Vault (architecture)

• 1. Introduction

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  A vault is one of the oldest engineering ideas in architecture, and its entire logic rests on a single moment. Until the final stone, the keystone, drops into place at the crown of an arch, nothing holds. The temporary wooden frame, called centering, bears the full weight of every voussoir laid before it. Remove the centering too soon and the whole structure collapses. Wait until the keystone locks the ring together, and you have a self-supporting arched form that can stand for thousands of years.

  The vault, in its many forms, did not develop in one place or one era. It stretches from Neolithic Cyprus around 6000 BCE through the Sumerians at Nippur, through pharaoh Ramesses II and his granaries at Thebes, through Roman bath complexes and Byzantine domes, all the way to prefabricated concrete shells lifted by chains in the 20th century. Each step was a response to a practical problem: how do you span a wide space without timber, without stone beams, and without the structure falling in on itself?

  This documentary follows the vault through those answers, from the corbelled beehive tombs of Mycenae to the double-shell illusions of St Paul's Cathedral in London.

• 2. Before the True Vault

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  Khirokitia, a Neolithic village on the island of Cyprus, holds what may be among the earliest known examples of any form of vaulting. Dating to around 6000 BCE, its circular buildings were roofed with beehive-shaped corbel domes made of unfired mud-bricks. They also carry the earliest evidence of settlements with an upper floor.

  Corbelled vaults work on a different principle from true arched vaults. Instead of wedge-shaped voussoirs radiating from a centre, corbelled construction uses horizontal layers of stone, each course projecting slightly beyond the one below it, until the two sides of a chamber meet at the top. Builders in Mycenae were constructing corbelled vaults by the 14th century BC, and this tradition persisted in various regions right up to modern times.

  The Mycenaeans used this method for their famous tholos tombs, also called beehive tombs, in underground structures with conical profiles. Similar beehive tombs, also known as tholoi, appear in Crete and Northern Iraq, though most of those examples differ from Khirokitia in that they appear partially buried and include a dromos, an open entry passage.

  The distinction between a corbelled vault and a true vault matters technically. A true vault uses radially joined stones where each piece presses against its neighbors along angled faces, distributing weight outward in a controlled thrust. That technique was already known to the Egyptians and Assyrians, and the Etruscans brought it into western building practice.

• 3. Barrel Vaults and Brick

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  The earliest known barrel vaults were built by the Sumerians, possibly beneath the ziggurat at Nippur in Babylonia, using fired bricks cemented with clay mortar. A barrel vault is the simplest arched form: semi-circular in cross-section, it resembles a tunnel cut lengthwise in half.

  In ancient Egypt, brick vaults appeared as early as the 3rd millennium BC. By the time of the 19th dynasty pharaoh Ramesses II, barrel-vaulted granaries had been built at Thebes, the ruins of which survive behind the Ramesseum. Those granaries had a span of 12 feet. The lower portion of each arch was built in horizontal courses up to about one-third of the height, and the upper rings were inclined at a slight angle so that each course of bricks adhered without any centering at all, producing an elliptical profile as a result of the construction method.

  A far more ambitious version of that approach appeared in the great hall at Ctesiphon, where the span reached close to 83 feet and the vault was nearly 5 feet thick at the crown. Four rings of large fired bricks or tiles, cemented in mortar, carried that load without centering.

  Assyrian palaces applied pitched-brick vaults made of sun-dried mudbricks to gates, subterranean graves, and drains. During the reign of king Sennacherib, similar construction was used for aqueducts, including those at Jerwan. In the provincial city Dur-Katlimmu, pitched-brick vaults created raised platforms. That tradition passed to the Sassanians, who built domed structures in Sarvestan and Firouzabad that still survive, likely abandoned at the Islamic invasion in the 7th century.

