How are domes built? From the Pantheon's concrete to Brunelleschi's bricks, the pendentives of Hagia Sophia, iron, geodesics and the stadium roof

Architecture · 4 September 2026

A dome is an arch rotated around its centre, a shell of masonry, concrete, iron, steel, timber or fabric that covers a round or polygonal space without internal supports; like an arch it stands by compression, its weight flowing down the curve to the base, and unlike an arch it also pulls itself apart around its lower rings, where the shell wants to spread outward and cracks from the base upward, which every dome from the Pantheon to St Peter's has done. The builders of each age solved the same problem differently: the Romans with a thick concrete base and light aggregate at the top, the Byzantines with pendentives that carried a round dome onto a square room, Brunelleschi with two shells and herringbone brick, the Baroque with iron chains around the base, the 19th century with cast iron and the 20th with thin concrete shells, geodesic triangles and cables holding fabric over stadiums. Below: the problem, and the answers in order, with the domes of the Pantheon, Hagia Sophia, Florence Cathedral, St Peter's and the Capitol in the Buildings by style collection.

The problem: a dome wants to burst

Cut a dome vertically and each slice is an arch, carrying its load down to the base in compression along its curve; but the slices also press sideways on one another around the dome, and this hoop force is compression in the upper part of the dome, where the rings are being squeezed together as they carry the load, and tension in the lower part, below an angle of about 52 degrees from the top, where the rings are being stretched as the shell tries to spread. Masonry cannot take tension, so a masonry dome cracks along its meridians in its lower part, splitting into wedges that lean on one another like the segments of an orange, which is what happened to the Pantheon and to St Peter's and what the builders always expected; the dome stands as long as the base is heavy enough, or tied tightly enough, to hold the wedges in. Everything in the history of domes is a way of handling that outward push: thickening the base, lightening the top, adding a drum and a ring, encircling it with chains, or making the shell so thin and continuous that the tension is small and the material, concrete with steel in it, can take it. Dome.

Rome: concrete on stepped rings

The Romans built domes in concrete from the 1st century BC, in the baths of Baiae and Nero's Golden House, and the Pantheon of AD 126 is the masterpiece: 43.3 m across, a hemisphere of concrete cast on timber centering in horizontal rings, 6.4 m thick at the base and 1.2 m at the oculus, with the aggregate graded from heavy travertine at the bottom to light pumice at the top, seven stepped rings on the outside adding weight where the tension is greatest, five rings of coffers on the inside taking weight away, and the 9 m oculus as a compression ring that the wedges lean on; the drum is 6 m thick and hollowed by arches and niches, and the whole thing is one mass with no iron in it. The cracks in its lower part are ancient and stable, the dome behaving as the Romans intended, as a set of arches meeting at the oculus. No masonry dome has been larger since. Its story is in who built the Pantheon.

Byzantium: the pendentive

A dome on a round room is easy; a dome on a square room, which is what a church needs, is not, and the answer of Hagia Sophia in 537 was the pendentive, a triangular section of a larger sphere that rises from each corner of the square between the four arches and curves inward to meet its neighbours in a circle on which the dome sits, so that the dome's weight goes down through the pendentives into four piers at the corners and the walls between them can be open. The dome of Hagia Sophia, 31 m across, is a shallow shell of brick and mortar with 40 ribs between 40 windows at its base, which lighten it and let the light in; the first dome, too shallow, pushed outward too hard and fell in 558, and the rebuilt one of 563 is 6 m taller, so that its thrust goes down more steeply, with half-domes east and west and buttresses north and south to take what push remains. The squinch, an arch across the corner of the square, is the older and simpler answer, used in Persia and in Islamic domes; the pendentive is the elegant one, and every Renaissance and Baroque dome, from St Peter's to St Paul's, sits on pendentives. Hagia Sophia's history is in Hagia Sophia history.

Florence: Brunelleschi without centering

Florence Cathedral was left in 1418 with an octagonal drum 42 m across and 52 m up, too wide for any timber centering that could be built and too high for buttresses, which the city would not allow; Filippo Brunelleschi won the competition with a plan to build the dome with no centering at all, and did so between 1420 and 1436. His answers: a pointed rather than round profile, which pushes outward less; two shells, an inner one 2 m thick that carries the load and an outer one 60 cm thick that keeps the weather off, joined by 24 ribs and 9 horizontal rings of stone, so that the whole is light and stiff; the bricks, four million of them, laid in a herringbone pattern, in which vertical bricks at intervals turn each course into a self-supporting ring so that the dome could be built up in rings that stood on their own as they closed, each ring supporting the workmen who laid the next; chains of sandstone beams clamped with iron, and one of oak, around the dome as hoops; and a hoist he invented, ox-driven with a reversing gear, that lifted the stone. The lantern, which loads the top and locks the ribs, was built after his death to his design. The dome is 45.5 m across the octagon, larger than St Peter's and still the largest brick dome ever built. Its story is in Renaissance architecture.

