Roof window sizes and how they are coded

The code is two dimensions in centimetres

Take a code apart and it resolves into a width and a height, both in whole centimetres, both describing the outer frame of the window. Every other number attached to a roof window, the structural opening, the glazed area, the clear openable area, is derived from those two figures by a published offset.

The convention is old and it survives because it works at the merchant’s counter. Two characters and a number get a unit, a flashing kit and a blind out of a warehouse without a drawing. It is only when a homeowner tries to reverse-engineer a code that the shorthand becomes obscure, and even then the answer is on the manufacturer’s size chart.

The width letters

On the VELUX system the first character is the width group, and there are six in common use. C is 55cm, F is 66cm, M is 78cm, P is 94cm, S is 114cm and U is 134cm. Those six numbers are not arbitrary. They are chosen to drop between rafters at the spacings British roofs are actually built to.

The second character in a code such as MK08 belongs to the size series rather than to a dimension, which is why almost every current size code carries a K in that position and why the letter can be ignored when reading the size. Confirm against the current chart before ordering, because ranges are revised.

The height numbers

The two digits give the height, again in centimetres, through a lookup rather than directly. In the common range, 02 is 78cm, 04 is 98cm, 06 is 118cm, 08 is 140cm and 10 is 160cm. There are shorter and taller codes at either end of the range.

The heights are less closely tied to structure than the widths, because a roof window can be made taller without touching a rafter. What limits height instead is the slope: a taller unit on a steep pitch pushes its head close to the ridge and its cill low over the floor, and both of those become the practical constraint before the catalogue does.

Reading a full code

A full product code carries more than the size. A typical string names the mechanism and glazing before the size, so a code will identify a centre-pivot or top-hung unit, a pine or polyurethane finish, a glazing specification, and then the size group. The size is only the tail of it.

When you are asked for the code of an existing unit, the whole string is what is wanted, not just the size. The glazing designation tells a supplier which sealed unit fits, and the frame designation tells them which blind and which flashing generation apply. Photograph the data plate rather than transcribing it.

The code is not the hole you cut

This is the single most consequential point on the page. The size code describes the outer frame. The structural opening, the hole in the roof between the trimmed rafters and trimmers, is larger, typically by 40 to 60mm on the width and a similar amount on the height depending on the range.

That allowance exists so the frame can be packed, levelled and squared in an opening that is never perfectly true, and so the insulation collar has somewhere to go. Cut the opening to the frame size and there is nowhere to pack, nowhere to insulate and no tolerance for a rafter that is 8mm out of plumb. Every manufacturer publishes the structural opening figure for each size code, and that is the figure a carpenter works to.

Glazed area, and the number that reaches the room

Neither the code nor the structural opening tells you how much daylight arrives. That is the glazed area, which is the pane inside the sash rebate, and it is smaller again than the frame. On a mid-size unit the frame and sash together account for roughly 100 to 130mm off the width and a similar amount off the height.

Two units with the same size code but different mechanisms can differ here as well, because a top-hung sash section and a centre-pivot sash section are not identical. If daylight is the reason for the job, compare published glazed areas rather than frame sizes.

Rafter spacing decides the width first

The width letters exist because rafters do. British roofs are commonly built at 400mm or 600mm centres, sometimes 450mm, and a roof window that fits between two rafters without cutting one is a fundamentally simpler installation than one that does not.

Work it out from the clear gap. Rafters at 600mm centres with 50mm rafters give a 550mm clear bay, which suits a 550mm frame with the trimming allowance eating into the rafter faces, or comfortably suits a 550mm unit where the rafters are packed. Anything wider means cutting a rafter and installing trimmers above and below to carry the load around the opening, which is a structural operation and needs to be designed rather than improvised.

When a size change becomes a structural job

Cutting one rafter in a traditional cut roof is routine, and doubled trimmers with the cut rafter hung off them is standard practice. Cutting two rafters starts to want a sizeable timber trimmer or a steel, and it wants somebody to size it.

