Do roof windows come in standard sizes?

Why standard sizes are worth using

They fit between rafters. The narrower standard sizes are dimensioned for typical rafter and truss centres, so no structural work is needed. That is the single largest saving available on this job.

Availability. Stock sizes are often available in days. Bespoke is weeks.

Price. Substantially cheaper, because there is no special fabrication.

Future replacement. A standard size today is a standard size in twenty years. A bespoke unit means a bespoke replacement.

Bespoke sizes

Available from most manufacturers within structural limits, and the legitimate reasons are retrofit ones: an existing opening that does not match anything current, a listed building where the opening cannot be altered, or an architectural requirement for a specific proportion.

Expect a premium and a longer lead time.

Obsolete sizes

The common problem in Colchester’s older loft conversions. A window fitted in the 1980s or 1990s may be a size no longer made.

Routes: nearest current size with minor carpentry to adapt the opening; another manufacturer’s unit matching the old dimension; or reframing to a current size. The last is more work now and standard for the next thirty years.

The practical advice

On a new opening, design to a standard size and set the opening out to suit. On a replacement, find the code on the data plate first, because it usually answers the question without anyone visiting.

Yes, and the codes are worth understanding

Every mainstream manufacturer works to a range of standard sizes identified by a short code, usually a letter and a number, that appears on the data plate of every window they have ever made.

The code identifies the structural opening the window needs, not the glass area or the internal reveal. That is why we ask for a photograph of the plate rather than a measurement: the number you can take from inside the room is not the number the manufacturer works from.

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.

The future. A standard opening is one that can be filled again in twenty five years without a special order.

When your size is no longer made

Ranges change over the decades, and a window fitted in the 1980s or 1990s is quite likely to be a size nobody makes now. This is normal rather than a problem, and there are three routes.

Adapt the opening to the nearest current size. Current sizes are frequently within 40mm or 60mm of an old one, so adapting 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. This is usually the right answer.

Find another manufacturer at the old dimension. Worth it where the opening is genuinely awkward to alter, for instance tight against a purlin or a party wall.

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

What nobody should do is force a unit into an opening it does not fit and pack the difference. It survives about a year before the packing settles, the frame goes out of square and the sash stops closing properly. If a quotation is noticeably cheaper than the others, this is sometimes why.

Made to measure

Genuinely necessary in some situations: a listed building where the position is constrained by historic timber, an awkward structural opening that cannot be altered, or a conservation unit in a non standard proportion.

It costs more, it takes several weeks, and it is the right answer when the alternative is compromising the installation.

Choosing a size for a new opening

Start from the structure rather than the catalogue. On a trussed rafter roof, a unit narrow enough to sit between two trusses at 600mm centres needs no structural work at all, and anything wider needs a designed trimmer.

That constraint frequently produces two or three realistic options rather than a free choice, which is a better conversation than starting from a size you liked.

Yes, and staying within them is worth real money

Every mainstream manufacturer publishes a range of stock sizes, identified by a short letter and number code on the data plate. The letter denotes a width group and the number a height within it.

Crucially, the code describes the structural opening, not the glass and not the internal reveal. That is why a supplier asks for a photograph of the plate rather than a measurement.

What staying standard buys you

Availability. Stock sizes are held and arrive in days. Made to measure runs to several weeks.

Price. 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 clip in blackout blind matched to a stock size fits exactly and costs a fraction of a made to measure blind on the reveal.

The future. A standard opening can be filled again in twenty five years without a special order.

Where made to measure is genuinely needed

Where historic timber constrains the position and the clear span is whatever the carpenter left. Where an opening cannot be altered because of a purlin, a steel or a party wall. Where a conservation officer has conditioned the proportion. And on lanterns and large flat rooflights, which are bespoke as a matter of course.

The one to avoid

Paying a bespoke premium and waiting weeks to avoid adapting an opening by 50mm on a rear slope, where the difference would be invisible from anywhere and the reveal is being replastered regardless.