Roman architecture: characteristics, examples, and how the Pantheon's dome was built

Architecture · 4 September 2026

Roman architecture is the building tradition of the Roman Republic and Empire from about 500 BC to AD 500, and its four great inventions, the arch, the vault, the dome and concrete, are the reasons the Romans could build what the Greeks could not: a dome 142 feet (43 m) across, an amphitheatre for 50,000, aqueducts that crossed valleys on three tiers of arches, and baths for 1,600 people under vaulted roofs. The Romans kept the Greek orders of columns but used them as decoration on the outside of structures that stood up by other means. Below: the seven characteristics with the buildings that show each in the Buildings by style collection, a step-by-step account of how the Pantheon's dome was built, the Colosseum and the engineering works, and the difference from Greek architecture.

The seven characteristics

1. The arch. A semicircle of wedge-shaped stones that turns the weight above into sideways thrust, allowing wide openings and long spans. The Etruscans and Greeks knew it; the Romans built everything with it: gates, bridges, aqueducts, and the triumphal arch, a building that is nothing but an arch.
2. The vault. An arch extended in depth. The barrel vault covers a corridor; two crossing barrel vaults make a groin vault that needs support only at the corners. The Baths of Caracalla (AD 216) roofed halls 80 feet (24 m) wide with groin vaults; medieval cathedrals learned from them.
3. The dome. An arch rotated. The Pantheon (AD 126) is the largest unreinforced concrete dome ever built and was the largest dome of any kind until the 15th century.
4. Concrete. Lime mixed with volcanic ash from Pozzuoli, which set even under water and gained strength for centuries, poured between brick faces into any shape. It is the material of the Pantheon, the baths, the harbours and the tenements of Rome. Modern analysis shows the lime clasts in it heal cracks as water enters, which is why Roman harbours are still standing.
5. The orders as ornament. Doric, Ionic and Corinthian columns, plus the Roman Tuscan and Composite, applied as half-columns and pilasters over arches: the Colosseum stacks the three Greek orders on its three arcaded storeys, purely as decoration.
6. Engineering for the public. Aqueducts, roads, sewers, bridges, harbours and baths, built for whole cities: the Pont du Gard (about AD 50) and the Aqueduct of Segovia (about AD 100) still stand without mortar.
7. The planned city. A grid of streets around a forum with a temple, a basilica for law and business, baths, a theatre and an amphitheatre, repeated from Britain to Syria; Diocletian's Palace at Split (AD 305) is a fortified city in one building, and old Split still lives inside it.

How the Pantheon's dome was built

The Pantheon was rebuilt by the emperor Hadrian about AD 118–126 on the site of an earlier temple, whose inscription, "M. Agrippa built this", was kept on the new porch. The dome is a hemisphere 142 feet (43.3 m) in diameter, sitting on a cylinder of the same height, so a sphere would fit exactly inside; the open oculus at the top is 27 feet (8.2 m) across and is the only source of light. Five things make it stand.

1. The walls. The drum is concrete 20 feet (6 m) thick, faced with brick, hollowed by eight great niches and hidden relieving arches that carry the load down to eight massive piers between them.
2. Graded concrete. The aggregate changes with height: heavy travertine and tufa at the foundations, tufa and brick in the walls, brick and light tufa in the lower dome, and volcanic pumice near the top, so the dome weighs least where it is hardest to hold up.
3. Thinning shell. The dome is 21 feet (6.4 m) thick at its base and 4 feet (1.2 m) at the oculus.
4. The coffers. The five rings of 28 sunken panels remove about a fifth of the dome's weight and were cast against timber formwork on a hemispherical scaffold; the formwork stayed until the concrete cured.
5. The oculus. A hole at the top of a dome is where a stone dome would be weakest, but in concrete it removes weight and ends the shell with a compression ring of brick; rain falls through it onto a floor that drains.

The result has stood for 1,900 years through earthquakes, the stripping of its bronze roof tiles and the removal of its gilded bronze ceiling by Pope Urban VIII in 1625; it has been a church since 609, which saved it. Brunelleschi measured it before building the dome of Florence, and every domed capitol in the United States descends from it. Pantheon overview.

The Colosseum

The Flavian Amphitheatre, begun by Vespasian in AD 72 and opened by Titus in 80 with 100 days of games, is 615 feet (189 m) long, 510 feet (156 m) wide and 157 feet (48 m) high, and held about 50,000 spectators who could enter and leave through 80 numbered arches in minutes. It is an ellipse of radiating travertine piers joined by concrete vaults, with tiers of seats on the vaults; the outer wall is three storeys of arcades with Doric, Ionic and Corinthian half-columns and a fourth storey of Corinthian pilasters, once fitted with masts for an awning. Beneath the arena a two-storey basement of corridors, cages and lifts, the hypogeum, sent animals and scenery up through trapdoors. It was built by Vespasian on the drained lake of Nero's palace, with the spoils of the Jewish War of AD 70 and, according to a reconstructed inscription, prisoners from it. About two thirds of the stone was quarried away in the Middle Ages for palaces and churches. Colosseum overview.

Aqueducts, baths and bridges

Rome had eleven aqueducts delivering something like a million cubic metres of water a day; most of the route ran underground, and the arches people photograph are the short stretches where the channel had to cross a valley while keeping its gradient of a few feet per mile. The Pont du Gard carried the aqueduct of Nîmes 160 feet (49 m) above the river on three tiers of arches; the Segovia aqueduct has 167 arches of unmortared granite and fed the city until the 20th century. The water went to fountains and to baths: the Baths of Caracalla covered 62 acres with hot, warm and cold halls, two gymnasia, libraries and gardens, heated by furnaces under the floors. Bridges like the Pons Fabricius in Rome (62 BC) and the Alcántara bridge in Spain (AD 106) are still in use.

Temples, houses and the rest

Roman temples kept the Greek form but raised it on a high podium with steps only at the front and columns engaged into the side walls: the Maison Carrée in Nîmes (about AD 5), the best-preserved, was Jefferson's model for the Virginia Capitol, and the Temple of Portunus in Rome shows the same type in the Republic. The Library of Celsus at Ephesus (AD 117) is a two-storey facade of columns and niches, the Roman "scenic front" that Baroque architects would revive. The Ara Pacis (9 BC) is the state art of Augustus in marble; Hadrian's Mausoleum is a drum 210 feet (64 m) across. Houses ranged from the atrium house of Pompeii, rooms around an open court with a pool, to the brick apartment blocks of Ostia, five storeys high with shops below, the ancestors of every European city street.

Roman vs Greek architecture

• Structure. Greek: post and lintel, stone beams on columns, so spans were short and interiors small. Roman: arch, vault and dome in concrete, so interiors could be vast.
• Columns. Greek: structural, carrying the roof. Roman: often decorative, applied to a wall or arcade that carries itself.
• Materials. Greek: marble and limestone in dressed blocks. Roman: concrete and brick faced with marble, stucco or travertine.
• Buildings. Greek: temples, theatres cut into hillsides, stoas. Roman: everything the Greeks built plus amphitheatres, baths, basilicas, aqueducts, triumphal arches, free-standing theatres and apartment blocks.
• Plan. Greek: buildings placed on sacred sites as the ground allowed. Roman: axial, symmetrical, planned around a forum.
• Legacy. The Renaissance, Baroque and Neoclassical styles all rebuilt Roman forms; see architectural styles in chronological order and, for the Greek side, the Parthenon in the collection.

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