Should you repair or replace old sash windows?

In most cases, repair. The Society for the Protection of Ancient Buildings has a strong presumption in favour of repairing old single-glazed windows and, where needed, adding secondary glazing rather than fitting double-glazed replacements. Its technical note points out that original windows survive from the 18th century and earlier, while many replacements fitted in the last 30 years have already decayed beyond repair. SPAB's rule for listed buildings is the same: repair, or replace like for like where repair is not viable.

The decision should rest on the state of the timber, and you can begin to judge that with a screwdriver and an hour on a dry day.

Start with a survey of your own

SPAB's guidance is that a window should never be condemned on surface appearance alone. Peeling paint can sit over perfectly sound wood. Wet rot, by contrast, shows as slight ripples and discolouration in the paint, with timber underneath that feels soft and crumbly when probed. Go round each window and press gently at these points:

• the bottom rail of the lower sash and the foot of its two stiles
• the joints where glazing bars meet the rails, especially where putty has failed
• the cill, along with the joint between timber and any masonry sub-cill
• the foot of the pulley stiles and the outer lining of the box

Write down what you find for each window on a simple sheet. A joiner will price from that far more accurately, and you may discover that three of ten windows need work while the rest need paint.

Why they rot, and why it matters

Rot needs moisture, so a repair fails if the source stays. The usual culprits are water lying on a badly falling cill, capillary action between timber and masonry, broken paint and putty, and persistent condensation. SPAB's sequence for wet rot is simple: find and stop the moisture, help the area dry quickly, remove decayed timber only as far as a repair requires, then use a durable timber. If you see heavy condensation, read our note on condensation and damp first.

Dry rot is a different matter. SPAB advises it usually should not be tackled without an independent timber decay consultant.

What a joiner can actually do

Most sash repairs are splices: cut out the decayed section and let in new wood to the same profile. Bottom rails can be replaced without dismantling the whole sash, and short lengths of glazing bar can be renewed, sometimes with the glass in place. Cills can often be repaired where they sit. SPAB's note gives principles worth putting in your brief:

• remove as little original timber as the repair allows
• match the species and, as closely as possible, the grain, since mixed species move differently and joints open up
• shape joints so water runs to the outer face rather than sitting on the joint
• use pegs, dowels or non-ferrous fixings along with glue
• number the glass and parts before anything comes apart

Do not let anyone dip windows in caustic stripping tanks. SPAB rules this out because it deforms timber and weakens joints.

Glass, putty and paint

Handmade crown and cylinder glass from the mid-19th century and earlier is thin and easily broken, and SPAB singles out general building work and scaffolding as a risk. Protect it before work starts. If a pane does need replacing, its thickness matters: the sash is balanced by its weights, so a heavier pane throws that balance off. This catches people out when they think about swapping thin glass for something thicker.

Reglaze with linseed oil putty, paint it within a day or two, and take the paint about 1mm onto the glass to seal the edge. Complete stripping is seldom necessary; remove only what is loose. Old paint may contain lead, so use wet abrasive paper when rubbing down and take precautions against fumes if any paint is burnt off. If you are unsure, have it tested first. Leave the running surfaces between pulley stile and sash stile unpainted, as SPAB recommends waxing them so the window slides easily.

Draughts and heat

Draughts usually come from worn beads and gaps at the meeting rails. Beads can be reworked or replaced, timber fillets can close gaps, and draught strips can be fitted, although SPAB warns against sealing a window completely. A house still needs air, and sealing up can worsen condensation. Keep kitchen and bathroom windows free of draught-proofing for that reason.

Secondary glazing is the next step, and the Planning Portal says internal secondary glazing does not usually need planning permission. On a listed house, ask the conservation officer before fixing anything to historic joinery.

If replacement is the honest answer

Some windows are too far gone, or are poor later replacements. Then the rules matter:

Ask for like-for-like joinery in those cases: same section sizes, same glazing pattern, same depth in the wall. SPAB notes that the depth at which a frame sits greatly affects the character of a building, and a new window set flush will look wrong however well it is made.

Sources

Opened and read by the editors in November 2026. Rules and guidance change, so check the current version.

• SPAB Technical Advice Note: Repair of Wood Windows (PDF)
• Planning Portal: doors and windows, planning permission
• Planning Portal: doors and windows, building regulations
• GOV.UK: Approved Document L, conservation of fuel and power