Double-shell dome

A double-shell dome is made of two separate shells, one inside the other. The inner shell forms the ceiling and sets the proportions seen from inside. The outer shell gives the building its silhouette and protects the inner one from the weather. A gap is left between them, which adds insulation and in some buildings gives access for maintenance.

Role in the building

In a single-shell dome the inner and outer forms are tied together: proportions that look right from inside may look squat from outside, or the other way round. A double shell separates the two. The inner dome can stay at a height suited to the room, while the outer one rises higher, pointed or bulbous. Structurally each shell carries its own weight; the outer one bears on the inner shell, on timber or masonry ties in the gap, or on the drum.

Double shells in Turkish architecture

Anatolian Seljuk tomb towers have a dome inside and a conical or pyramidal roof outside, which is the most familiar double-layered covering in Anatolia. Tall outer shells are common in Iranian and Central Asian architecture. Ottoman domes, by contrast, are generally single-shell masonry covered externally with lead.

• Inner shell: the dome over the space
• Gap: air space and connecting members
• Outer shell: dome, cone or pointed roof

Survey and 3D modeling

Measurements from outside and inside give two different surfaces; the space between them is often only understood in section. Building the inner and outer shells on separate layers in a 3D model makes sections and restitution proposals far clearer.