What is a Transom?
A transom is a horizontal cross rail that divides a frame vertically into a transom light (fanlight) above and a window or door below. The transom sits between the sill and the head of the frame, separating the fixed upper light from the operable window or door section beneath.
Function
The transom serves multiple purposes:
- Structural — distributes the load across the frame
- Aesthetic — gives a frame a classic, layered appearance
- Functional — allows a fixed upper light above an operable window or door
Transom vs. sill vs. lintel
| Element | Position | Function |
|---|---|---|
| Transom | Horizontal, mid-frame | Separates window from upper light |
| Sill | Bottom of frame | Water drainage, sealing |
| Lintel | Above wall opening | Carries wall load above the frame |
Cross window
A frame with both a transom (horizontal) and a mullion (vertical) is called a cross window. This type was standard in British and European construction from the 17th to the early 20th century and is now returning to popularity for its classic appearance.
Materials
- Timber — traditional, good insulation, requires maintenance
- uPVC — maintenance-free, good thermal performance
- Aluminium — slender profiles, durable
Dimensions
- The transom height determines the ratio between the upper light and the window
- Classic proportion: the upper light is ⅓ of the total frame height
- Minimum transom height for a workable window below: depends on the direction of opening
Related terms
- Window frame
- Sill
- Cross window
- Reveal
- Fanlight / upper light
- Daylight opening
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