What is a helical vault?
A helical vault, also called a spiral or screw vault (Dutch: schroefgewelf), is a vault that winds upwards like a spiral (screw). It is found mainly beneath a brick or natural stone spiral staircase in a stair tower: the vault rises at an angle around a central newel and carries the steps. Structurally, it is a rising annular barrel vault.
The steps rest on the top of the helical vault. They may be of brick (built up on the vault), natural stone or timber, or of brick with a stone or timber facing. From the stair below, you can see the spiral underside of the vault.
A simpler variant, found mainly in square stair towers, consists of short horizontal barrel vaults that each span a quarter of the turn and are set progressively higher. Sometimes a handrail is recessed into or cut out of the masonry of the newel or the wall.
Examples in the Netherlands and Flanders include the helical vault in the Sint-Annakapel in Kampen and the vaulted spiral staircase in the town house Den Grooten Reghenboghe in Antwerp. A famous international example is the ‘vis de Saint-Gilles’ in the abbey church of Saint-Gilles in southern France.
Application
Helical vaults are rarely if ever built today; they matter mainly in the restoration and maintenance of historic buildings:
- Monitor cracks: cracks in the spiral or loose stones on the underside may indicate settlement of the newel or the tower. Have suspicious cracks assessed by a structural engineer or restoration specialist.
- Keep moisture out: stair towers with leaking roofs or failed joints let water into the vault; frost then pushes stones loose.
- Limit loads: do not place heavy objects or new installations on a historic vault without an assessment.
- Use the right mortar: repair with a mortar that matches the original material, usually lime-based; mortar that is too hard, such as cement mortar, can damage the masonry.
Related Terms
- Vault bay
- Spiral staircase
- Lime mortar
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