A flying buttress is a sloping arch, carried through open air, that takes the lateral thrust of a vault and transfers it over the aisle roof to a heavy buttress outside the building. In French arc-boutant, in Dutch luchtboog, in German Strebebogen.
The flying buttress is the structural breakthrough of Gothic architecture. Without it, church walls had to be thick and largely solid to resist the thrust of a vault. With flying buttresses that thrust could be caught outside the building, which is why Gothic churches could be opened up with enormous windows.
How it works
A stone vault does not only push downwards; it also pushes outwards. That lateral component, the thrust, is what makes a wall lean.
- The vault springing in the nave delivers the thrust to the clerestory wall
- The flying buttress picks it up and carries it diagonally outwards and downwards
- The buttress converts it into a vertical force down to the foundation
- A pinnacle — the decorated turret on top — adds weight, so the resultant force runs more steeply and stays within the core of the buttress
On large churches flying buttresses are sometimes arranged in two tiers: the lower one takes the vault thrust, the upper one mainly the wind load on the high roof.
Important to know
- A flying buttress is a compression member: it only works while it is loaded. Removing or unloading one can allow the vault to spread
- In conservation work the top surface of the arch and its gutter junction are the most vulnerable points: water gets into the structure there and washes out the mortar
- The modern equivalent of the same principle is a tie rod, which resists the thrust in tension rather than compression
Related terms
- Buttress
- Tympanum
- Truss
- Battlement
- Roof structure
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