St Paul's Cathedral: The Building That Refused to Burn Twice

Stand at the bottom of Ludgate Hill and look up. The dome above you is the second-largest in any cathedral in the world, smaller only than St Peter's in Rome. It is 111 feet in diameter, 365 feet from the ground to the cross at the top, and weighs roughly 65,000 tons. Almost everything about it is structural deception. What looks like one dome is three. What looks like stone is brick. What looks like a finished masterpiece is the result of two near-destructions, three hundred years apart, and a man who watched his own cathedral rise for nearly half his life.

What was here before

There has been a Christian cathedral on this hill since 604, when the Saxon bishop Mellitus founded the first wooden church to a plan from Pope Gregory the Great. That building burned. The second was Norman, begun in 1087 by Bishop Maurice and consecrated in 1240. It was one of the largest churches in Europe. Old St Paul's was 585 feet long, with a spire reportedly 489 feet tall, taller than the present cathedral. It dominated the medieval skyline of London for four hundred years.

The spire came down in 1561 after a lightning strike and was never rebuilt. By the early seventeenth century the cathedral was in poor structural condition, and Inigo Jones was hired in 1633 to refurbish the west front. Jones built a classical Portico in front of the medieval Gothic body, which was visually incoherent but at least kept the building standing.

In September 1666 the Great Fire reached the cathedral on its fourth day. The lead roof melted and ran down Ludgate Hill in streams. Booksellers in St Paul's churchyard had moved their stock into the crypt for safekeeping. The crypt vault held for a few hours, then collapsed, and the books burned. Stones cracked from the heat and exploded. Eyewitnesses described the noise as artillery. By the morning of 5 September, the medieval cathedral was a pile of charred stone.

Wren's commission

Christopher Wren was thirty-three years old in 1666 and not a trained architect. He was Savilian Professor of Astronomy at Oxford, a founding member of the Royal Society, and a man who had already designed two buildings, the Sheldonian Theatre at Oxford and the Pembroke College Chapel at Cambridge. The Great Fire gave him a blank canvas the size of a city. He designed fifty-one new parish churches across the City of London and started work on the new St Paul's in 1675. He was given the job over more experienced candidates partly because of his Royal Society connections to the King.

The design went through three versions. The first, the Greek Cross design, was rejected as too radical for an Anglican congregation that wanted a long nave. The second, the Warrant design, was approved in 1675 and was a compromise. The third, the design that was actually built, was Wren's own, which he developed during construction by exploiting a clause in the warrant permitting "ornamental variations". The cathedral that exists is essentially the third design.

Construction took thirty-five years. Wren laid the first stone in 1675 and watched his son lay the last stone on the lantern in 1710. He was hauled up to the dome in a basket twice a week in his old age to inspect the work. He died in 1723 at ninety years old. His tombstone in the crypt, directly beneath the dome, reads in Latin: Lector, si monumentum requiris, circumspice. Reader, if you seek his monument, look around you.

The dome problem

A dome of this size cannot be one thing. The weight of a stone dome 111 feet across, with a stone lantern weighing 850 tons on top, would crush any normal masonry support. Wren's solution was three concentric structures performing different jobs.

The inner dome you see from inside the cathedral is the visual dome. It is hemispherical, brick, faced with plaster and frescoed by James Thornhill. It carries no load above itself. Its only job is to be beautiful from underneath.

The outer dome you see from the street is the visual dome from outside. It is also brick, sheathed in lead, and it shapes the silhouette of London. It also carries no real load. It is a skin.

Between them is the cone. A hidden brick cone, invisible from inside or outside, runs from the drum at the base up to the lantern at the top. The cone carries the entire 850-ton weight of the stone lantern down to the eight piers at the dome's base. Without the cone, the inner dome would crack and the lantern would collapse. With it, both domes are decorative.

This three-layer system is the heart of the engineering. Wren and Robert Hooke worked the structural mathematics out together. Hooke, the same scientist who designed the Monument to the Great Fire as a failed telescope, helped calculate the catenary curves of the cone. The cone's profile follows the same line a hanging chain takes under gravity, inverted. It is a mathematically optimal load path. The dome stands because the cone is shaped like the curve a chain makes when held at its two ends.

The Blitz, 29 December 1940

On the night of 29 December 1940, the Luftwaffe targeted the City of London with the most concentrated incendiary raid of the war. Roughly 30,000 incendiary bombs fell on the Square Mile that night, plus high explosive. The fire that started in the City became the worst since 1666. Photographs from the next morning show whole blocks reduced to chimneys and rubble.

Twenty-eight incendiaries hit St Paul's specifically. One burned through the outer lead skin of the dome and lodged in the wooden timbers underneath, where the lead was attached to the structural cone. If it had set the timbers alight, the heat would have warped the cone, and the cone collapsing would have pulled down both the inner and outer domes. St Paul's would have been destroyed exactly as the medieval cathedral was destroyed in 1666, for the same reason: a fire in the roof becomes a fire in the structure.

The cathedral was saved by the St Paul's Watch, a volunteer fire brigade of about three hundred men, mostly architects, clergy, students, and Royal Academy fellows, organised by the cathedral's surveyor Walter Godfrey Allen since 1939. They patrolled the roof in shifts throughout the war. On the night of 29 December they were already on station when the incendiaries hit. The bomb in the dome timber was reached by two watchmen, Vernon Ridley and Ron Smith, who crawled across the lead skin on the outside of the dome with sandbags and hand pumps. They smothered the bomb before the wood caught.

The photograph taken from Ludgate Hill the next morning, "St Paul's Survives", shows the dome standing untouched in clouds of smoke from the burning city around it. Herbert Mason of the Daily Mail took it from the roof of the newspaper building. It became the iconic image of the Blitz. Winston Churchill had personally ordered that St Paul's must be saved at all costs. He was acting on a calculation: the morale value of the dome standing was higher than the military value of any of the fire engines deployed elsewhere that night. The calculation proved correct.

What the engineering and the volunteers had in common

The same structural logic that lets the dome stand is the reason it was savable. A monolithic stone dome of this size, of the sort built in medieval times, would have been impossible to monitor from above. Wren's three-layer system, with the outer skin separated from the cone by a structural cavity, meant the watchmen could climb between the layers and reach incendiary fires before they touched the cone. The cathedral was built to be inspected. That accessibility, designed in 1675 for the maintenance of a brand-new building, made it accessible to volunteers with sandbags in 1940.

This is the small irony of St Paul's. The dome that looks most permanent in London is the dome most carefully designed to be looked after. It survived because Wren built it to be maintained, and because three hundred volunteers in December 1940 maintained it.

The cathedral was completed in 1710. The dome stood without major incident for two hundred and thirty years. It then stood through six years of war that destroyed most of the buildings around it. It is now in its three-hundred-and-fifteenth year of standing, on the same hill, at the end of the same Roman street, looking out at a city that has rebuilt around it three times.