Semi-dome

A semi-dome is half a dome, cut along a vertical plane. Its open face is framed by a large arch, while its back rests on a semicircular wall or on further arches. It has a long history as the covering of apses and niches, and in large domed buildings it is used to support the main dome and to extend the space beneath it.

Role in the building

A main dome pushes outward at its base. A semi-dome leaning against one side of it works like a buttress, carrying that thrust down and outward along its own curvature into its piers and into smaller semi-domes. At the same time it opens the main space in that direction, creating a flowing interior that reaches beyond the limits of a single dome. At Hagia Sophia the main dome is flanked by two semi-domes to the east and west, and in Ottoman mosques the number and position of semi-domes is one of the main factors that define the plan type.

• Semi-dome over an apse or mihrab niche
• Large semi-dome supporting the main dome
• Smaller exedra semi-domes along the edges of a large one

Survey and 3D modeling

Where a semi-dome meets the main dome and its arch, the intersection is complex. In survey, record that junction line and the height of the semi-dome’s crown. Modeling a semi-dome as half a surface of revolution is straightforward, but historic profiles are rarely true hemispheres, so compare the model with the point cloud.