The science of timber preservation is relatively new. However, many timber sash windows of the 18th and 19th centuries continue to provide excellent service. In contrast, some of the new timber windows of the 1960s and 70s are already deteriorating.

Part of the reason for this situation lies in the choice of timber. Heartwood, from the centre of the tree, is naturally durable, and should always be preferred. Since 1945 it has been common practice to use poor quality wood for many joinery tasks. It is therefore important to retain old joinery wherever it is sound, or, if repair or replacement is necessary, to ensure that the wood chosen is heartwood or, at the very least, well-treated sapwood. It is not true that all modern softwood is low-grade: British, Scandinavian, and North American softwood from farmed, properly managed sources can still last for decades and even centuries, particularly if treated before use.

When repairing window joinery, it is always a good idea to deal with the source of the problem, such as water ingress first. If  you need to apply preservative treatments, these can be brushed onto the affected area after the decayed wood is has been cut out. A more sophisticated method is pressure-inject organic, solvent-based preservative into the timber through non-return valves, which are later filled. This is best done by a specialist and is not really economical for fewer than 5 windows.

The insertion of preservative rods containing water soluble chemicals (usually boric acid), which diffuse into the surrounding timber is also highly effective, but again is best carried out by an experienced person.

For more detailed advice on timber improvement services, please contact Timeless Wood Windows.