Voussoier

Voussoier is a French architectural verb meaning to place or set voussoirs, the wedge-shaped masonry units that form arches and vaults. A voussoir is a wedge-shaped block, typically cut to fit the curve of an arch or vault. When arranged in a radial sequence, the voussoirs create a voussoir arch, with the central block—the keystone—locking the whole structure in place.

The function of voussoirs is to transfer loads through compression along the curve of the arch. Each voussoir bears against its neighbors, and the inner face follows the arch’s curvature while the outer edge tapers toward the supports. The thrust produced by the load is directed outward and downward along the arch, enabling stability without reliance on lateral thrust.

Construction typically involves careful shaping of each voussoir to its precise angle and face, followed by placement from the springing of the arch toward the crown. Mortar or lime putty may be used in traditional masonry, while modern applications may employ dry joints or precast concrete voussoirs in archways and vaults.

Historically, voussoirs are a defining feature of Roman architecture and continued through medieval and Renaissance masonry across Europe, in bridges, churches, and aqueducts. The concept also translates to brickwork and contemporary practice, where precast or cast-in-place voussoirs replicate traditional arch forms in modern materials. The act of fitting and shaping these elements remains central to the construction and restoration of arch structures.