Buttresses and Flying Buttresses: How the Gothic Buttress System Works

A buttress is a heavy reinforcement on the outside of a wall that takes the thrust from the vaults inside the church. A flying buttress is a half arch that springs from such a pier, over the roof of the aisle and in towards the wall of the nave, and leads the thrust of the vaults out of the building. Together they are called the buttress system. It meant that the cathedrals of the Gothic could grow tall and the walls could be dissolved into windows: it is not the wall that carries the vault, but the stone skeleton outside.

Facts

The Problem: the Vault Pushes Outwards

Stone withstands compression but not tension. That is why an arch or a vault carries by each stone being pressed against its neighbours, and the load is led down into the supports. But an arch pushes not only downwards but also outwards, like a book standing leaning against another. The outward force is called lateral thrust, and if nothing takes it up the supports slide apart and the vault falls. The basics on arches and vaults explain why.

The simplest way to take up the lateral thrust is to make the wall thick. That is how the master builders of the Romanesque built: walls more than a metre thick, small windows and heavy, dim rooms. The higher the vault, the longer the lever the thrust gets and the thicker the wall must be. Anyone who wanted a tall and bright church interior therefore had to find another way of resisting.

The drawing of the line of thrust below shows the idea. The thrust takes a definite path through the stones, and the arch stands as long as that path fits inside them. The pointed arch leads the thrust more steeply downwards than the round arch, which was one of the reasons the Gothic chose it. But even the pointed arch pushes outwards.

The Buttress: the Wall Made Thick Where It Needs To Be

The Gothic rib vault gathers the thrust of the vault at a few points, at the corners of each bay. Then the wall does not need to be thick everywhere, only where the thrust arrives. The buttress is exactly that: a thick piece of masonry projecting from the wall at each vault springing, often in stages that become narrower upwards. Between the buttresses the wall can be thin and opened for windows.

Buttresses exist long before the Gothic, as flat reinforcements of the wall on Romanesque churches, and they remain long after, on churches, castles and barns. What makes the Gothic buttress special is the depth: it can project several metres from the wall. In many cathedrals chapels were later built in the space between the buttresses, as in Notre-Dame in Paris, where the chapels were completed in the 14th century. From outside the buttresses can then be hard to see.

A clear example is Sainte-Chapelle in Paris, Louis IX's chapel from the 1240s. It has no aisles and therefore no flying buttresses. The vaults are carried by buttresses on the outside and by iron bands that tie the windows together, and the walls of the upper chapel are almost nothing but glass.

The Flying Buttress: the Thrust Carried over the Aisle

A cathedral has a tall nave and lower aisles. The vaults of the nave sit high above the roofs of the aisles, and that is where the support is needed. A buttress directly against the wall of the nave would fill the aisle. The solution was the flying buttress: a half arch that springs from a freestanding pier outside the aisle, over its roof and in towards the wall of the nave, exactly where the thrust of the vault acts. Often there are two arches one above the other, or two piers in a row when there are double aisles.

The flying buttress emerged in the region around Paris in the second half of the 12th century. Notre-Dame, begun in 1163, is among the first churches where it was used to carry high vaults, and the arches were rebuilt as early as the 13th century. At Chartres, rebuilt after the fire of 1194, the flying buttresses were part of the structure from the start. There the master builders could give each vault bay two large pointed windows and a rose window above, and the three-storey wall became the pattern for the High Gothic.

On top of the buttresses stand pinnacles, small spires that look like pure decoration. But their weight presses the pier down and helps it resist the oblique thrust that the flying buttress delivers. Most things on the outside of a cathedral have a function, even those that look playful. The water is also led away: a channel often runs along the back of the flying buttress and ends in a waterspout, which may be carved as a grotesque.

How High Could One Go?

With the buttress system, height became a matter of ambition. The nave of Amiens, begun in 1220, reaches about 42 metres to the vault, and almost the entire wall between the piers is glass. At Beauvais they tried to go higher still. The choir was given vaults at about 48 metres, but in 1284 part of them collapsed, and the choir had to be rebuilt with more supports. The nave was never built. Beauvais shows where the limit lay with medieval technology.

The master builders did not calculate as an engineer does today. They used geometric rules and experience from earlier buildings, and they learned from mistakes. Only in the 17th century did the English natural philosopher Robert Hooke describe that an arch stands best if it has the shape of a hanging chain turned upside down, the same principle on which the drawing of the line of thrust is based. The medieval builders had nevertheless found their way to forms that lie close to it.

Cologne Cathedral shows how long the idea lived. Building began in 1248 on French models and was completed in 1880 following the medieval drawings, with flying buttresses in several tiers round the choir. On Milan Cathedral one can walk on the roof between the flying buttresses and the spires, and it is the best way of seeing the buttress system close up.

Buttresses in Sweden and After the Middle Ages

In the Nordic countries the Gothic was built mostly in brick and on a smaller scale, and flying buttresses are unusual. Buttresses, on the other hand, are found on a multitude of churches. Uppsala Cathedral, the largest church in the Nordic countries, has buttresses and the same three-part wall as the French cathedrals. Many Swedish parish churches were given buttresses afterwards, when brick vaults were built in during the 15th century and the old walls needed help to resist the thrust.

Another way was the hall church, in which the aisles are as high as the nave. Then the vaults of the aisles support the central vaults, and the thrust is taken up by buttresses in the outer walls without flying buttresses. Vadstena Abbey Church is such a hall church.

After the Middle Ages architects often wanted to hide the buttress system. Christopher Wren gave St Paul's Cathedral in London flying buttresses over the aisles but built tall screen walls in front of them, so that the church looks classical from the outside. The hallmarks of the whole style are in the guide to recognising Gothic, the building process in the guide to how a Gothic cathedral was built, and the terms in the glossary.

Look Out For

Exercise

What does a flying buttress do?