Dome

A dome is a roof or vault in the form of the hollow upper half of a sphere, or a related curved shape, rising from a circular, polygonal or square base. Because its curvature runs in every direction, a dome carries loads by compression along its surface and can span large spaces without internal supports. The word cupola is sometimes used as a synonym, and also for a small dome on a roof or turret.

Parts of a dome

• Drum: a cylindrical or polygonal wall that raises the dome and often contains windows; drums are frequently much thicker than the dome itself.
• Pendentives and squinches: triangular or arched transitions that allow a round dome to sit over a square space.
• Oculus: an opening at the apex.
• Lantern: a small structure over the oculus admitting light and often topped by a cupola.

Landmark examples

• Pantheon, Rome: described as the most famous, best preserved and largest Roman dome, with an open oculus.
• Hagia Sophia, Istanbul: Justinian's 6th-century church combined a central dome with semi-domes over a basilica plan; earthquakes caused partial collapses requiring repairs.
• Florence Cathedral: Filippo Brunelleschi's octagonal Renaissance dome.
• Geodesic domes: lattice shells of triangles associated with Buckminster Fuller, who patented them in the United States although he did not invent the form.

Coverings

Masonry and concrete domes may be left exposed or covered. Timber and lightweight domes have traditionally been clad in sheet lead or copper, and Byzantine domes used everything from ceramic tiles to lead sheeting. Modern domes use standing-seam or flat-lock metal, membranes, or specialised shingles. The doubly curved surface means sheets must be tapered or shaped to fit, and drainage at the base must handle water running off in all directions.

Structural behaviour

Domes push outward at their base, so traditional masonry domes rely on thick drums, buttresses or tension rings. Onion domes and other decorative domes are often light timber or metal frames on top of a structural vault rather than true structural domes.