Why Florence Left Its Cathedral With a Giant Hole in the Roof for 140 Years

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In 1296, Florence began building what it hoped would be the greatest cathedral in the world. Workers laid the foundations, raised the walls, and built the drum that would support the dome. Then construction stopped — and stayed stopped for over a century. Not because of money. Not because of war. But because nobody on Earth knew how to build the roof.

A Cathedral With a Hole in Its Head

The cathedral was designed with an octagonal drum 42 metres wide. Medieval builders knew how to construct high walls and pointed arches. They did not know how to span a gap that size.

The standard method was centring: a temporary wooden frame built across the full width of the dome, which held the bricks in place while the mortar dried. For the Florence dome, this would have required a staggering quantity of timber — trees felled from across Tuscany — and years of work before a single brick of the dome could go up.

Several proposals circulated over the decades. One architect suggested filling the cathedral with soil mixed with coins, so that locals would dig it out themselves once the dome was finished. Nobody was convinced.

The Competition Florence Could Not Solve

By 1418, the cathedral authorities were out of ideas. They opened a competition and invited proposals from across Europe. Whoever could explain how to build the dome would win the commission.

Florence in the early 15th century was the cultural heart of Europe. The same powerful merchant families who were sponsoring Botticelli and Ghiberti were watching their cathedral sit open to the rain, decade after decade. It was an embarrassment for the city.

The man who answered the competition was not an architect. Filippo Brunelleschi had trained as a goldsmith and sculptor. He had spent years in Rome studying ancient buildings — measuring walls, tracing arches, trying to understand how the Romans had built things that still stood a thousand years later. When he presented his proposal to the guild, they asked him to explain it fully. He refused. He said that if he explained, his rivals would steal the method. The guild, having no better option, eventually let him build a scale model instead.

The Double Shell

Brunelleschi’s central idea was to build two domes instead of one.

The inner shell was thinner and carried most of the structural weight. The outer shell was what the city would see — taller, more dramatic, clad in the terracotta tiles that still define the Florence skyline. Between the two shells was a gap just wide enough for a worker to move through. This is the space visitors pass through today when climbing the dome’s 463 steps.

Eight large stone ribs ran vertically from the base to the lantern at the top, with sixteen smaller ribs between them. These ribs — visible from outside as white marble lines against the terracotta tiles — gave the dome its distinctive striped silhouette, recognisable from miles around.

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The Herringbone Trick

Even with a double shell, the dome still had to support its own weight during construction. Every horizontal ring of bricks had to bear the weight of everything above it, without a wooden frame underneath.

Brunelleschi’s answer was a bricklaying technique called spinapesce — herringbone. Every ninth row of bricks was laid vertically rather than horizontally. This locked each ring of brickwork into a self-supporting circle that could hold its own weight before the mortar dried. No centring required.

The technique had been used in ancient Rome but was long forgotten. Brunelleschi appears to have rediscovered it during his years studying Roman ruins — and kept it close. Construction proceeded ring by ring, each layer resting securely on the one below, rising steadily above the city.

The Machine Above the City

Getting materials to the construction site was its own challenge. The dome sat 55 metres above street level. Hundreds of tonnes of brick, stone, and mortar had to be lifted there every week throughout sixteen years of work.

Brunelleschi invented a hoisting machine powered by oxen walking in a circle to wind a lifting rope. The cleverest feature was a reversible gear: the oxen could walk in the same direction to both raise and lower loads, simply by shifting a lever. It cut waiting times and kept the work moving. Leonardo da Vinci, who later transformed Milan’s canal network, sketched detailed hoisting devices of similar design in his notebooks, suggesting the machine’s influence reached well beyond the construction site.

What the Dome Changed

The dome was completed in 1436. At the time it was consecrated, it was the largest brick dome ever constructed — and it still holds that record today.

For generations after Brunelleschi, architects made the journey to Florence to study it. Michelangelo examined it carefully before designing the dome of St Peter’s Basilica in Rome. Christopher Wren visited Italy and took notes before designing St Paul’s Cathedral in London. The double shell, the herringbone brickwork, the idea of building without centring — these concepts spread across Europe and shaped how builders thought about domes for centuries.

Brunelleschi never fully explained his method. He died in 1446 having taken some of his secrets with him. Engineers and historians are still working out the details.

Climb the steps inside the dome today and you pass through the gap between the two shells — a narrow passage where the walls are close enough to touch on both sides. It is one of the most extraordinary spaces in Italy: a place that was not supposed to exist, built by a goldsmith who refused to take no for an answer. Standing at the top, looking down over Florence through the circular windows, the city spread out below the terracotta rooftops, it becomes easy to understand why the city waited 140 years. Some buildings are worth every one of them.

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Plan Your Italy Trip

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