The Anatomy Of A Sash


The anatomy of a sash window: Credit Le Lay Architects

By the Georgian period the sash window was an engineering feat. Elegant and slim in appearance externally, behind the scenes it contained a complex form of operation; an intricate weighting mechanism that allowed each sash to hang at varied heights providing the Georgian home with improved ventilation and temperature regulation. To appropriately deal with an historic sash window it is important to understand the intricacies of this mechanism and more generally the make-up of its parts, or its anatomy, as explained below.


Components and Function

A typical sash window is made up of two glazed sash frames, each comprising horizontal top (1) and bottom (2) rails and vertical stiles (3). Commonly all the rails and stiles will be equal in width, except for the bottom most rail which would usually be equal or greater in height.

Vertical and horizontal glazing bars (4) subdivide each frame further, with each subdivision housing individual panes of equal sized glass (5). The closer the proportions of the glass created by the glazing bars are to the golden ration (height = 1.6 x width), the more elegant the window design.

Historically, the glass was secured to the glazing bars in a bed of putty (6) and pinned using glazing sprigs. More putty was then applied to waterproof the joint before decoration. More recently wooden beads have been used in place of putty.

For the window to open, both sash frames are contained between the frame lining (7) and a staff bead (8) and are offset by a p­­arting bead (9) allowing each to closely slide in front/ behind the other. Each frame sits in a vertical channel forming one edge of the sash box.



To facilitate operation, the weight of the glazed frames is counter- balanced by a heavy metal- steel, lead or iron- counter-weight (10)- separated by a loose-fitted parting slip (11)- which is contained, and moves freely, within the sash box. The counter-weight is connected to the window by a braided cotton sash cord that runs over a pulley at the top of the frame. (Note, the archetypal sash window would have simply been held open with wooden wedges rather than the more advanced counter-balance system). In the event of the pulley system malfunctioning, an opening in the sash box, known as the ‘pocket’ (12) provides access to the system for maintenance. The loose fitted parting slip is likely to be sacrificially broken at this point, requiring replacement.

As with most things however, the sash window is not infallible and will over time being to fail. Only once the anatomy of the sash is correctly understood can we consider appropriate forms of remedy or repair. The next blog in this series will consider just that.