Florence Cathedral, Florence

A dome that could not be built

When the city of Florence began constructing its new cathedral Santa Maria del Fiore in 1296, the design called for a dome larger than any built in Europe since antiquity — 45 metres in diameter, roughly the size of the Pantheon's dome in Rome.

For the next 124 years construction proceeded up to the level of the drum and then stopped, because no architect of medieval Europe knew how to build a dome that large. The standard medieval method required wooden centring — a temporary timber framework on which the masonry was laid until it could support itself — but the required centring would have taken more timber than was available in Tuscany.

A goldsmith who solved the problem

Filippo Brunelleschi was not an architect.

He was a Florentine goldsmith and clockmaker who had lost the competition to design the bronze doors of the Florence Baptistery to his rival Lorenzo Ghiberti in 1401.

Embittered, Brunelleschi went to Rome and spent fifteen years studying the ruins of Roman architecture, particularly the Pantheon's dome. When the Florence cathedral commission reopened in 1418, Brunelleschi submitted a dome design that required no centring at all. The dome would be self-supporting at every stage of construction.

The commissioners did not believe him.

A double shell and a herringbone pattern

Brunelleschi's solution was unprecedented.

The dome would be a double shell — an outer dome and an inner dome with a hollow space between them, both supporting each other. The eight visible white marble ribs on the exterior anchor and stiffen the structure. The bricks of the inner shell were laid in a spina di pesce — herringbone — pattern, courses of horizontal bricks alternated with single bricks stood on end, locking each new course in place.

Brunelleschi also invented several machines specifically for the project. A hoist with a reversible gear, so loads could be raised and lowered without unyoking the oxen. A lantern-positioning crane that he refused to explain in detail because he did not want the design copied.

Completed sixteen years before he died

Brunelleschi began work in 1420. The dome was structurally complete in 1436, sixteen years later, and consecrated by Pope Eugenius IV that year. The lantern was finished in 1461, fifteen years after Brunelleschi's death in 1446, following his original design.

The dome remains the largest masonry dome ever built.

Modern engineering analyses have confirmed Brunelleschi's calculations were almost exactly right. The dome carries its load with very small safety margins, but the margins are positive. Six centuries on, the structure is still standing because it was designed correctly, not because it was overbuilt.