A dome drum — also called a tambour — is the cylindrical or polygonal wall on which a dome rests. It sits between the substructure and the dome shell, lifting the dome so that it becomes visible from outside and floats higher above the space within.
The drum exists for two reasons. First, appearance: a dome without a drum disappears behind the roof, while a dome on a drum defines a city skyline. Second, light: a ring of windows can be set into the drum wall, lighting the space below from above and from all sides.
Build-up
- Substructure — often square, with pendentives or squinches making the transition to a circle
- Drum — a solid or window-pierced ring wall, frequently articulated with columns or pilasters
- Dome — the shell standing on the drum, sometimes closed with a lantern or an oculus
The structural point
The drum has to take the thrust of the dome. Every window opening weakens the ring action exactly where the tensile force runs. That is why drum walls are heavily dimensioned, the piers between the windows relatively wide, and a tension ring or ring beam is often built in at the level of the dome springing.
Where you find it
- Domed churches and mosques
- Mausolea and burial chapels
- Palaces, parliament and court buildings in the classical tradition
- Modern concrete domes with a continuous glazed band instead of windows
Practical tips
- Cracks in the piers between drum windows point to dome thrust and should be assessed by an engineer
- Maintain the windows seriously — rain drawn in around the frames attacks the load-bearing part of the structure
- Watch for condensation — warm air collects in the dome and settles on the drum glazing; ventilation at that level protects paintwork and plaster
Related terms
- Dome
- Pendentive
- Vault
- Oculus
- Roof lantern
- Calotte
- Cloister vault
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