• 4. Roman Engineering at Scale

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  Groin vaults, which form when two barrel vaults cross at right angles, allowed the Romans to cover spaces of extraordinary width. The tepidarium of the Baths of Caracalla had a span of 80 feet, more than twice the width of an English cathedral nave. The Roman reservoir at Baiae, called the Piscina Mirabilis, used five aisles intersected by twelve cross aisles, with the vaults carried on 48 piers and thick external walls.

  The researches of M. Choisy, published in his work L'Art de batir chez les Romains, reconstructed the method from surviving fragments still in place. On a comparatively slight centering of trusses placed about 10 feet apart and covered with planks, workers first laid two layers of Roman brick measuring nearly 2 feet square and 2 inches thick. Transverse rings of brick were then built on top, with longitudinal ties at intervals. Concrete was thrown in horizontal layers over the whole assembly, the haunches filled solid and the surface sloped and covered with a low-pitch tile roof laid directly on the concrete.

  One ingredient made the difference: a volcanic deposit found near Rome called pozzolana. When mixed into the concrete and set, pozzolana made the material as solid as natural rock and, to some extent, neutralized the lateral thrust of the vaults. The Romans did not appear to recognize this benefit, since they also built extensive cross walls and buttresses as though the thrust were unreduced.

  The Pantheon and the Basilica of Maxentius survive as outstanding examples of this tradition. The Basilica of Maxentius, completed by Constantine, was the last great vaulted work carried out in Rome before the city's decline.

• 5. The Rib Vault and the Gothic Turn

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  The rib vault changed everything about how medieval builders approached a stone ceiling. In earlier groin vaults, the diagonal groins were elliptical surfaces formed by the intersection of two curved geometries, generally weak and difficult to center correctly. The innovation attributed to the medieval builder was to reverse the sequence: set up the diagonal ribs first as permanent arched frames, then lay the stone webs between them, taking their shape from the ribs.

  This shift is first documented at Cefalù Cathedral, where pointed arch ribs appear and pre-date the choir aisles of the abbey of Saint-Denis, built by the abbot Suger in 1135. Cefalù is a Romanesque building whose masons tried Gothic rib arches before that approach was widely adopted across western church architecture. The pointed arch itself was employed because it exerts less lateral thrust on the walls than a semicircular arch, and because any span could be brought to the same crown height simply by adjusting the degree of point.

  The problem of spanning a nave, which was twice the width of the aisles, led to the sexpartite vault, where an intermediate rib divided each square bay into six cells. The Abbaye-aux-Hommes at Caen, built by William the Conqueror originally for a timber roof, had its upper nave walls partly rebuilt nearly a century later to carry one of the earliest examples of this vault. The cathedral of Soissons introduced the quadripartite vault in 1205, reducing the cells from six to four to avoid throwing unequal loads on alternate piers.

  In England, sexpartite vaults survive at Canterbury, set out by William of Sens around 1175, at Rochester, Lincoln, and Durham. English and French builders diverged on the treatment of the web between the ribs. English masons ran courses of uniform height from side to side; French masons laid horizontal courses of increasing height toward the diagonal rib, giving the web a slight domical rise that made the ridge rib unnecessary in France until the 15th century.

• 6. Fan Vaults and Faux Domes

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  Fan vaulting is peculiar to England. No true equivalent exists in France, Germany, or Spain, only an approximate pendant in the Lady-chapel at Caudebec-en-Caux in Normandy. The form emerged when builders began using a single curved centering for all the ribs rather than separate centerings for each rib type, and when the four-centred arch made the lower portion of the conoid easy to form as part of the fan itself.

  The earliest surviving example is the east walk of the cloister at Gloucester, where intricately decorated stone panels rise from the vault's springers in conical structures. In King's College Chapel, Cambridge, the great dimensions required transverse ribs to provide additional strength. At the Divinity School at Oxford and in Henry VII's chapel, a further development appeared: the complete conoid is detached from the wall and treated as a hanging pendant, correcting the visual defect at Gloucester where the vault appears half-buried in the wall.

  The vault over the staircase at Christ Church, Oxford, though not built until 1640, demonstrates the tradition surviving well into the 17th century, carried through, likely, by the late vaulting of Oxford college gateways.

