What is a T-window?
A T-window is a window whose layout forms a T: a fanlight across the full width at the top, with two casements side by side below, separated by a central mullion. The horizontal transom and the vertical mullion together create the T shape.
The type became popular in the second half of the nineteenth century, when industrially made glass became cheaper and available in larger panes, so that even modest houses could have bigger glazed areas. The fanlight was often a single pane, either fixed or top-hung. The lower sashes were casements, which in the Netherlands usually opened outwards; sometimes they were fixed and only the fanlight opened.
The T-window is typical of many Dutch workers’ houses, townhouses and farmhouses from roughly 1870 to 1930. In the UK a similar layout is common in casement windows with a top light.
In practice
- Keep the layout when replacing. For listed buildings and conservation areas, authorities often require the original T layout and profile sizes to be retained.
- Choose slim insulating glass. Narrow old profiles often cannot take standard double glazing. Slim heritage glazing or vacuum glass may fit the existing rebate.
- Check the transom. Water lies on this horizontal member. Make sure it has a fall, a drip and sound paintwork to prevent rot.
- Repair rather than replace. A joiner can often splice in new timber for rotten parts, keeping the original frame.
Common mistake: replacing a T-window with one large tilt-and-turn sash without divisions. That changes the character of the facade dramatically.
Related terms
- Fanlight
- Casement window
- Top-hung window
- Sash window
- Vertical pivot window
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