Standard sizes and made to measure units

Stock sizes

Manufacturers hold a range of common dimensions, typically in 100mm or 200mm steps from about 600 by 600 up to around 2000 by 3000.

They are cheaper, because there is no bespoke fabrication, and they are quicker, often available in days rather than weeks. On a new extension, where the opening is being built anyway, designing the kerb around a stock size is free money: you get the same rooflight for less and you get it sooner.

Made to measure

Any dimension within the manufacturer’s structural limits. More expensive and slower, typically two to four weeks.

The reason it exists is retrofit. Where an opening is already formed, or a kerb already built, or a failed unit is being replaced into an existing aperture, the unit has to suit what is there. Adjusting a structural opening is far more expensive than paying for a bespoke unit.

The decision, put simply

Building new: pick a stock size and build the kerb to it. Replacing or retrofitting: measure what is there and order to suit.

The mistake is committing to a bespoke size on a new build because a drawing showed a round number, when moving the kerb 40mm would have hit a stock size.

Structural limits

Made to measure is not unlimited. Every system has a maximum pane size governed by glass thickness, wind load and snow load. Past it, the answer is two units with a structural bar between them rather than one larger pane.

Which is often better anyway: two rooflights distribute light across a long room more evenly than one, and a failed pane is cheaper to replace.

Lead time and sequence

Made to measure units are ordered from a measured kerb, not from a drawing. That means the kerb is built first, then measured, then the unit ordered. Building that fortnight into the programme avoids the alternative, which is a unit that does not fit.

What you gain by staying standard

Four things, and together they make a strong case for adapting an opening to fit a stock size rather than ordering a unit to fit the opening.

Availability. Stock sizes are held in the country and arrive in days. Made to measure runs to several weeks, and longer for an unusual proportion or a specialist product.

On a job with a fixed date, a builder’s programme or a room that needs to be usable, that is frequently decisive.

Price. A stock size is meaningfully cheaper than the same window made to order, and the gap widens on specialist products.

Accessories. Flashing kits, blinds and insulation collars are all size specific and made for the standard range.

That last one is underrated. A clip in blackout blind matched to a stock size fits exactly, goes on without tools, runs in side channels so light does not leak past the edges, and costs far less than a made to measure blind fitted to a plaster reveal.

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

What made to measure gets you

A unit that fits an opening you cannot or should not alter, and that is a real requirement in specific situations.

Where historic timber constrains the position. On a listed timber framed building the frame is frequently part of what is listed, so an opening is placed to avoid original rafters and purlins. The available clear span is whatever it is, and it rarely matches a catalogue size.

Where an existing opening genuinely cannot be altered. Tight against a purlin, a steel, a party wall, or a structural member that cannot be cut.

Where a conservation officer has conditioned the proportion, for instance to match a surviving unit elsewhere on the building.

Where a design calls for a size no manufacturer makes, which on lanterns and large flat rooflights is normal rather than exceptional: those are made to your dimensions as a matter of course.

Where made to measure is not necessary

Where a standard size would fit and the difference is preference rather than requirement.

Current sizes are frequently within 40mm or 60mm of an old one, and adapting the opening means packing out or trimming back and making good the reveal. Since the reveal is being replastered on a replacement regardless, the additional work is modest and it is invisible when finished.

On a rear slope, a difference of 60mm either way is not visible from anywhere, and paying a bespoke premium plus several weeks of lead time to avoid it is not good value.

How size codes work

Every mainstream roof window carries a data plate with a short code identifying the range and the size, usually on the top edge of the sash or on the frame beside the hinge.

Manufacturers use their own systems, most working on a letter and number arrangement where the letter denotes a width group and the number a height. Codes are comparable within a range and not across manufacturers.

The critical point: the code describes the structural opening, not the glass and not the internal reveal. That is why we ask for a photograph of the plate rather than a measurement, because the dimension the manufacturer works from is behind the lining and under the covering.

Photograph it rather than transcribing it. The characters are small, several are easily confused, and a mistyped code buys the wrong window.

Obsolete sizes

Ranges are revised every decade or so. A sealed unit lasts fifteen to twenty five years. So most replacements arrive after the original size went out of production, and on anything fitted before about 2000 you should expect it.

Three routes, in order of preference.

Adapt the opening to the nearest current size. Usually within 40mm or 60mm. Invisible when finished, and it leaves a standard size for the next thirty years.

Find another manufacturer at the old dimension. Ranges overlap, and a size one maker has dropped may be current with another. Worth it where the opening is genuinely awkward to alter, at the cost of a mixed set of windows on one roof.

Reframe to a current size. More work now, and right where several windows are being standardised across a slope or where the timber needs replacing anyway.

What never to do

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

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.

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

Choosing a size for a new opening

Start from the structure rather than the catalogue.

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.

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

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 want has to be measured rather than assumed.

Then consider going taller rather than wider. Lengthening 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.

Lead times, and the sequence

Stock size: days to a couple of weeks.

Made to measure roof window: several weeks from order.

Conservation rooflight in a non standard size: several weeks, and longer for an unusual proportion.

Lantern: several weeks minimum, and considerably longer for a large or unusual configuration.

Where consent is needed, that comes first and runs to months. Where structural design is needed, the engineer’s drawing comes before the order because the opening dimensions depend on it.

Survey, permission, structural design, order, then book the fitting to the confirmed delivery date. Booking the fitting first and hoping the unit arrives is how jobs slip.

What to record afterwards

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

In twenty five years somebody will be standing in the room with a torch trying to read a plate at height, and a piece of paper saves them a survey.

The lead time difference, which frequently decides it

A stock size roof window is held in the country and arrives in days. A made to measure unit runs to several weeks, and longer for an unusual proportion or a specialist product.

On a job with a fixed date, a builder programme, or a room that needs to be usable, that gap is frequently the whole argument.

It also affects risk. A stock size that turns out to be wrong can be exchanged. A bespoke unit cannot be returned and has no resale value.

The twenty five year argument

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

That is a genuine gift to whoever deals with it next, and frequently to you, because sealed units last fifteen to twenty five years and somebody will be standing under this window with the same question.

A bespoke opening means every future replacement is bespoke too, along with every blind and every flashing kit.

Where made to measure is genuinely required

Historic structure that cannot be altered. On a listed timber framed building the frame may be part of what is listed, so the opening is placed to avoid original rafters and purlins and the available clear span is whatever it is.

An opening constrained by something immovable. Tight against a purlin, a steel, or a party wall.

A conditioned proportion, where a conservation officer has required the unit to match a surviving original elsewhere on the building.

Lanterns and large flat rooflights, which are bespoke as a matter of course because terrace and extension openings are never a catalogue size.

Where it is not required and gets bought anyway

Where a standard size would fit and the difference is preference.

Current sizes are frequently within 40mm or 60mm of an old one. Closing that means packing out or trimming back and making good the reveal, which is being replastered anyway because the plaster comes off at the frame regardless.

On a rear slope, 60mm either way is invisible from anywhere. Paying a bespoke premium and waiting weeks to avoid it is poor value.

The accessories point

Blinds, flashing kits and insulation collars are all size specific and made for the standard range.

A clip in blackout blind matched to a stock size fits exactly, goes on without tools, runs in side channels so light does not leak past the edges, and costs a fraction of a made to measure blind fitted to a plaster reveal.