Reading a VELUX size code and finding your product code

Every VELUX roof window has a data plate, and it is not on the outside where you might expect it. Open the window fully, either by swinging a centre-pivot sash right over or lifting a top-hung sash, and look along the top of the frame. On most models the plate sits on the head of the frame or on the top edge of the sash, printed on a silver or white label. You will see it clearly once the window is open and you are looking up into the reveal.

The plate holds three things you care about. There is the type designation, a group of three or four letters such as GGL or GGU. There is the size code, two letters and two digits such as MK04. And there is a longer product number, often with a four digit glazing code such as 0070 or 2070 on the end. Underneath sits the manufacturing information: a serial number and a date, usually shown as a week and year, which tells you roughly when the window was made and therefore which generation of fittings it takes.

If you cannot open the window far enough to read the plate, do not guess from the glass. Measure instead, which the size table further down turns into a code, and confirm the type by how the window pivots. A sash that spins around a central hinge is a centre-pivot. One that hinges at the top and opens outwards from the bottom is top-hung. That single observation already narrows the type letters considerably.

Take a typical full string: GGL MK04 3070. It looks like a registration plate and it reads like one, in three chunks, each answering a different question.

The first chunk, GGL, is the type. It tells you the material of the frame, how the window opens, and whether it is manual, electric or solar powered. This is the part that decides which spare sash or which motor kit fits.

The second chunk, MK04, is the size code. The letter M is the width and the number 04 is the height. Together they describe a standard opening that has not changed in decades, which is why a blind sold for an MK04 today fits an MK04 from twenty years ago.

The third chunk, 3070, is the glazing and variant code. The last two digits describe the sealed glass unit inside the sash, and the first two describe the pane package and any coatings. This is the chunk that governs how the window performs against winter heat loss and, more to the point on a south-facing Essex roof, against summer solar gain.

You will not always see all three chunks together. An older window might show the type and size on the plate and carry the glazing code only on a separate sticker on the glass spacer bar. If the glazing digits are missing, the frame and size are still enough to order a matching unit, and the glass specification can be chosen fresh.

The size code is the part people most need and most often misread. It is always a letter followed by a two digit number. The letter is the width of the window frame. The number is the height. Neither figure is the size of the hole in your roof exactly, because the frame sits inside a structural opening with a tolerance around it, but the codes map to fixed nominal dimensions.

Here are the width letters and the frame widths they stand for.

And here are the height numbers.

So MK04 is 78cm wide by 98cm tall. UK08 is 134cm wide by 140cm tall. CK02 is a small 55cm by 78cm window, the sort that goes over a stairwell or a small landing. Once you have the two lookups in front of you, no size code is a mystery.

Put the common combinations together and you get the sizes you will actually meet on a survey. These are the codes that turn up again and again on the 1930s semis and post-war roofs across south Essex.

One caution about older stock. VELUX ran an earlier numbering system before the current letter-and-number codes, using plain numbers such as 102, 304 or 606. If your window predates the change you may see one of those instead. A 606 in the old system is close to an MK06 in the new one, but the earlier frames are not always interchangeable with current sashes, so an old-number window is worth having looked at rather than ordering blind.

The three-letter type code carries more than the size code does, because it fixes the material, the opening action and the operation all at once. The first letter is nearly always G for a roof window. The second and third letters do the work.

The pine versions, ending in L, are the natural choice for a bedroom or living space where the timber is on show and gets a warm look under the light. The polyurethane versions, ending in U, are white, wipe clean and hold up to moisture, which is why they suit bathrooms, kitchens and utility rooms. The frame outside is the same aluminium regardless, so the letter is really about the face you see from inside.

Operation is the other thing the code hides. A manual window is opened by hand or with a pole on the control bar. An electric window, branded INTEGRA, has a concealed motor and a wall switch or remote. A solar-powered window runs the same motor from a small roof-mounted panel and a battery, with no wiring to bring down through the ceiling. All three can share the GGL or GGU body letters, and they are told apart by a variant marking on the data plate and by a longer product number. If you are matching a broken control or a motor, the body code alone is not enough: you need the full product number off the plate, because the electric and solar parts are specific to the generation of the window.

The four digit code on the end, the 3070 in our example, is where the window’s real behaviour is written. The last two digits name the sealed glass unit. The standard current double-glazed pane reads as 70. Triple glazing reads as 73. Older and specialist units carry other endings such as 66 or 68, generally laminated safety build-ups. The first two digits describe the pane package: the coatings, the gas fill and any noise or heat control layers stacked on top of the base unit.