  Meanwhile, across Europe, the relationship between structural vault and visible surface was quietly separating. Roman vaults and Byzantine domes were unprotected: what you saw inside was the same shell exposed to weather outside. Gothic builders covered their stone vaults with wooden roofs, so the vault was never visible from the exterior. Renaissance architects pushed this further. Michelangelo's dome for St Peter's Basilica, redesigned by Giacomo della Porta between 1585 and 1590, comprises two domes, only the inner of which is structural. St Paul's Cathedral in London goes further still: it has a structural catenary vault of brick that is invisible from both inside and outside, an outer dome that is a lightweight wooden frame, and an inner dome of plaster on wood, giving visitors the impression they are looking at the same structure seen from the street.

• 7. Hagia Sophia and the Pendentive

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  Two centuries after the Basilica of Maxentius, the next great leap in vaulting appeared in Constantinople in the Church of the Holy Wisdom, Hagia Sophia, built by Justinian. The central challenge was to place a round dome over a square floor plan, a problem the Romans had solved only by making the Pantheon's supporting wall 20 feet thick and virtually windowless.

  The solution at Hagia Sophia used pendentives: spherical spandrels formed where a hemispherical dome is cut by four vertical planes and then by a horizontal plane at the level of the tops of the four arches. That leftover curved triangle between each pair of arches provides a smooth transition from a square base to a circular one, without requiring a massive continuous wall.

  The first and second domes of Hagia Sophia both fell. Justinian chose to raise the replacement higher, partly for structural reasons and partly to bring more light into the interior. Forty windows were pierced around the base of the dome, and the effect, as described by the historian Procopius, was that the dome appeared to be suspended in the air. The dome's diameter reached 107 feet, far exceeding anything attempted in Byzantine construction afterward; later Byzantine churches imitated the plan of Hagia Sophia but never approached its scale.

  The Mosque of Damascus, built by Byzantine workmen for Al-Walid I in 705 CE, used a different pendentive: large niches in the angles created an octagonal base for the dome. Justinian's church of Saints Sergius and Bacchus in Constantinople took yet another approach: its central area was octagonal, with a dome divided into sixteen compartments alternating between flat bands and concave cells, giving the roof the appearance of an umbrella.

  Unlike the Pantheon's concrete dome, Byzantine domes were built in brick, which was lighter and thinner but more vulnerable to the outward forces the dome generates. That tradeoff between weight and vulnerability would continue to drive experimentation for centuries.

• 8. Beyond Europe

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  Two vaults in the Indian city of Bijapur represent a tradition of ribbed construction that developed independently of European practice. The Persian term for these vaults is Karbandi. One carries the central dome of the Jumma Musjid, built in 1559; the other is at the Gol Gumbaz, the tomb of Muhammad Adil Shah II, who reigned from 1626 to 1660.

  The Gol Gumbaz vault was built over a hall 135 feet square. Its ribs, rather than simply crossing the corners of the square to provide an octagonal base, continue all the way to the far pier of the octagon, intersecting one another and reducing the central opening to 97 feet in diameter. The masonry carried by those ribs acts as a counterweight to the dome's thrust. The internal diameter of the completed dome is 124 feet, its height 175 feet, and the ribs are struck from four centres with their springing point 57 feet above the hall floor.

  The Jumma Musjid dome was built on a square of 70 feet, with a diameter of 57 feet, carried on piers rather than the massive walls used at the tomb. Any remaining thrust was transmitted across aisles to the outer wall.

  The 20th century brought a different kind of vault altogether. Advances in reinforced concrete and a better mathematical understanding of hyperbolic paraboloids allowed extremely thin, curved shells to be cast in shapes impossible in masonry. The vaults in the Church of Saint Sava are made of prefabricated concrete boxes that were assembled on the ground and then lifted to a height of 40 metres on chains, a method that would have been incomprehensible to the masons who built Hagia Sophia.