Repair or replace: how to decide
Repair, in nine cases out of ten. Original sash windows were made from slow-grown softwood that is denser and more rot-resistant than modern timber. Rot is usually confined to the bottom rail and cill and can be cut out and spliced. Replacement makes sense only when the box frame itself has failed, or when the window has already been replaced once with something poor.
What rot looks like and where it starts
Rot starts where water sits. On a sash window that means the bottom rail of the lower sash, the cill beneath it, and the feet of the pulley stiles where they meet the cill. The cause is almost always failed paint or putty: a crack lets water into the end grain, the timber stays damp behind paint that still looks sound, and the decay works outward from inside.
The early signs are easy to miss. Paint that has lifted or crazed along the bottom rail, putty that has cracked away from the glass, a dark stain at a joint, a cill that dips where water should run off. Press a screwdriver into the timber at those points. Sound wood resists; rotten wood gives, and often comes away in soft, broken fibres.
Rot found early is a small repair. Rot left for a few more winters spreads along the rail and into the joints, and the repair grows with it. That is the main reason to have windows looked at before they fail rather than after, and the free digital survey from your photographs is a reasonable first step.
What can be spliced and what can't
Splicing is the repair at the centre of sash window restoration. We cut out the decayed section back to sound timber and glue in a new piece, matched to the moulding profile and with the grain running the same way, so that it moves with the original rather than against it. Once primed and painted, a well-made splice is hard to find.
Most of the timber in a sash window can be spliced: bottom rails, the lower ends of stiles, meeting rails, glazing bars, beads, and the cill itself, which can often be part-replaced rather than renewed whole. Original sash windows were made from slow-grown softwood that is denser and more rot-resistant than modern timber, so keeping as much of it as possible is worth the effort.
When rot has run through most of a rail, or into both the rail and the stiles at a joint, the sash would end up with more new timber than old, and at that point a new sash made to the original profile is the better repair. We say which applies window by window, at the survey and in the quotation.
When the box frame is the problem
The box frame is the fixed part of the window: the pulley stiles the sashes run in, the linings either side, the head and the cill. It holds the weights and it is built into the wall, so it is the part people most fear losing. In practice it is often the most durable part of the window, because the sashes shelter it.
It fails at the bottom. Water that gets past a cracked cill or a failed joint to the masonry sits in the foot of the box and rots the lower ends of the pulley stiles and linings. Some of this can be spliced like the sashes. When the cill and both pulley stiles have rotted through, or the frame has moved with the wall so the sashes no longer run square, repair stops making sense.
That is when replacement is the honest answer: new sashes in a repaired box or, where the box itself has gone, a new box sash window made to the original section. It is less common than people expect. Most replacements are single sashes into the original box, and whole windows are the exception. Replacement sashes and windows explains the three levels.
What replacement costs you in character (and in consent)
An original sash window is a record of when the house was built: the section of the glazing bars, the shape of the horns or their absence, the putty line, often the glass itself, slightly uneven and catching the light differently from pane to pane. A replacement made with care can match the section and the pattern. It cannot bring back the original timber or glass.
Replacement also brings consent into the picture. Repair that does not change the appearance of a building does not usually need permission. Replacing windows can: on a street covered by an Article 4 direction, in a flat or maisonette, where householder permitted development rights do not apply, and almost always in a listed building, where works that affect its character need listed building consent. Our page on conservation areas, Article 4 and listed buildings sets out the detail.
The practical cost is time. A consent application needs drawings and a decision, and the work cannot start until it is granted. Repair can usually start once the quotation is agreed.
The honest answer on double glazing
Most of the cold and noise from an old sash window comes through the gaps around the sashes. So the first step is nearly always draught-proofing: brush and fin seals set into the sashes and beads, fitted from inside. It does not insulate the glass itself, but on a sound window it stops draughts and rattles completely.
If you want the glass insulated as well, slimline double glazing fits a slim sealed unit, typically 10 to 14 mm thick, into your existing sashes. Standard double glazing is 24 to 28 mm thick and needs a modern sash to carry it, which changes the sightlines, and a change of appearance like that can need planning permission. A slim unit insulates slightly less than a standard one; it is still a large improvement on single glass.
Not every sash can take a slim unit. Fine Georgian sashes are often too thin to take a unit, which the survey will show. Where no change to the window is permitted, secondary glazing on the inside is the usual answer. Replacing sound original windows in order to double glaze them is rarely necessary.
What to ask any firm that quotes you for replacement
If a firm recommends replacing your windows, a few questions are worth asking before you agree.
Has each window been assessed for repair, and can they show you where the rot is? Is the quotation itemised by window, so you can see which are being replaced and why? Will the new sashes match the original section, horns, glazing bar pattern and putty line, or will the glazing bars be applied to the face of the glass? What timber are they made from, and who makes them?
Does the work need planning permission or listed building consent, and who prepares the application? What happens to the box frame: is it being repaired, or replaced as well? What guarantee comes with the work, and is it in writing?
Ours are answered in the quotation. Our replacements are made by Barnsbury Joinery, the group's manufacturing studio, to the original profile, and fitted by the team that surveyed the window. The guarantee is set out in your quotation.
Book a survey
Send us photographs of your windows and we'll tell you what needs doing and what doesn't, with a written quotation. The digital survey is free.