Once you’ve built an arch, you know how to make a doorway or a window without having to use big expensive beams. But how do you make the whole roof that way?
Easy: you stick a whole lot of arches together in a row, and that makes a roof. That’s a barrel vault: an arch over and over. (This one is in the Roman baths at Cluny in Paris).
They’re called barrel vaults because they look like the inside of a barrel.
The earliest barrel vaults were small ones in Mesopotamia, where architects used them for small drainage tunnels and tombs. The Elamites used bigger barrel vaults to roof buildings at Susa. By around 2600 BC, Egyptian architects used mud-brick barrel vaults. At Nineveh, the Assyrians used barrel vaults in their fortification walls. People in the Parthian Empire also used barrel vaults. The Romans also loved barrel vaults; they used concrete to build barrel vaults all over Europe and North Africa.
In the Middle Ages, architects kept up the West Asian tradition and used barrel vaults to put stone roofs on Umayyad castles in West Asia, like these barrel vaults on the castle at Qasr Amra, from the early 700s AD. The Al-Aqsa mosque in Jerusalem, from the 1100s AD, also used small barrel vaults, though the main nave has a flat wooden roof.
Further west, in Europe, architects used much bigger barrel vaults to cover Romanesque churches and cathedrals. This Romanesque barrel vault is from a big church in Toulouse, in the south of France, about 1100 AD. By the late 1100s AD, however, most architects in Europe were using a more complicated kind of stone roof called a groin vault.
In India and China, people used barrel vaults for small, often underground structures like tombs and sewage pipes, but not for houses or big public buildings. But in South Africa, North and South America, people didn’t build barrel vaults at all.
Arches to Zigzags: An Architecture ABC, by Michael J. Crosbie (2000). Shows what an arch is, or a gable, or an eave. Easy reading.
Eyewitness: Building, by Philip Wilkinson, Dave King, and Geoff Dann (2000). Lavishly illustrated, like other Eyewitness books, and with good explanations of most architectural terms.
Cathedral: The Story of Its Construction, by David Macaulay (1981). Beautiful drawings and clear text explain exactly how medieval craftsmen built a groined vault. Easy reading.