Extrados

Extrados is a term used in architecture and structural engineering to describe the outer surface or curve of an arched, vaulted, or curved element. It is the convex side opposite the intrados, which is the inner, concave surface. In arches, vaults, and similar structures, the intrados bears the load and forms the interior profile, while the extrados represents the exterior profile that is visible from the outside.

The distinction between intrados and extrados is common in masonry and brickwork. For a voussoir arch, the extrados is formed by the outer faces of the voussoirs and defines the exterior contour of the arch. The thickness, curvature, and surface treatments on the extrados can influence exterior appearance, weather exposure, and, in some cases, the distribution of stresses near the crown of the arch.

In practical terms, engineers and conservators reference the extrados when assessing exterior profiles, plaster or facing work, and the overall architectural expression of an arched or vaulted element. While the intrados concerns the interior space and illumination of a structure, the extrados relates to its exterior form and structural envelope.

Etymology traces to architectural vocabulary in European languages, with the term entering English in early modern usage. See also intrados, voussoir, arch, vault, and keystone for related concepts.