How to restore sash windows

If you’re planning to improve your period property, follow experienced renovator Michael Holmes’ top tips to repair your sash windows sympathetically.

Timber sash windows are a key part of the character of many period properties, especially those from the 18th and 19th centuries, and as such, they are well worth preserving.

It can be tempting to replace sash windows that are difficult to open or stuck fast, draughty, with rattling panes of glass, or frames that are beginning to rot, but often such problems can be resolved with simple repairs. As the cost of good-quality sash windows starts from £1,200–£1,600 plus VAT, it is also invariably cheaper to repair or upgrade original windows than to replace them.

If your sash windows are curved, repair will almost certainly be the most cost-effective option, as bow-shaped windows are expensive to manufacture.

When a sash window shows signs of rot, the damage will most likely be to the most vulnerable parts, typically the horizontal elements, such as the bottom rail, or the meeting rail. The ‘boxes’ (the frame in which the sashes slide) are often intact and can be left in situ while other components are repaired, although the sill can also be vulnerable.

Damaged sections of timber can either be replaced (including the whole sash in some instances), or repaired by cutting out the section of damaged timber to at least 5cm beyond the furthest point of decay and then treating the remainder and splicing in a new piece of matching seasoned timber. This is followed by sanding, filling and repainting.

Draught and noise problems can be signifi cantly reduced by fitting the sashes with new seals and replacing the parting beads (a vertical strip on each jamb between the sashes), which will also ensure smoother operation. The cost of draught-proofing will generally be recouped within five to 10 years.

If external noise is a problem, or when superior energy efficiency is required, it is possible to add new double-glazed replacement sashes into the original box frames, or secondary glazing can be added internally. Very thin double-glazed units are available from Thindow, Slimlite and Slenderpane.

All prices and estimates correct at time of publishing