This is the chunk to slow down on if your window faces the estuary. Southend looks south across the Thames, and a south-facing roof plane collects far more solar energy through the summer than the same window would on a north slope. Two windows with identical GGL MK04 bodies can behave completely differently in July depending on which glazing code sits on the end. One might let more than half the sun’s energy straight through. Another, with a solar-control coating in the pane, cuts that figure sharply while keeping the room bright. That difference is measured as the G-value, and it is the number a bare size code will never tell you.

So when you read a glazing code off an existing window, do two things with it. Note it, so you can match the physical unit if you are replacing a failed sealed pane with the same size. But before you commit to repeating it, check what that pane actually does. If the window overheats a south-facing loft every summer, matching the old glazing code exactly repeats the problem. The frame and size can stay the same while the glass specification is upgraded to a solar-control unit. Choosing that pane deliberately, plane by plane, is the whole basis of solar-control glazing work, and it is why we read the compass before we read the catalogue.

Ordinary clear double glazing lets roughly 60 per cent of solar energy through. A good solar-control unit brings that down towards 30 per cent while holding daylight up where you want it. On a north-facing plane the higher figure is fine and the extra light is welcome. On a south-facing plane over a kitchen-diner, the lower figure is the difference between a room you use in August and one you avoid. The glazing code is where that decision gets recorded.

The window is only half the order. A VELUX sits in a flashing kit that seals it to the surrounding roof covering, and the flashing has its own code that must match both the window size and the roof material. Get the window right and the flashing wrong and you still cannot install it.

Flashing codes start with ED for a single window and follow with a letter for the roof type. The common ones are EDW for profiled tiles up to about 120mm deep, EDL for slate and other low-profile coverings, and EDJ for a recessed installation that sets the window deeper into the roof for a flatter look. Each flashing is then sized to the window, so an EDW comes as an EDW MK04 to suit an MK04 window. On the plain concrete interlocking tiles and the natural slate you find across the south Essex roofscape, EDW and EDL between them cover most jobs.

Under the flashing sit two more coded items worth knowing. A BDX kit is the insulation collar and underfelt that seal the frame into the timber before the flashing goes on. A BFX is the underfelt collar and drainage gutter on its own. Both are sized to the window in the same way. When two or more windows go in side by side or one above another, the flashing changes again to a combination kit, coded EKW or EKL with a number for how many windows it joins. None of this is guesswork once you have the window size code, because every accessory keys off it.

Blinds and shutters follow the same rule. A blackout blind, a roller blind or an external awning is all ordered against the size code, so the MK04 you read off the plate is the MK04 you type into the blind order. The one thing to double check is the generation, because the mounting brackets changed over the years, and the manufacturing date on the data plate settles which fitting a blind needs.

Reading the code is the easy part. Deciding what to do with it is where it pays to think about the roof rather than just the label. If you are replacing a window that has failed, misted between the panes or is simply past its service life, the size code gives you a like-for-like frame, and the structural opening usually needs no change. That keeps the work quick and the roof covering largely undisturbed.

The moment worth pausing on is orientation. A window being replaced on a south or west-facing slope is a chance to correct the glazing that a previous installer never specified for the aspect. The same opening, the same size code, a better pane. On a north-facing slope the priority flips towards daylight and winter warmth, and the higher-transmission glass is the right call. A roof with windows on more than one aspect should not carry the same glazing code on every unit, and anyone quoting a single specification across the whole roof has not looked at which way each slope faces.

If your window is a genuinely old-number unit, or the plate is unreadable, or you are adding a window rather than swapping one, the code stops being a shortcut and the survey takes over. Size, pitch, roof covering, the rafters either side of the opening and the aspect all feed into the specification. This is the ground covered in detail on the VELUX window installation service, and it is why a measured survey beats a code copied off a plate when anything about the opening is changing. For a like-for-like swap on a pitched roof, the code alone will usually do.

Copy the full string down before you do anything: type, size and glazing code, plus the date off the plate. That single line of characters is enough to price a matching unit, order the flashing, specify a blind and settle whether the glass is right for the way the roof faces. If you would rather have someone read the roof as well as the label, send us the code and your postcode and we will work out what belongs in that opening, aspect included.