Roof window sizes and how they are coded

What the code refers to

The outer frame dimension. That is the number that has to fit the structural opening, and it is what a replacement has to match.

Common sizes exist because they suit standard rafter spacings. A window sized to fit between rafters at typical centres needs no structural work at all, which is the cheapest version of this job.

Glass area is smaller

Worth knowing when estimating light. The frame, sash and glazing bead all take width, so the visible glass is meaningfully smaller than the code suggests, typically 120mm to 180mm less in each direction.

On a small window that is a large proportion. Two small windows give considerably less glass than one window of the combined size, because you pay the frame penalty twice.

Where to find the code

On a data plate on the unit, usually visible on the top edge of the sash or on the frame beside the hinge once the window is opened. Photograph it and a replacement can usually be priced without a visit.

Where the plate is missing or unreadable, the opening is measured instead, which means a survey.

Obsolete sizes

Size ranges change over the decades, and windows fitted in the 1980s and 1990s are often sizes no longer made. That comes up constantly on Colchester’s older loft conversions.

Three routes: the nearest current size with minor carpentry to adapt the opening, another manufacturer’s unit that matches the old dimension, or reframing the opening to a current size. The third is more work but leaves you standard for the next thirty years.

Choosing a size for a new opening

Rafter spacing first, because staying within one bay avoids structural work entirely. Then cill height, because that decides whether you can see out. Then glass area.

What the code on the plate means

Every mainstream roof window carries a data plate with a short code identifying the range and the size. It is usually on the top edge of the sash or on the frame beside the hinge, and on a centre pivot window rotating the sash brings it into easy view.

That code is the single most useful piece of information about your window, and it is worth photographing rather than transcribing. The characters are small, several are easily confused, and a mistyped code buys the wrong window.

Manufacturers use their own systems. Most work on a letter and number arrangement where a letter denotes a width group and a number a height, so codes within a range are comparable with each other but not across manufacturers.

The critical point: it describes the structural opening

Not the glass, not the frame, and not the internal reveal.

That is why we ask for a photograph of the plate rather than a measurement. The dimension a manufacturer works from is the size of the hole in the roof, and that is behind the lining and under the covering where you cannot get at it.

Measuring the visible glass gives a number. Measuring the plaster reveal gives a different number. Neither is the structural opening, and ordering from either produces a window that does not fit.

The three dimensions, in order

The structural opening, which the size code describes and which the roof has to accommodate.

The frame size, slightly smaller, sitting within that opening with a tolerance for packing and squaring. That tolerance is deliberate: a frame forced into an opening with no room to adjust cannot be set square, and a frame that is not square gives a sash that catches at one corner for the rest of its life.

The visible glass, smaller again, and on a modest unit a surprisingly large proportion of the opening is frame rather than glass.

That last point is worth remembering when comparing one large unit against two small ones. Frame width is roughly constant regardless of size, so two small windows do not deliver the same glass area as one of twice the size, even though the structural openings add up.

Why standard sizes matter

Availability. Stock sizes are held and can be delivered in days. Made to measure runs to several weeks.

Price. A stock size is meaningfully cheaper than the same window made to order.

Accessories. Flashing kits, blinds and insulation collars are all size specific and made for the standard range. A bespoke unit frequently means bespoke or reveal fitted accessories too, and a blind fitted to a plaster reveal costs more, never sits as neatly and lacks the side channels that make a blackout blind genuinely black out.

The future. A standard opening is one that can be filled again in twenty five years without a special order. That is a real gift to whoever deals with it next, and frequently to you.

When the size has been discontinued

Ranges are revised every decade or so: sizes rationalised, profiles slimmed, occasionally a whole range withdrawn. On anything fitted before about 2000 you should expect the size to be obsolete.

It is normal rather than unlucky, and it is entirely solvable. Three routes.

Adapt the opening to the nearest current size. Usually within 40mm or 60mm, so it means packing out or trimming back and making good the reveal. Invisible when finished, and it leaves a standard size for the next thirty years. Usually the right answer, particularly since the reveal is being replastered anyway on a replacement.

