VELUX window sizes and codes explained

Each code has two parts. The first part, the letters, identifies the width of the frame. The second part, the two digits, identifies the height. So MK04 and MK06 are the same width and different heights, and CK04 and MK04 are the same height and different widths.

The digits are a label, not a measurement. You cannot read 04 as four of anything, and the step from one number to the next does not follow a rule you could work out on the back of an envelope. The size of each code is simply listed in the manufacturer's range, which is why people keep a table to hand.

The code describes the size of the window frame. It does not describe the opening type, which is shown by a separate part of the product name. That is the part with letters such as GGL, GGU, GPL and GPU, and it tells you how the window opens and what the sash is made of.

GGL

Centre-pivot, white-painted pine.

GGU

Centre-pivot, white polyurethane.

GPL

Top-hung, white-painted pine.

GPU

Top-hung, white polyurethane.

A full product reference therefore reads as an opening type, then a size code, so a centre-pivot pine window in the MK04 size is a GGL MK04. Our answer to what VELUX GGL means goes through the first half of that reference.

The VELUX range is wide, but a handful of sizes cover most houses. The sizes below are frame sizes, width by height, in centimetres. They are the nominal figures in the manufacturer's range, so check the current listing before you order.

That table is a sample, not the full range. There are narrower, wider, shorter and taller sizes, and some are made in only certain opening types. If your window is not in the table, do not guess: find the code on the window itself, as described below, or send us a photo of the label.

If you are replacing a window, the quickest route to the right size is the label on the old one. The label sits on the frame, usually on the inner edge of the sash or the side of the frame, and you may have to open the window fully to see it. It carries the product type and size code, along with a date. Photograph it, and send us the photo.

If the label has gone or is unreadable, the frame can be measured instead. Measure the outer width and outer height of the frame, not the glass, then compare the figures with the range. Note, too, that an older window may have been made in a size that is no longer produced. In that case there is a nearest current size, and what that means for the opening is covered in our short answer on how to find your VELUX size code.

When a nearby size is chosen, the flashing, the lining and often the roof covering around the frame change with it. That is why we confirm the size at the survey, with the roof in front of us, and do not leave it to the order form.

The size you can fit is limited by the roof as much as by the room. There are four things to check.

The rafters

A roof window sits between rafters, and the usual positions are set by their spacing. A window that fits the gap exactly needs no structural change. A wider window means cutting a rafter, or more than one, and trimming around the opening with new timbers. That takes time and may need an engineer's input, so the cheapest route is often to choose a width that works with the roof you have.

The pitch and the height of the opening

A tall window on a shallow roof sits further up the slope than the room can easily use, and a short window on a steep roof leaves a lot of wall. Where the sill sits matters for how the window works and, in a loft room, for whether you can see out of it from where you sit. We look at where the sill should land in a loft room in our guide to planning a loft room around VELUX windows.

The ceiling

On a sloping ceiling, the window is lined directly. Under a flat ceiling with a loft above, the lining becomes a shaft, and a wide window means a wide shaft with a lot of surface to insulate and finish.

The covering

The roof covering around the frame has to be cut and relaid, and a bigger window means more of it. With slate that is fairly easy. With interlocking tile, the width has to work with the tile width, or you end up with cut tiles beside the frame.

A bigger window lets in more daylight, but only up to a point, and the point is set by the room. A common rule of thumb in loft rooms is that the total glazed area should be a fair proportion of the floor area, and a small room with one wide window can feel brighter than a big room with a window too small for it. Rather than lean on a rule, think about what the room is for.

• A bedroom needs enough glass for morning light and enough opening for ventilation, but it also needs a sill low enough to reach the handle.
• A study or a landing needs steady daylight, and a taller window pushes it further into the room.
• A bathroom often does better with one smaller window than a large one, since privacy and heat loss both matter.
• A stairwell is the place for the narrowest size, because the opening is more about daylight on the stairs than about the room.

Two smaller windows side by side often look better and work better than one very large one. They fit between standard rafters, the ceiling opening can be lined as a single wide splayed reveal, and the cost of structural trimming is lower. For loft conversions in particular, our guide to roof windows for loft conversions goes into the layout in more depth.

