What is Roman architecture?

Roman architecture is the building art of the Roman Empire, roughly from the third century BC to the fifth century AD. Where the Greeks thought in columns and beams, the Romans built with the arch, the vault and the dome — and with a pourable building material that had no successor until the nineteenth century.

The two great inventions

Opus caementicium — Roman concrete. A mix of burnt lime, pozzolana (volcanic ash, named after Pozzuoli near Naples), water and rubble aggregate. It sets slowly and is remarkably durable; pozzolana also makes the mortar hydraulic, so it works under water. That explains the Roman harbour works. The concrete was almost always cast against a permanent facing, with different facing bonds on show: opus incertum (irregular rubble), opus reticulatum (a diamond net) and opus testaceum (flat brick).

The arch and what follows from it. From the round arch the Romans developed the barrel vault, the groin vault and the dome. For the first time large spaces could be spanned without a forest of columns. The Pantheon in Rome, completed around AD 126 under Hadrian, has a concrete dome just over 43 metres across — still the largest unreinforced concrete dome in the world.

Other characteristics

• Five orders. The Romans took the Doric, Ionic and Corinthian orders from the Greeks and added the Tuscan and the Composite.
• The order as decoration. Column and capital often carry nothing; they articulate a load-bearing concrete wall, as on the Colosseum (completed AD 80).
• Infrastructure at scale. Aqueducts, roads, bridges, baths and sewers, with very shallow but strictly maintained gradients.
• Underfloor heating. The hypocaust: a floor on small piers with hot flue gases led beneath it — the ancestor of modern underfloor heating.
• Dimensioning. Buildings were set out in Roman feet (pes), about 29.6 centimetres.

The thinking behind it survives in Vitruvius’s De architectura, written in the first century BC: a building should be firm, useful and beautiful.

Application

In north-west Europe Roman building is largely archaeology — Hadrian’s Wall, the Rhine limes, the baths at Bath. The technique, though, remains current: hydraulic lime with pozzolana is still the starting point for conservation mortars, because it stays soft, vapour-open and able to move — exactly where modern cement fails. And almost every masonry arch you build still follows Roman geometry.

Related terms

• Round arch
• Vault
• Lime mortar
• Column
• Capital

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