VELUX Size Codes Explained

Take MK04, one of the most common codes. The first letter, M, stands for the width of the frame. The two digits, 04, stand for the height. The second letter, K, is part of the maker's coding for its current range and does not change how you read the size.

An M frame is 78 cm wide and a 04 frame is 98 cm high, so an MK04 is 78 cm by 98 cm. A CK02 is 55 cm by 78 cm. A UK08 is 134 cm by 140 cm. The sizes are always given as width first, then height, and they are the dimensions of the outer frame as the maker states them.

The width is the dimension across the slope, from one side of the frame to the other. The height is the dimension up the slope, from the lower edge of the frame to the upper edge. People sometimes swap the two when they measure from inside the room, so it helps to remember that a window on a roof is measured the way the roof is climbed: width across, height up.

Because the code states the frame size, two windows with the same code have the same outer dimensions even when their glazing, finish or opening style differ. That is what makes it possible to replace one model with another in the same opening.

VELUX uses a short set of width letters. Each stands for one width, and the widths step up in uneven stages, from 11 cm to 20 cm apart.

The narrow end of the range, C and F, suits tight spaces: a gap between close-set rafters, a small landing or a bathroom with little wall to spare. M is the middle width, wide enough to light a bedroom and still narrow enough to fit in most roofs without disturbing much structure. P, S and U are the wide frames. They give a lot of glass and a good view, and they need a wider opening in the roof, which brings the rafters into play.

How the width relates to the rafters is covered in the guide to roof window sizes and rafter spacing. The short form is that a window can sit between two rafters when it is narrower than the clear gap, and a wider one needs a rafter trimmed and the load carried by new timbers. The survey measures the real gap, because rafter centres vary from house to house.

The common heights run from 02 up to 10 in steps of two, and each pair stands for one height. The maker's current size list is the place to confirm which codes are made in which model.

Height has a marked effect on the room because the glass is tilted. A taller window reaches further up the slope and further down it, so it lights more of the floor and gives a longer view from a seated position. A short window of 78 cm gives a small view of sky. A tall one of 140 or 160 cm reaches from near the floor of a knee-wall room to a good way up the ceiling.

Height also feeds into the escape question for a bedroom in a bungalow roof, because the clear opening of the open window depends partly on the frame height. The code gives the frame size, and the actual clear opening is read from the maker's data for the chosen model with the sash open.

Height also bears on the roof structure. A tall window may cross a purlin or a collar tie, or sit across a joint in the rafters, and the survey checks for these before it settles on a code. Ordering a tall window from a catalogue without looking at the roof is how a window ends up not fitting its opening.

These codes give the pattern. In each case the width letter is read first, then the height digits.

An FK06 is a slim, tall window, 66 cm wide and 118 cm high, which suits a narrow gap in a roof where the height is available. A PK10 is the same idea at a larger scale: 94 cm wide and a full 160 cm high. An SK06 is wide and moderately tall, a good shape for a landscape view over a garden from a bedroom. A UK08 is at the top of the width range and covers the greatest area of the six.

The two sets of letters and digits combine freely, which is why there are so many codes in the range. Not every combination is made, and the range changes from time to time. The catalogue at the time of your order is the authority on what is available.

When you see two windows in a row on a roof, they will often share a code. A mixed pair, say a wide one beside a narrow one, can also be fitted; the maker's data shows whether a combination flashing kit suits that pairing or whether each window takes its own kit.

The code is printed on a label that VELUX calls the type sign. On its roof windows it sits on the top of the sash, in the right-hand corner, behind the ventilation flap. VELUX says every roof window it has made since 1968 carries one, and it shows the type, the size, the variant and a production code.

If the label has worn off, or the window is out of reach, the size can still be found by measuring. Measure the outer edge of the frame across and up the slope, then compare the two measurements against the tables above. A frame that measures about 78 cm across and 98 cm up is an MK04. Measurements taken from inside the room read smaller than the frame, because the lining hides the outer edges, so an outside measurement is the safer reading.

Older windows carry codes in earlier formats, and an old code may not match the current letter-and-digit pattern. In that case the measured size is the thing to go by. We take the frame size from the roof and match it to the current range, and the survey sheet records both the label as it reads and the measured size.

• Open the ventilation flap at the top of the sash and look in the right-hand corner
• Photograph the label, which makes it easy to read and to share
• If there is no label, measure the outer frame across the slope and up the slope
• Measure at the widest point in case the frame is out of square
• Note the model name, the glazing and the opening style along with the code

A photograph of the label is the quickest first step when you ask for a quote. It lets us match the window, and the survey then confirms the size on the roof.

The code states the frame. The opening in the roof is a different size, and the two are related but not equal. The opening has to be a little larger than the frame to leave room for the flashing and the fixing, and inside the room the lining is formed to suit the thickness of the roof.

This matters when you replace a window. The old frame sits in an opening made for it, and a new frame of the same code will fit that opening. A frame one size up will not fit without enlarging the opening, which means altering the roof timbers. Our answer on fitting a bigger VELUX in the same roof goes through when that is possible.

The flashing is bought to the same code and also to the roof covering. A tile roof, a slate roof and a profiled roof each need a different flashing kit, and a kit made for the wrong covering will not shed water. The answer on which VELUX flashing kit you need sets out the choice.

Blinds follow the code too. A blind is made for a particular frame size, so a blind from an MK04 fits an MK04. Fitting blinds at the same time as the window is covered in the answer on blinds at installation.

If you are replacing a window, the code is the starting point. Matching the existing code is the simplest route: the new frame is the same size as the old one, so the opening stays as it is, and the work is limited to the frame, the flashing and the lining.

Some people use replacement as a chance to change the size. A move up in height from 04 to 06 adds 20 cm of glass up the slope, and it needs the opening extended upwards. A move up in width needs more of the rafters. Either is a structural change, and it is priced and notified as one. The process for the like-for-like case is described in the guide to replacing an old VELUX window.

Two more details are worth writing down at the survey. The first is the covering the window sits in, since the flashing changes with it. The second is the position of the window in relation to the rafters, because an earlier fitter may have trimmed a rafter to suit the old frame, and that trimming sets what the opening can take now. We photograph the label, measure the frame, and enter the code, the measured size and the covering side by side on the sheet, so that anyone reading it later can see how the match was made.

In every case, the code on the quote and the survey sheet should be the same code you read from the label or the tables. That small check, done before ordering, is what prevents a window arriving a size out.