Arches and Vaults: How Buildings Carry Their Roofs

An arch is a curved construction of wedge-shaped stones that carries a load over an opening by pressing the stones against each other. A vault is an arch broadened into a roof or stone superstructure. Arch and vault make it possible to span large rooms without timber beams.

Facts

The arch: how it works

A straight stone beam cannot span far, because stone withstands compression but is poor in tension, and the beam cracks in the middle. The arch solves the problem by turning the forces so that all the stones are pressed against each other. The wedge-shaped voussoirs come from two sides and meet at the keystone at the top. When the keystone is in place the arch locks itself by its own weight. That is why large openings can be spanned with small stones.

But an arch also pushes sideways. The support must therefore be heavy and stable, or a counterweight must take the side thrust. The basis of all vault building is the struggle between weight and counterweight. Read more about the parts of a building under arches and vaults.

The Romans: the round arch and concrete

The Romans made the round arch their hallmark. It is seen everywhere, in the storeys of aqueducts such as the Pont du Gard and in the façade of the Colosseum, where arcades carry the spectators’ seating. They also invented the groin vault, in which two barrel vaults cross at right angles, and with Roman concrete they could build domes of enormous size.

The dome of the Pantheon in Rome, finished around AD 125, has a span of 43.3 metres and is still the world’s largest dome of unreinforced concrete. The concrete becomes thinner and lighter towards the top, and in the middle of the dome is an opening, the oculus. It is Roman building at its most famous; read more in the guide to Roman architecture.

Medieval vaults: Romanesque and Gothic

In Romanesque architecture barrel vaults and groin vaults of stone were used over churches, for example in the cathedral at Speyer. They require thick walls and small windows. At Durham one of the first great rib vaults in England was built around 1093–1133: stone ribs are stretched diagonally and filled with lighter stone panels, so the vault becomes lighter.

Gothic took the rib vault further and combined it with the pointed arch. The pointed arch pushes more directly downwards than the round arch, which makes it possible to build higher and narrower. The side thrust is led out through flying buttresses to piers, and the walls can be opened for large windows; see Saint-Denis. How it was done is described in the guides on building and on how to recognise Gothic.

Domes, fans and the modern

The dome is a vault rotated around an axis. Byzantine architects placed domes over square rooms on pendentives in Hagia Sophia; see Byzantine architecture. Brunelleschi built the dome of Florence Cathedral in 1420–1436, the dome of Florence, with two shells and bricks laid in herringbone pattern so that it could be raised without a full wooden centering.

In England the fan vault developed, in which the ribs spread like a fan; King’s College Chapel in Cambridge, built 1446–1515, has one of the most famous. Antoni Gaudí used hanging chains in the Sagrada Família to find the ideal forms: a freely hanging chain makes a catenary curve that, turned upside down, becomes an arch in pure compression. See also the guide to Gaudí and the cathedral.

How to recognise it

Exercise

What is the stone at the top that locks an arch called?