How Sash Windows Evolved


Chelsea Window. Sash set back with thick frame visible, likely dated 1709-1774. Source: Le Lay Architects

Picture a sash window in your mind and it is probably set in brickwork; likely soot-stained yellow stock bricks or Victorian reds. Whist the sash window was introduced just after the Medieval period where timber frame houses prevailed, today you will seldom see an original sliding sash set into a timber building. The reason for this dates back to 1667. Just a year before, tall timber-framed, jettied houses overhung the narrow streets of London. As the population grew, space within the city walls became cramped with storey built upon storey, creating an overlapping line of combustible homes. It would only take a strong wind and a baker’s oven left on heat to cause one of the worst disasters in the city’s history.

The building of houses out of timber and thatch was eradicated following the Great Fire when Charles II introduced the 1667 Rebuilding of London Act requiring all future buildings to be built in brick or stone. Additionally, window sills were first introduced as a method for deflecting rising fire around the window frame- Medieval windows were not so well equipped. Legislation began dictating the form of the sash window.


A Setback for the Better

Further fire-proofing methods were introduced in the 1709 Act which required all sills to be a minimum of four inches deep and more importantly for the window frames to be set back into the wall by the same distance (the equivalent of an imperial brick depth). Still not content with their buildings’ resistance to fire, the government introduced a third Act in 1774 which aimed to standardise the quality and construction of their buildings by introducing further fire-resisting methods. Sash boxes were required not just to be set back into the wall, but also to be concealed within the wall (behind the outer layer of brickwork) resulting in only a small amount of frame being visible and therefore susceptible to fire (usually of similar width to the glazing bars).


Dating Guidance

And so for dating purposes, any original sash window set flush in a brick or stone wall will likely be pre 1709; set back four inches with thick frame visible should be post 1709 but pre 1774; set back with slim frame visible should be post 1774. This development of window legislation spanned the Georgian period, resulting in the symmetrical, carefully proportioned sash window, classical in style, forming the archetypal window of the Georgian era. The addition of ornate embellishments to the window reveals and surrounds, often set in patterned brickwork, would date the window more towards the more recent Victorian or Edwardian period.