The frame size is a roof measurement. What you see from below depends on how the lining is formed, and that depends on the depth of the roof build between the frame and the ceiling.

On a sloping loft ceiling the lining is short, so the opening you see is close to the frame size and the choice of window is very nearly the choice of how big the opening looks. Under a flat ceiling with a deep shaft, a splayed lining can make the ceiling opening noticeably wider than the frame, so a smaller window can read as a generous one. In both cases the plaster, the bead at each corner and the paint line are what turn the frame into a finished edge.

That is why a size is never settled in isolation. On a Georgian ceiling in Exeter or a cob cottage near Crediton, a wider window might mean cutting more lath and plaster than the room can tolerate, and a slightly smaller window with a splayed reveal can do the job with less damage. On a plasterboard ceiling in a 1930s semi or a Cranbrook new build, the wider choice is usually simple. We write the chosen size, the lining shape and the finish into the finish schedule that comes with the quote, so the opening you see after the work is the one you agreed to.

Where a window is being replaced, the simplest outcome is a new window of the same size code in the same opening. The frame lands where the old one was, the rafters need no change, and the flashing kit is the one for that size and your roof covering. The ceiling opening stays as it was, so the lining often needs only a clean edge and a fresh paint line.

It does not always work that way. Older windows may have been made in a size that has since been dropped, and the nearest current size can be a few centimetres wider or taller. A few centimetres sounds small, but it moves the frame onto a different set of courses, and the roof covering around it has to be cut back and relaid. If the new size is narrower, the gap beside it needs filling and finishing, inside and out. Going the other way, to a wider window, can mean cutting a rafter. Our short answer on whether a VELUX window can be replaced like for like sets out when the swap is straightforward.

The type of opening is a separate decision from the size. You can keep the size and change from a top-hung window to a centre-pivot one, or the other way, though the handle position and the clearance at the head change with it.

A few errors come up again and again, and all of them are easy to avoid once you know them.

The first is measuring the glass and not the frame. The glass is smaller than the frame by the width of the sash, so a measurement taken through the pane gives a size that is too small and points to the wrong code.

The second is choosing the biggest window that fits without asking what it does to the room. A window that is too large for its ceiling can put glare in the room at the wrong time of day, and a tall window on a low roof can leave the sill at an awkward height.

The third is deciding the size before anyone has looked at the ceiling. A size that suits the roof can still be wrong for a lath and plaster ceiling or a deep roof build, because the lining and the plaster have to be finished around it.

The fourth is assuming that the same code means the same window. It means the same frame size. Glazing, opening type, finish and extras vary on top of that, and the choices are laid out in our guide to VELUX glazing options.

Size has a small part to play in the rules. Rooflights on a house are usually permitted development, provided they project no more than 150mm beyond the roof plane and sit no higher than the highest part of the roof. Neither of those limits is about the frame size, so a larger window is not in itself a planning problem. Conservation areas, listed buildings and national park land are different, and the local planning authority is the place to check.

Building Regulations are more concerned with size in one case: a window meant to serve as an escape from a loft room. That is governed by Approved Document B, which sets size and position rules, so the frame size needs to be chosen with that in mind. Our answer on whether a roof window can be a fire escape covers it. A new window in an existing roof is also notifiable building work, and we handle the Building Control notification.

Before you settle on a code, work through this list. It takes ten minutes and saves most of the trouble that comes later.

1. Measure the rafter gaps in the loft or from inside the roof, and note which window widths fit between them.
2. Decide where the sill should sit relative to the floor, and what you want to see from a chair or the bed.
3. Check the roof covering, and whether a wider window will land on whole tiles or cut ones.
4. Think about the ceiling and how deep the lining will run, since a deep shaft changes how big the window feels.
5. Photograph any label on an existing window, and the roof from outside.

With those to hand, we can confirm the size at the survey, order the right kit and agree the finish before the roof is opened. For the wider picture of what we fit and how, see our VELUX window installation page, then ask for a quote with your photos and measurements and we will write the size into your finish schedule.