A recent survey indicated that, on average, only 5% of timber in windows that were being replaced was affected by decay. Yet a 1991 Gallup Poll revealed that 46% of home owners replaced wood windows because of ‘rotten timber’, and only 20% to reduce draughts and heating bills.

There are no inherent defects in the original design of the sash window, an extremely sophisticated piece of technology that has lasted, with minor modifications, for 350 years. It is also quite feasible to apply modern repair and maintenance techniques to our stock of existing sash windows. Therefore whenever possible original sash windows should be repaired rather than replaced. Permanent repair of a window may be less expensive than wholesale replacement, and no facsimile can be no other than new work. It is also possible to bring original windows up to modern enviromental standards without harming any features of historical value.

It is important to remember that where necessary:

  • permanent repairs can be made using appropriate materials and timber preservatives
  • the use of modern paints and methods can lengthen the time between redecorations
  • draughtproofing, secondary glazing, and even traditional shutters and heavy curtains improve energy efficiency and reduce noise transmission
  • it is possible to hinge a bottom sash to allow easy and safe cleaning
  • modern locking devices are available to deter intruders and to restrict opening

If sash windows need repair, it may well be sensible to upgrade them at the same time.


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.


Useful information on the Sash Window History by the English Heritage.

Single-hung sash windows (where only the bottom frame moves and is supported by wooden gates or pins when open) were probably copied by the British from France some time in the mid-seventeenth century. The double-hung sash, however, appears to have been an British invention. This was an ingenious technological breakthrough that enabled a far more subtle and sophisticated system of ventilation to be achieved than was possible with the old, side-hung casement. It used a system of hidden, counterbalanced weights to allow both top and bottom sash frames to be moved independently. The earliest double-hung sash discovered so far dates from 1701; however, by 1720 double-hung sashes had spread only as far as Holland and the British and Dutch colonies.Until the early eighteenth century sash frames were usually made of native oak or a similar hardwood. By 1700, oak was becoming rarer and thus more expensive, while softwoods from Scotland, the Baltic, and North America were more widely available. From the 1720′s most sash joinery used deal, a generic term for pine or fir softwood.

As a precautionary measure against fire the 1709 Building Act required that the corners of a sash box frame be hidden behind the face of the brick or stone masonry and that ‘no door or window frame of wood… shall be set nearer to the outside face of the wall than four inches’. In 1774 this distance was increased to nine inches, and nearly all of the frame had to be hidden behind the face of the wall. While this legislation was initially only applicable to London, within a few years its provisions were being taken up throughout Britain and America.

By 1730 segmentally arched windows had largely been replaced by square-headed varieties that were cheaper to make. The glazing patterns inserted into these frames often took the form of six panes over six, although this was by no means the rule. Nor were the dimensions of each pane necessarily dependent on the principle of the golden section: in some cases, individual panes were broader than they were tall. The overall size of the window was, nevertheless, always kept in strict proportional harmony with the rest of the facade, in accordance with Palladian theory.

Early glazing bars were thick and robust, to support and protect the fragile glass. Most seventeenth – and early eighteenth-century examples were based on the ovolo, or quarter-circle moulding. As glass technology improved and glass became stronger, glazing bars became increasingly slender, with pointed (gothic) and lamb’s tongue mouldings becoming very popular. By 1820 some glazing bars were only 12.5mm wide (although, to provide lateral strength, these could be up to 38mm deep).

As with so many elements of the Georgian house, glazing-bar patterns and profiles varied according to the social status of the window. Thus, for example, basement or attic windows, used only by servants, were often fitted with old-fashioned, obtrusive ovolo glazing bars and inferior-quality glass.

With the introduction of cheap, strong plate glass in the 1830′s glazing bars became less necessary and views became accordingly less cluttered. By 1850 many sash-window frames had no internal glazing at all. However, the great weight of these frames and the absence of any internal supports necessitated the introduction on the upper frame of sash horns, extensions of the stiles which helped to strengthen the vulnerable joints at either end of the meeting rail.