Matching a replacement to an obsolete size

Adapt the opening to the nearest current size

Usually the best answer. Current sizes are frequently within 40mm or 60mm of an old one, and adapting means packing out or trimming back the opening, then making good the reveal.

Invisible when finished, and it leaves a standard size for the next thirty years so the following replacement is simple.

Find another manufacturer at the old dimension

Ranges differ, and another maker sometimes still produces something at or very near the original size.

Worth it where the opening is genuinely awkward to alter, for example tight against a purlin, a steel or a party wall. The trade is a mixed set of windows on one roof.

Reframe to a current size

Alter the structural opening properly. More work now, and on a trussed roof it needs care about what is being cut.

Right where several windows are being done together and standardising makes sense, or where the timber around the opening needs replacing anyway.

What we will not do

Force a unit in and pack the difference with foam and sealant. 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 often why.

Do them together

Where a loft has three or four of the same obsolete size, one visit shares the access cost and standardises the whole roof.

Three routes, and one thing never to do

Size ranges change over the decades, and a window fitted in the 1980s or 1990s is quite likely to be a size nobody makes now. It is normal rather than a problem, and it is entirely solvable.

1. Adapt the opening

Current sizes are frequently within 40mm or 60mm of an old one, so adapting means packing out or trimming back the opening and making good the reveal.

Invisible when finished, and it leaves you a standard size for the next thirty years, which matters because the next person replacing it will thank you. This is usually the right answer.

2. Find another manufacturer at the old dimension

Several makers overlap, and a size discontinued by one may still be current with another. Worth it where the opening is genuinely awkward to alter, for instance tight against a purlin, a steel or a party wall.

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

3. 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. If we are cutting out decayed trimmers regardless, reframing to a standard size costs very little more.

What never to do

Force a unit into an opening it does not fit 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 gasket, and water gets past. By then the covering is back on and nobody can see what was done.

Where a quotation is noticeably cheaper than the others on a replacement with an obsolete size, this is sometimes why.

How we establish it

From a photograph of the data plate, before anybody visits. The size code tells us what the structural opening is, which is not a dimension you can take from inside the room.

Where the plate is painted over, worn away or missing, the opening is measured instead, and that means a survey rather than a phone call.

The upgrade that comes with it

A unit fitted twenty five years ago has no low emissivity coating, no argon, an aluminium spacer and no solar control. The replacement has all four as standard, so the room will be warmer, cooler in summer and quieter in rain without paying extra for any of it.

Establishing what you have

From the data plate on the top edge of the sash or the frame beside the hinge. Rotate a centre pivot sash to bring it down to eye level, and photograph the plate close up with the flash on rather than transcribing it.

A supplier will tell you within a minute whether that size is still made. Where the plate is unreadable, a survey establishes the structural opening directly, and on a job of any value that half hour removes all the risk.

The three routes, ranked

Adapt the opening to the nearest current size. Usually the right answer. Current sizes are frequently within 40mm or 60mm of a discontinued one, and closing that gap means packing out or trimming back and making good the reveal, which is being replastered anyway. Invisible when finished, and it leaves a standard opening for the next thirty years.

Find another manufacturer at the original dimension. Ranges overlap without matching, so a dropped size may be current elsewhere. Worth it for a single window in a difficult opening. Less good on a slope with three matching windows already, because frame proportions differ enough between makers to be noticed from inside.

Reframe to a current size. Right where several windows are being standardised at once, or where the surrounding timber needs replacing anyway.

The thing that must not happen

A modern unit forced into an old opening with the difference packed loosely. The packing settles, the frame goes out of square, the sash stops meeting its gasket evenly, and it draughts and then leaks, usually within two winters.

Where a quotation for a known obsolete size is unexpectedly cheap, ask specifically how the difference is being handled.