where the code is

On the data plate, which is a small label on the top edge of the sash or on the frame beside the hinge.

To see it on a centre pivot window, rotate the sash so the top edge comes down to eye level and lock it in the cleaning position. On a top hung window, open it and look at the frame head.

The plate carries the model designation, the size code, and usually a manufacture date. All three are useful.

Photograph it, do not transcribe it

This is the single piece of practical advice that matters most on this page.

The characters are small, they are frequently faded after twenty years of sunlight, and several of them are easily confused. A photograph taken close up with the phone flash on removes all of that risk, and it lets a supplier read the whole plate rather than the part you thought was relevant.

A mistyped code buys the wrong window, and a wrong window is discovered on installation day when the roof is already open.

How the code is structured

The size code is a short combination of a letter and a number.

The letter indicates a width group. The number indicates a height within that group. So two windows sharing a letter share a width, and two sharing a number share a height.

That structure is why a code is quick to work with once you know the range. A supplier hearing the code knows the structural opening immediately, without measuring anything.

Alongside the size code sits the model designation, which is a longer alphanumeric string identifying the type of window: the operation, the glazing specification, the frame material and the generation of the product.

The critical thing the code describes

The structural opening. Not the glass, not the internal reveal, not the visible frame.

The structural opening is the hole in the roof timbers that the window sits in, and it is behind the plasterboard and under the tiles. You cannot measure it from inside a finished room, which is exactly why the code exists.

This is the source of most confusion. Somebody measures the glass, or the visible aperture from inside, orders a window to that dimension, and receives something considerably too small. The frame occupies space, the lining occupies space, and the difference between what you can see and what the roof actually holds is substantial.

Codes are not portable between manufacturers

Every major manufacturer uses its own coding system. A code from one maker means nothing to another, and coincidental similarities between systems are traps rather than shortcuts.

If you are moving between brands, the conversion goes through actual millimetre dimensions rather than through code matching. Any competent supplier will do that conversion, and it is worth having them confirm it in writing before ordering.

What to do if the plate is unreadable

It happens. Sun bleaching, paint over the frame during decorating, or a plate that has come away entirely.

Three routes.

Measure the visible frame externally and internally and let a supplier work back to the likely structural opening. Approximate, and usually enough to narrow the candidates to two or three.

Check the other windows. On a loft conversion where several went in at once, they are usually the same size and at least one plate will be legible.

Have it surveyed. Somebody who fits these regularly will identify the model and size on sight, and can confirm by lifting a tile or two at the head if there is any doubt. On a job of any value, that half hour eliminates the risk entirely.

Reading the date

The manufacture date on the plate tells you two useful things.

Whether the size is likely to still be current. Ranges are revised every decade or so, and anything from before roughly 2000 has a good chance of being an obsolete size.

And whether the failure you are seeing is expected. A sealed unit that has misted at eighteen years has reached the end of its service life and the window is a replacement candidate. The same failure at four years is a warranty conversation.

What else the plate tells a supplier

The model designation identifies the glazing specification, which matters for a like for like replacement. A window originally fitted with a basic double glazed unit and one fitted with a high performance unit look identical from the ground and perform very differently.

It identifies whether the window was centre pivot or top hung, which affects both the replacement choice and whether the existing opening suits it.

And it identifies the frame material, which on an older window is frequently timber where the owner assumed it was something else.

What to record for next time

When a new window goes in, write down the manufacturer, the model designation, the size code, the glazing specification and the date, and keep it with the house paperwork.

In twenty years somebody will be standing in that room with a torch and a phone trying to read a faded plate at height. A piece of paper saves them the exercise, and saves them a survey.

What the plate carries besides the size

Three pieces of information, and all three are worth having.

The model designation. A longer alphanumeric string identifying the type of window: the operation, the glazing specification, the frame material and the generation of the product. This is what tells a supplier whether the unit was centre pivot or top hung, whether it was timber or a coated finish, and what glass it was fitted with originally.

The size code. A short letter and number combination where the letter denotes a width group and the number a height within it.

The manufacture date. Which tells you whether the size is likely to still be current, and whether the failure you are looking at is expected or premature.

The date is more useful than people realise

A sealed unit that has misted at eighteen years has reached the end of its service life. That is a replacement conversation, and the age tells you the hinges, gaskets and mechanism are the same age and heading the same way.

The same failure at four years is a warranty conversation, and the date on the plate is the evidence.

The date also predicts the size question. Ranges are revised every decade or so, so anything from before roughly 2000 has a good chance of being an obsolete size, and knowing that early changes the lead time you should plan for.

Why codes do not travel between manufacturers

Every major maker uses its own system, developed independently, and the similarities between them are coincidental rather than meaningful.

A code that looks like it means something in another manufacturer range is a trap rather than a shortcut. Conversion between brands goes through actual millimetre dimensions, and any competent supplier will do it and confirm it in writing before you order.

That written confirmation is worth having. A cross brand size assumption that turns out to be wrong is discovered on installation day with the roof already open.

Getting the plate photographed safely

On a centre pivot window, rotate the sash until the top edge comes down to a comfortable height and engage the barrel bolt or catch that holds it in the cleaning position. That catch is not optional. Reaching into a rotated sash that is not locked risks it swinging back.

On a top hung window, open it and look at the frame head beside the hinge.

Use the flash even in daylight. The plate is frequently in shadow under the sash edge, the characters are small, and twenty years of sunlight fades the print.

When it cannot be read

Photograph the whole window from inside showing the full frame and how the sash sits within it, and from outside if that can be done safely. The external cladding profile and the frame proportions identify most mainstream products to somebody who fits them regularly.

Check the other windows on the same slope. Where several went in together they are usually identical and at least one plate will be legible.

Then, on any job of value, have it surveyed. A half hour on site removes the risk entirely, and the alternative is discovering the error with the covering off.

Recording it for next time

Write down the manufacturer, model designation, size code, glazing specification and date, and keep it with the house paperwork rather than in a drawer.

In twenty five years somebody will be up a ladder with a torch trying to read a faded plate. A single sheet of paper saves them a survey and saves you nothing at all, which is rather the point of doing it for whoever comes next.