11/08/2026
What actually makes a heritage sash window?
There is a growing difference between a timber sash window and a sash window that has been properly made for a period building.
This photograph is a good example.
The neighbouring properties have very similar architecture, but the treatment of the windows is quite different. The house with the green door has recently had replacement double-glazed sashes fitted. Next door, we are installing new slimline double-glazed sashes into the existing box frames.
Traditional sash windows were remarkably delicate pieces of joinery. Narrow glazing bars, fine meeting rails and restrained sightlines were all part of the original design. When those proportions become heavier, the character of the whole elevation changes.
Our new sashes are being made with 22mm solid timber glazing bars and 12mm slimline double-glazed units, allowing us to retain much more traditional proportions while introducing modern glazing.
The glazing bars are part of the sash itself — not planted or stuck onto the glass.
Externally, the glass is finished traditionally in putty, rather than with modern glazing beads. The sashes are fully hand painted, and particular attention is paid to the thickness of the meeting rails, to match with the original windows on the property.
When installing new box frames external cheeks should only be showing around 10–15mm from the concrete reveals. Once this becomes substantially wider, the sashes begin to look smaller and not a like for like replacement.
Compare that with many modern replacement “heritage” windows and you will often find chunky meeting rails, planted glazing bars and sprayed finishes. They may technically be timber sash windows, but visually they are quite different from the windows they are intended to replace.
For listed buildings and properties in conservation areas, this distinction matters.
The aim shouldn’t be to manufacture a modern window and add traditional-looking details afterwards. It should be to understand how the original window was constructed and proportioned, retain historic material wherever possible, and introduce improvements without unnecessarily changing its appearance.
That is why retaining a sound original box frame and making new sash windows to suit, can be such a good conservation approach.
Heritage joinery is not about making something look old. It is about understanding why the original looked right in the first place.