Repairing the Sash


Chelsea Window. Slim aesthetic beauty of a Georgian sash. Source: Le Lay Architects

Draughts, heat loss, noise, rotten sills, rattling operation… for period property owners problems like these are the main arguments for the quick and full replacement of their historic sash windows (leaving listed building owners infuriated when legislation requires them to be kept!). But as architects we recognise the importance of preserving our heritage (despite the red tape, the listed building system is a great thing) and we would question the ‘get rid and get new’ approach adopted by many.

Before we consider the preservation of existing windows as a valid environmental and financial option, it is important that any improvement measure is considered as part of a holistic, whole-house approach rather than in isolation as there may well be longer term effects resulting from seemingly minor changes. STBA provide a very helpful guidance wheel which considers how various improvement measures interact with others. The combined introduction of draught-proofing, secondary glazing and thermal blinds/ curtains for example, can all but match the performance of double-glazing, for significantly less cost. The renovation of existing shutters can further improve performance. In light of this we can now consider sash window renovation options knowing that any changes will be considered within the bigger, holistic picture.



At the sight of rotten sills and cracked paintwork it is very easy to write off a tired and worn looking historic timber window. However, this could be a costly decision. Traditional refurbishment methods can bring tired frames back to life and to a very high standard. Defective putty can be cut off and replaced. Rotten timber can be cut away and new timber carefully spliced in its place using epoxy glues and traditional fixings. Not only is the historic nature of the windows retained in place (sash windows can date back to late 17th Century) but significant savings can be made; the average cost of a new sash window is around £1,300 whereas the cost for renovating an original window back to full health can be half that price.

Rotten timber being cut away from an original sash window. Source: Fortis & Hooke


Generally, conservation officers do not look favourably on standard double glazing and with good reason- it significantly alters the appearance of a window. Sash glazing bars were historically very slim, usually only around 40mm deep. To replace the original single glass panes (4-6mm thick) for standard double glazing (14-24mm thick) will result in chunkier, modern glazing bars being installed in their place to the detriment of the historic window aesthetic. However, subject to listed building permissions, there are specialist double glazed products available that provide improved energy thermal performance whilst (almost) retaining the thickness of single glazing- see for 9-12mm double glazed products. Such products can be installed in existing sash frames with a small amount of modification. Note, the sash counter-weights will require adjustment due to the increased weight of the glass.

Slimlite Glass glazing profiles incorporating 12mm double glazing. Source: Slimlite Glass


Windows are painted for two reasons. Firstly, for decorative effect, and secondly, to protect the timber substrate from environmental conditions. Without a protective coating the timber would quickly deteriorate. Early windows would have only been protected by an oil preparation or limewash. Effective protection of the substrate has become more important as window design has progressed. The early 18th century sashes were very expensive commodities and understandably their protection was essential to ensure the longevity of the fenestration. Lead oil paint made of linseed oil, turpentine, driers and lead white pigment became the staple form of protection for windows until the early twentieth century, though each decorator would apply their own ratios when it came to material quantity. The flexibility of the oil paint allowed it to follow closely the subtle deformations and irregularities in the timber providing a greater level of protection from moisture ingress. Historic paint would have been a brittle coating prone to cracking and consequently allowing water ingress. Modern paints however, are more elastic and therefore allows the coat to expand and contract with the timber resulting in longer paint guarantees. Applying a new layer of paint will significantly increase the window’s durability and life span.

Window frame redecoration following timber splicing and epoxy resin repair work. Source: Fortis & Hooke


In summary, the flexible nature of timber as a natural material, compared to more recent plastics and metals, means that any defects can be remedied at the source, rather than requiring wholesale replacement of the window. With regular maintenance a timber window’s ability to accept piecemeal repairs over time can result in a life expectancy of over 50 years. The same cannot be said about modern uPVC windows which will require complete replacement after only 25-30 years. Consider this alongside their limited aesthetic qualities, limited colour range and poor environmental impact, it seems the renovation of an historic timber sash is by far the most environmentally friendly, cost-effective and honest approach to window repair.

Unsightly rotten window sills may wrongly condemn the whole window… Source: The Decorators Stamford

…yet the splicing in of a new sill, localised repairs and full redecoration can resurrect the window to new life. Source: The Decorators Stamford