What is a flying buttress? How it works, why Gothic cathedrals needed it, where to see the best ones, and what it is not

Architecture · 4 September 2026

A flying buttress is an arch, usually a half-arch, that springs from a free-standing pier outside a building and leans against its upper wall, carrying the outward push of the stone vault inside over the roof of the aisle and down into the pier and the ground; it is called "flying" because it crosses open air. It was the device that made the Gothic cathedral possible: a stone vault 30 or 40 m up pushes its walls outward with a force that a wall thick enough to resist would leave no room for windows, and the flyer takes that push outside the building, so that the wall between the piers carries almost nothing and can be glass. The first were built on the nave of Notre-Dame de Paris about 1180, the first designed from the start at Chartres in 1194, and the most daring at Amiens and Beauvais; every cathedral of the 13th century has them, and so, in disguise, do domes from Hagia Sophia to St Paul's. Below: the problem, the ordinary buttress, how the flyer works, the pinnacle, the history, the parts, the physics in plain words, the buildings to see, and what a flying buttress is not, with the style in Gothic architecture and the building process in how the cathedrals were built.

The problem: a vault pushes outward

An arch or vault of stone stands by compression: each stone is squeezed by its neighbours and passes its weight down the curve to the supports, and at the supports the load arrives not straight down but at an angle, pushing outward as well as down; this outward push is the thrust, and it grows as the arch gets flatter, heavier or wider. A round barrel vault over a nave 10 m wide pushes out along its whole length, and the Romanesque builders of the 11th century held it with walls 2 or 3 m thick, few windows and small ones, and dark churches; a rib vault, the Gothic invention of about 1100, gathers the thrust into the four corners of each bay, so that the wall between the corners carries nothing, and the problem becomes how to hold four points 30 m up. The higher the vault, the longer the lever, and the thrust at the top of a tall wall will push it over unless something pushes back. Flying buttress.

The ordinary buttress and its limit

A buttress is a mass of masonry built against a wall to stiffen it, thicker at the bottom, and the Romanesque churches have them as shallow pilasters; making them deeper works for a low wall, but a cathedral nave has aisles on both sides, and the buttress against the nave wall would have to stand inside the aisle, blocking it, or the aisle roof would have to be raised to hide it, as was done at Paris and Laon with a gallery above the aisle whose vaults propped the nave wall from inside, at the cost of a dark church and a low clerestory. The Roman solution, in the baths and in the Basilica of Maxentius, had been to build the aisle vaults as buttresses to the nave, hidden under the roof; the Gothic solution was to take the prop outside and into the air.

How the flyer works

• The pier. A tall buttress of masonry stands in the open, outside the aisle wall, on its own foundation, with its foot in the ground and its head at the level of the springing of the high vault or above.
• The flyer. A half-arch of stone springs from the head of the pier and lands against the nave wall at the point where the vault's thrust comes out, usually just below the springing, so that the thrust runs along the curve of the flyer into the pier and down; the flyer must meet the wall at the right height, too low and the wall bends above it, too high and it pushes the wall in.
• Two tiers. Many cathedrals have two flyers one above the other from the same pier: the lower takes the vault's thrust, the upper the wind load on the high roof, which in a storm is as dangerous as the vault; Chartres has three arcades of spokes between its flyers, Amiens two tiers with tracery.
• The pinnacle. The stone spire on top of the pier is not decoration: its weight pushes the pier's load down and inward, so that the resultant of the flyer's sideways push and the pier's weight stays inside the pier's base and it does not tip; it is a counterweight, and the more the flyer pushes, the taller the pinnacle needs to be.
• The channel. The top of a flyer often carries a gutter that drains the high roof down to gargoyles on the pier, so that the flyer is also a water channel and the gargoyles spit at the end of it.
• The whole. A Gothic cathedral is a stone frame: piers inside, ribs above, flyers outside, with the thrust of the vault flowing through the ribs, across the flyers and down the piers to the ground, and the wall doing nothing but keeping the weather out, which is why it can be glass.