Find another manufacturer at the old dimension. Ranges overlap, so a size discontinued by one may be current with another. Worth it where the opening is genuinely awkward to alter: tight against a purlin, a steel, a party wall or a historic timber that cannot be cut.

The trade is a mixed set of windows on one roof, which on a rear slope is irrelevant and on a front elevation may not be.

Reframe to a current size. More work now, and right where several windows on a slope are being standardised, or where the timber needs replacing anyway. If decayed trimmers are coming out regardless, reframing costs very little more.

What never to do

Force a modern unit into an old opening and pack the difference.

It survives about a year. The packing settles, the frame goes out of square, the sash stops closing evenly against the compression gasket, and water gets past. By then the covering is back on and nobody can see what was done.

Where a quotation for a replacement with an obsolete size is noticeably cheaper than the others, it is worth asking directly how the size difference is being dealt with.

Where the plate has gone

Painted over, weathered away, or removed at some point. Then it comes down to measurement, and that means a survey rather than a phone call.

A photograph of the window from inside and outside still helps a great deal. The profile of the frame, the hinge arrangement, the flashing type and the cladding colour frequently identify the range even without a plate, and narrowing it to two candidates makes the survey quicker.

Sizing a new opening

Start from the structure rather than the catalogue, because that is what constrains you.

On a trussed rafter roof, trusses are typically at 600mm centres. A unit narrow enough to sit between two of them, touching neither, needs no structural work at all. Anything wider needs a designed trimmer and an engineer, and that is where the price steps up.

That constraint is why so many rooflights on modern estates are tall rather than wide, and it frequently produces two or three realistic options rather than a free choice.

On a traditional cut roof, trimming is ordinary carpentry so the constraint is looser, but rafter spacings on older buildings are irregular and the clear span at the position you actually want has to be measured.

Then think about going taller rather than wider. Lengthening an opening up the slope does not cut rafters, so a tall narrow unit frequently delivers what a wide one was being asked for at a fraction of the cost.

What to record

The manufacturer, model, size code and glazing specification of whatever goes in, with the house paperwork rather than in a drawer.

In twenty five years somebody will be standing in this room with a torch trying to read a plate at height, and a piece of paper saves them a survey. We keep our own record of what we fitted for the same reason.

The structure of a size code

A short combination of a letter and a number. The letter denotes a width group, the number a height within that group.

That structure means two windows sharing a letter share a width, and two sharing a number share a height, which makes the range quick to work with once you know it. A supplier hearing a code knows the structural opening immediately without measuring anything.

Alongside it sits the model designation, a longer string identifying the operation, the glazing specification, the frame material and the product generation.

The critical point about what is being described

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

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

This is where most confusion comes from. Somebody measures the glass, orders a window to that dimension, and receives something considerably too small. The frame occupies space, the lining occupies space, and the gap between what you can see and what the roof holds is substantial.

Codes do not translate between manufacturers

Every major maker uses its own system, developed independently. Similarities between them are coincidental, and treating a code from one range as meaningful in another is a trap.

Conversion between brands goes through actual millimetre dimensions. Any competent supplier will do it, and it is worth having confirmed in writing before ordering, because a cross brand assumption that turns out wrong is discovered with the roof open.

Why staying within the standard range pays

Stock sizes arrive in days rather than weeks. They cost less. And every accessory, blinds, flashing kits and insulation collars, is size specific and made for the standard range.

That last point matters for twenty five years. A clip in blackout blind matched to a stock size fits exactly, goes on without tools, and costs a fraction of a made to measure blind fitted to a reveal.

And a standard opening can be filled again in a quarter of a century without a special order.

Reading the plate

Photograph it rather than transcribing it. The characters are small, frequently faded after twenty years of sunlight, and several are easily confused. A mistyped code buys the wrong window.

Then record the manufacturer, model, code and glazing specification with the house paperwork, so the next replacement starts from a document rather than a ladder.