Dome drum

A dome drum, kasnak in Turkish, is the cylindrical or polygonal ring of wall beneath a dome that raises it above the transition elements. Lifting the dome makes it more prominent on the skyline, and windows in the drum bring light into the interior from high up. Not every dome has one; many sit directly on a band of pendentives or squinches.

How the load travels

The drum is an intermediate layer between dome and transition zone. It takes the vertical load of the dome and passes it down into the pendentives, squinches or Turkish triangles below. The outward thrust at the dome base also lands on the drum, so drums are often strengthened with buttresses, thick walls or tension rings around the outside. Windows weaken the drum’s bearing section, so their number and size always have to be balanced against its strength.

• Cylindrical drum: circular in section
• Polygonal drum: octagonal or with more sides
• Windowed drum: pierced by openings
• Buttressed drum: braced from outside by small piers

In survey and restoration

The drum is where the first signs of dome deformation usually appear. Vertical cracks in window arches and in the wall between windows are often linked to dome thrust and are mapped carefully in a damage assessment. Survey the drum separately from outside and inside, without leaving out window spacing or frame profiles. In a 3D model, building the drum apart from the dome lets you check the window rhythm and the springing level of the dome clearly.