What is a mirror vault?
A mirror vault is a vault with curved sides that do not meet at a crown or ridge at the top, but merge into a flat central panel. That flat panel is called the mirror. In Dutch it is a spiegelgewelf, in German Spiegelgewölbe. You can think of a mirror vault as a trough or cloister vault with its top cut off horizontally. The mirror is usually rectangular, sometimes oval. The word mirror here means a panel set within a frame; the frame is formed by the four curved vault surfaces.
Characteristics
- Two long and two short sides. Over a rectangular room, the vault has two long, curved surfaces and two shorter, trapezoidal surfaces.
- Flat central panel. The mirror is ideally suited to a ceiling painting or stucco work, which made the mirror vault popular in the Renaissance and Baroque periods.
- Lower construction height. Because the top is flattened, a mirror vault takes up less height than a full trough vault.
- Construction. It can be built of masonry, but in houses and castles it is often a plaster ceiling on a timber support structure that imitates the shape of a vault.
Application
- Establish what it is. Have it determined whether it is a masonry vault or a plaster ceiling on laths or reed. This determines the repair method and what load it may carry.
- Watch for horizontal thrust. A masonry mirror vault pushes sideways against the walls. Have any alterations to those walls assessed by a structural engineer first.
- Protect paintings. When working above the vault (for example services in the loft), do not load the vault and avoid vibrations.
- Document cracks. Have cracks in the vault webs or loose plaster investigated before any repair.
- Listed building? Mirror vaults occur mainly in listed buildings; check with your local authority, as listed buildings usually need consent for alterations.
Common mistake: mistaking a plaster ceiling in the shape of a mirror vault for a masonry vault and standing on it, or placing heavy items on the loft floor above it.
Related terms
- Trough vault
- Cloister vault
- Vault web
- Vault bay
- Baroque
- Renaissance architecture
- Horizontal thrust
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