What Is a Flying Buttress?

Stand outside a great Gothic cathedral and you see, leaning against its upper walls, a series of arched stone props springing out from the building to massive piers below. These are flying buttresses, and they are one of the most ingenious structural inventions in the history of architecture. The flying buttress let medieval builders raise impossibly tall, light-filled cathedrals by solving a single, stubborn problem: how to stop a high stone vault from pushing its own walls apart.

This article explains what a flying buttress is, the structural problem it solves, exactly how it carries load, why it made Gothic architecture possible, and where you see it. The engineering is genuinely elegant, and once you understand the forces, the strange external skeleton of a cathedral suddenly makes complete sense.

What a flying buttress is

A flying buttress is an external structural element consisting of an inclined arch or strut — the “flyer” — that springs from the upper part of a building’s wall and reaches across open space to a freestanding masonry pier set away from the building. Its job is to brace the high wall against an outward push, carrying that horizontal force out to the pier, which then channels it safely down to the ground.

The “flying” part of the name comes from the fact that the supporting arch does not sit against the wall along its whole length, as an ordinary buttress does. Instead it leaps across an open gap, often spanning above a side aisle, to meet the wall high up exactly where the support is needed. The result is the distinctive external network of arches and piers that defines the silhouette of a Gothic cathedral.

The problem it solves: outward thrust

To understand the flying buttress you have to understand the problem of thrust. A masonry vault — the great arched stone ceiling over a cathedral nave — does not press straight down on its supports. Because it is built of arches, its weight travels down the curved surface and arrives at the base pushing both downward and outward. This outward horizontal push is the thrust, and it tries to spread the tops of the walls apart.

If nothing resists that thrust, the walls bow outward and the vault cracks and collapses. The old solution was simply to build very thick, heavy walls strong enough to resist being pushed over — but thick walls are dark, because there is no room for large windows, and there is a limit to how high and thin you can build that way. Gothic builders wanted the opposite: walls that were tall, thin and filled with glass. To do that they had to deal with the thrust some other way.

How a flying buttress works

The flying buttress resolves the thrust by intercepting it and carrying it outside the building. Here is the chain of forces:

The vault pushes outward against the upper part of the nave wall.
The flying buttress — the inclined arch — meets the wall exactly at the height where the thrust acts, and pushes back against it, holding the wall in place.
That outward force travels along the flyer, across the open span, to the top of the external pier (the “buttress” proper, a tall mass of masonry standing clear of the building).
The pier, helped by its own great weight and often topped by a heavy pinnacle, redirects the combined forces steeply downward and carries them safely to the foundations.

Crucially, the whole system works in compression — the stone is being squeezed, which is exactly what stone does well. The heavy pinnacles often placed on top of the piers are not just decorative: their weight adds downward force that helps keep the line of thrust safely within the masonry, preventing the pier from being pushed over. The flying buttress, in short, takes the vault’s outward push, moves it clear of the wall and delivers it to the ground through a freestanding pier — leaving the wall itself free of the job of resisting thrust.

Why it made Gothic cathedrals possible

This is the key consequence. Once the flying buttress takes over the job of resisting the vault’s thrust, the nave wall no longer needs to be thick and heavy to hold the vault in. The wall can become thin, and — most importantly — it can be opened up into vast windows, because it is no longer doing the structural work of resisting outward thrust; the external buttresses are.

This is exactly why Gothic cathedrals can soar to such heights and dissolve their upper walls into enormous expanses of stained glass. The rib vault and pointed arch concentrated the loads and thrust onto specific points; the flying buttress reached in and braced those points from outside; and the wall between, relieved of its structural burden, could open to the light. The structure and the luminous, vertical aesthetic of the Gothic are inseparable — the flying buttress is the device that connects them. Reasoning through forces like this, tracing compression and thrust through a structure, is the heart of what a structural engineer does.

Where you see flying buttresses

Flying buttresses are most famously found on the great Gothic cathedrals of medieval Europe. Notre-Dame de Paris is among the best-known examples, its choir and nave braced by a striking array of flyers; the cathedrals of Chartres, Reims, Amiens and many others display the system at full development. They appear wherever builders sought to combine very tall vaults with thin, glass-filled walls.

While the flying buttress belongs primarily to the Gothic era — modern steel and reinforced concrete can resist tension and bending directly and so no longer need external props to manage thrust — it remains a vivid lesson in structural thinking, and it is still very much a live concern for anyone responsible for a historic masonry building. The thrust those buttresses resist is still acting today, and if a buttress, pier or foundation shifts, the balance can be disturbed. Assessing the stability of historic vaulted structures and their buttressing is specialist work that our structural-engineering consultancy can undertake.

Frequently asked questions

What is a flying buttress?

A flying buttress is an external arched strut that springs from the upper wall of a building across an open space to a freestanding masonry pier, bracing the wall against the outward thrust of a vault and carrying that force safely down to the ground.

What problem does a flying buttress solve?

It resists the outward thrust that a masonry vault exerts on the walls supporting it; without something to take that horizontal push, the high walls would bow outward and the vault would collapse, so the flying buttress carries the thrust away to an external pier.

How does a flying buttress carry load?

The inclined arch meets the wall where the vault’s thrust acts and transmits that outward force, in compression, across to the top of a tall external pier; the pier, weighed down by its own mass and often a heavy pinnacle, redirects the force steeply down to the foundations.

Why did Gothic cathedrals need flying buttresses?

Gothic builders wanted very tall, thin walls filled with stained glass, which could not resist a vault’s thrust on their own; flying buttresses took over that job from outside, freeing the walls to open into large windows and allowing the cathedrals to soar.

Are flying buttresses still used today?

They are rare in new construction because modern steel and reinforced concrete can resist tension and bending directly without external props, but they remain a vital structural concern on historic buildings, where the thrust they resist is still acting and must be kept in balance.

Responsible for a historic vaulted or buttressed building and need its stability assessed? Talk to our consultancy or get in touch.