Drums, lanterns and chains

• Drum. The cylindrical or polygonal wall on which a dome stands, with windows, which lifts the dome to be seen from outside and takes the thrust down through a thick ring; from Byzantium onward, and the Baroque made it tall, with columns.
• Lantern. The small structure on top of the dome, over the opening, which lights the interior and, by its weight, presses the ribs together at the top; without it the ribs would spread.
• Chains. Iron or timber rings set in the masonry around the base of the dome to take the hoop tension the masonry cannot; Brunelleschi's stone-and-iron chains, Michelangelo's two iron chains in St Peter's, and the five more added in 1743–44 when Poleni and the "three mathematicians" found the dome cracked, the first structural analysis of a building by calculation, which gave the answer that the dome was safe and needed hooping.
• Double and triple shells. An inner dome shaped for the room and an outer one shaped for the skyline, with a stair between: Florence and St Peter's have two; Wren's St Paul's of 1710 has three, a shallow inner dome of brick for the interior, a brick cone in the middle that carries the 850-ton stone lantern, and a timber-and-lead outer dome for the city, with an iron chain around the cone's base, so that the dome seen from outside is not the dome that holds anything up.

Iron and steel

The 19th century made domes of iron, which takes tension: the Halle au Blé in Paris got a cast-iron dome of 39 m in 1811 after its timber one burned; the Capitol in Washington got Thomas U. Walter's cast-iron dome of 1855–66, nearly 9 million pounds of iron ribs and plates on a masonry drum, an inner and an outer shell with the ribs between, painted to look like stone, which is why it could be built during the Civil War and taken apart if it had to be; St Isaac's in St Petersburg of 1858 has an iron frame under its gilded copper; and the reading room of the British Museum of 1857 and the Reichstag of 1894 are iron and glass. Steel came with the Galerie des Machines of 1889 and the railway station; the Devon dome of the Houston Astrodome of 1965 is a steel lattice 196 m across, the first domed stadium, and the Louisiana Superdome of 1975 is 207 m. A steel or iron dome is a frame, not a shell, and its ribs and rings take the tension that the masonry dome could not.

Concrete shells, geodesics and fabric

• Reinforced concrete shells. Steel in the concrete takes the tension, so a dome can be a thin shell: the Jahrhunderthalle at Breslau of 1913, 65 m across, the first larger than the Pantheon; the Zeiss planetarium of 1926 at Jena, 6 cm thick on a steel mesh, the first thin shell; Nervi's Palazzetto dello Sport of 1957, 60 m, of precast ribs; the Kingdome of Seattle of 1976, 201 m, the largest concrete dome, demolished in 2000. A shell dome is thin because a curved surface in compression is stiff, as an eggshell is.
• Geodesic domes. Buckminster Fuller's patent of 1954: a sphere made of triangles of struts in a network that spreads every load through the whole frame, so that the dome gets stronger as it gets bigger relative to its weight; the Montreal Expo dome of 1967, 76 m, the Spruce Goose dome at Long Beach, 126 m, and thousands of radar domes, greenhouses and houses; Fuller thought he could cover Manhattan.
• Tension and air. The stadium roofs since the 1970s hang fabric or cables from a compression ring: the Pontiac Silverdome of 1975 was fabric held up by air pressure, the Georgia Dome of 1992 a cable net 227 m across, the Millennium Dome of 1999 a fabric tent on masts 365 m across, the largest "dome" in the world and not a dome at all, being held up by tension rather than standing in compression.
• Timber. The Tacoma Dome of 1983, 161 m, is glued laminated timber, the largest wooden dome; medieval and Renaissance domes had timber outer shells, as St Paul's has.

The records

• Masonry. Pantheon, 43.3 m, concrete, AD 126, unbeaten in unreinforced concrete; Florence, 45.5 m across the octagon, brick, 1436, largest brick; Gol Gumbaz at Bijapur, 44 m, 1656, the largest dome in India; St Peter's, 42 m, 1590.
• Iron. Capitol, 29 m inside, 1866; Halle au Blé, 39 m, 1811.
• Concrete. Kingdome, 201 m, 1976, demolished; Breslau, 65 m, 1913, still standing.
• Steel. Superdome, 207 m, 1975; Nagoya Dome, 187 m, 1997; Singapore National Stadium, 310 m, 2014, the largest fixed dome, a steel lattice with a retractable centre.
• Geodesic. Fukuoka Dome, 216 m, 1993, retractable.
• Tension. Millennium Dome, 365 m, 1999.
• Oldest standing. The Treasury of Atreus, Mycenae, 1250 BC, corbelled, 14.5 m; the Pantheon for a true dome.

Versions and theories

• Brunelleschi's secret. He kept his method to himself and the biographers of the 15th century said he had a secret; the herringbone brick, the rings and the double shell, all visible now, are the secret, with the pointed profile and the hoist; scholars still argue about the exact geometry he used to keep the courses aligned without centering, the "slack line" method being the current answer.
• The Pantheon has no cracks. It has many, ancient and stable, in the lower dome and the drum, which is how a masonry dome works.
• Domes are the heaviest roofs. Masonry ones are; a geodesic or shell dome is the lightest roof per area there is.
• The Capitol dome is stone. It is cast iron, painted white; the drum below is masonry.
• Michelangelo's dome is as he drew it. Della Porta raised the profile by about 7 m in 1588–90 for strength; Michelangelo's model shows both the hemisphere and the pointed version.
• The Millennium Dome is the biggest dome. It is the biggest roof of its kind and hangs from masts in tension; the biggest structure that works as a dome, in compression, is the Singapore stadium.

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