A vault is an arched ceiling or roof structure, usually made of stone or concrete. It is the three-dimensional version of an arch. Where an arch spans an opening, a vault spans a space. The principles are the same: compression, thrust, and the careful arrangement of materials.

The simplest vault is a barrel vault, a continuous semicircular arch. It is strong but heavy, and it pushes outward on the walls that support it. A groin vault is formed by intersecting two barrel vaults. It directs thrust to the corners, allowing openings in the side walls. A rib vault uses stone ribs to carry the weight of the ceiling, with thinner panels between them. This was a key innovation of Gothic architecture.

Vaults are dramatic. They create soaring interior spaces with a sense of enclosure and permanence. They also require skilled labor and careful engineering. A vault that is not properly buttressed will collapse. That is why historic vaulted buildings often have thick walls, flying buttresses, or both.

Modern vaults use reinforced concrete and steel, which can span farther with less material. But the old stone vaults remain among the most impressive structures ever built.