In a trussed rafter roof the calculation is different again, because a truss is a designed structural element and cutting any member of one invalidates the design. That does not mean it cannot be done. It means it needs an engineer’s detail, and it means the price is a different price. Going up a size on an existing opening is covered on can you replace a roof window with a bigger one.

Manufacturers do not share a code system

There is no common standard. A code from one maker will not find a product from another, and there is no reliable cross-reference table that works in both directions, because the actual dimensions differ by a few centimetres even where the codes appear to correspond.

What does translate is millimetres. Give a supplier a frame width and height in millimetres and they can find the nearest size in any range. That is the safe currency for any conversation across brands, and it is what we use when we are matching a replacement to an existing opening.

Fakro and Keylite numbering

Fakro uses a two-digit size number in place of the letter and digits, so 01 through to the mid teens each map to a width and height pair on a chart. Keylite uses a comparable numeric system of its own. In both cases the number is a lookup with no internal logic, so it has to be read off a table.

Roto uses its own scheme again, with dimension figures embedded in the product string. None of this is difficult, but it does mean that “size 06” is a meaningless instruction unless the brand is stated alongside it. We install VELUX, Fakro, Keylite and Roto units and quote in millimetres for exactly this reason.

Finding the code on a unit already in the roof

Almost every unit carries a data plate, and on a hinged roof window it is usually on the top edge of the sash, visible only when the window is open, or on the sash face behind the handle. Open the window fully and look along the head of the sash.

The plate gives the full product code and often a manufacture date. If it has faded past reading, the fallback is to measure the outer frame at the roof surface, in millimetres, width and height, and to photograph the hinge arrangement and the frame profile. Between those, a supplier can usually identify the range.

Old codes and discontinued sizes

Size ranges are revised. Sizes are added, and occasionally a size is withdrawn, which is why a twenty-year-old opening does not always have a current unit that drops straight into it. Where the nearest current size is slightly smaller, the opening is packed and lined down. Where it is slightly larger, the opening is enlarged, which is a carpentry job on the trimmers.

Either is normal work on a replacement, and neither is a reason to hunt for a discontinued product. It is a reason to have the opening measured before anything is ordered rather than after.

Two smaller units or one large one

Two units in adjacent bays give more daylight per pound than one wide unit that requires structural work, and they spread the light across a longer stretch of ceiling, which usually reads better in a long room. They also need two flashing kits, or one combination kit, and a defined gap between the frames.

One large unit gives an uninterrupted view and a single opening, which suits a room where the window is the feature. The cost curve is not linear: the jump from a bay-width unit to one requiring trimmers and a steel is where the price moves, not the extra 200mm of glass.

Height, pitch, and how much sky you see

A taller unit does not simply give more glass. It changes what the glass looks at, because a roof window’s view is a function of the pitch and where the cill sits. On a shallow roof, extra height mostly buys more sky. On a steep roof, extra height at the bottom of the unit buys ground and horizon.

So on a steep slope, dropping the cill by taking a taller unit is often worth more than it appears from the size chart. This interacts with the escape opening band and with head height, and it is worked through on roof window cill height and the view out.

Ordering the flashing and the blind to match

The size code follows the window everywhere. The flashing kit is ordered in the same size, and the blind is ordered in the same size, and neither will adapt to a neighbouring one. A blind for an MK06 will not fit an MK08 sash, and a flashing for a 780mm frame will not stretch to a 940mm one.

Where a code has been transcribed by hand at some point in the chain, this is where the error surfaces, usually on the morning the tiles are already off. We order the unit, the kit and any shading together against one code. Which flashing type goes with which covering is set out on roof window flashing kits by roof covering.

What we measure at survey

We measure the clear bay between rafters, the rafter depth, the existing frame in millimetres if one is there, the pitch, and the finished floor level. From those five figures we can name a size code, a structural opening dimension and a cill position, and say whether a rafter has to be cut.

That measurement is what turns an email range into a fixed price. We give a range by email from a description, then a fixed price after we have been on the roof, because the difference between a unit that fits a bay and one that does not is the difference between a day and three. Ring 01865 704245 or email info@heritageskylightsoxford.co.uk with the room and the roof and we can start with the range.