The history

• Before. Roman and Byzantine builders used hidden buttressing, and Durham (1093–1133) has quadrant arches under the gallery roofs that do the work of flyers in the dark; Cluny III had exposed flyers added about 1130.
• Notre-Dame de Paris, about 1180–1200. The first flyers designed as such, added to the nave during construction when the 33 m vaults proved too high for the gallery to hold, in a single sweep from the outer aisle wall over both aisles, 15 m long, the boldest of the 12th century; the present ones around the choir, with their long single spans, are 13th-century replacements, restored after the 2019 fire.
• Chartres, 1194–1220. The first cathedral planned with flyers from the start, which let the master drop the gallery and open the clerestory to the height of the arcade; the flyers of the nave are three tiers with radiating spokes.
• Bourges, 1195–1230. Flyers so steep and thin, over double aisles, that they look like flying ribs.
• Reims (1211), Amiens (1220–70), Beauvais (1225). The High Gothic system complete, with pinnacles and two tiers; at Beauvais the vault, at 48 m the highest ever built, fell in 1284, probably because the flyers' piers were too far apart, and was rebuilt with extra piers, the limit of the system found by trial.
• England and Germany. Used more sparingly in England, at Westminster Abbey and Lincoln, and hidden under aisle roofs in much of German Gothic; Cologne has them in the French manner.
• After Gothic. The Renaissance hid them again, in the scrolls of Florence's Santa Maria Novella and the drum of St Peter's; Wren's St Paul's has flyers concealed behind a screen wall; the 19th century, with Viollet-le-Duc, revived them, and steel made them unnecessary.

The physics in plain words

Stone is strong in compression and weak in tension: a column can be loaded almost without limit, but a beam of stone snaps if it is bent, and a wall of stone falls if it is pushed sideways beyond the point where the line of its weight leaves its base. A vault's thrust at the top of a wall is a sideways push at the top of a lever, and the wall's weight is the only thing holding it; the flyer adds a second push, inward, at the same point, so that the two cancel and the wall's stones are only squeezed, which they can bear; the flyer itself is in compression along its curve, so it too is only squeezed; and the pier below it is loaded by the flyer's push, slanting outward, plus the pinnacle's weight, straight down, whose resultant must fall within the pier's foundation, which the pinnacle ensures. Everything in the system is in compression, nothing is in tension, and no mortar is needed for strength: the medieval masons had no calculation but knew from experience and from failures like Beauvais where the lines of force had to go, and Robert Mark's photoelastic models of the 1970s showed that the flyers of Chartres and Bourges put the thrust almost exactly where it needed to be.

Where to see the best

• Notre-Dame de Paris. The choir's flyers from the Square Jean XXIII behind the apse, the longest single spans, restored in 2019–24.
• Chartres. The nave's three-tier flyers with spokes from the south side, and the apse's from the bishop's garden.
• Bourges. The steepest, from the archbishop's garden, over the double aisles.
• Amiens. The most complete system, with traceried flyers and pinnacles, from the south.
• Beauvais. The tallest, at the choir, with the piers added after 1284 and the timber props of 1993 still in place.
• Reims. Flyers with angels on the pinnacles.
• Cologne, Milan, Westminster Abbey, Washington National Cathedral (1907–90), St John the Divine in New York, St Patrick's in New York, whose flyers are ornamental, the roof being of plaster.

What a flying buttress is not

• A decoration. It carries the vault; take it away and the wall bends. The 19th-century Gothic Revival added fake ones to buildings with timber roofs, St Patrick's in New York among them, which are decoration.
• A buttress. A buttress is the mass against the wall; a flying buttress is the arch that flies from a detached pier to the wall. "Flying buttress" is the arch and pier together.
• Gothic only. The device is Gothic; the idea, of an external prop taking a vault's thrust, is Roman, and the hidden quadrant arches of Romanesque galleries are its ancestors.
• What holds up the roof. The timber roof sits on the walls; the flyers hold up the stone vault below the roof and resist the wind on the roof.
• A sign of weakness. Renaissance critics, Vasari and Wren, called the flyers crutches and hid them; the modern view is that a structure that shows how it stands is honest, and that the Gothic cathedral is the first building whose skeleton is its form.
• The reason for the pinnacles. The pinnacles are the reason for the pinnacles: their weight is what keeps the pier standing, and the sculpture on them is the medieval mason's way of making a counterweight beautiful.

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