Flying Buttress is an arched exterior support that transfers a roof's outward thrust to the ground.
A flying buttress is an arched, freestanding structure that carries the lateral thrust of a vaulted ceiling or roof away from a building's walls and down into a pier planted some distance from the structure itself. The device let Gothic masons do something Romanesque builders could not: replace thick, load-bearing walls with soaring expanses of stained glass. Notre-Dame de Paris, Chartres, and Reims all owe their height and luminosity to rings of flying buttresses hidden — and sometimes proudly displayed — around their choirs and naves. Because the buttress absorbed the thrust that would otherwise have to be resisted by wall mass, architects could push vaults higher and windows wider with each generation, a competitive escalation that produced the tallest cathedrals in medieval Europe. Before its development, masonry vaults exerted enormous outward thrust on the walls that supported them, which meant Romanesque churches needed thick, heavy walls with small windows to resist that force — a structurally necessary but visually dim, fortress-like interior. The flying buttress solved this by carrying the vault's lateral thrust outside the building entirely, across an open arch, down into a freestanding pier reinforced by its own weight (often a stone pinnacle). Freed from having to resist thrust themselves, the walls between buttresses could be thinned dramatically and opened up with the large stained-glass windows that define Gothic architecture's luminous interiors — Chartres, Reims and Notre-Dame de Paris all depend structurally on their exterior buttress systems, even though visitors inside experience only the soaring height and colored light those systems make possible. The device evolved rapidly across the Gothic period: early flying buttresses (12th century) were relatively simple single arches, while later cathedrals developed multiple tiered, elaborately pierced buttresses combined with decorative pinnacles that were themselves structural, adding weight exactly where needed to keep the thrust angled safely into the ground. Structurally, the flying buttress is a direct ancestor of the tension-and-compression logic in Gustave Eiffel's ironwork and, later, in reinforced concrete and steel-frame skyscraper construction — different materials solving the same fundamental problem of getting a tall structure's forces safely to the ground.