The Flying Buttress As External Structural Support

A flying buttress is an external, arched support that carries the outward-pushing thrust generated by a high stone vault across open space to a freestanding pier set some distance from the main wall, an arrangement that lets the building's outer wall itself be built thinner, taller and more heavily pierced by windows than it could safely be if it had to resist that same thrust entirely on its own. The device solved a specific structural problem inherent to tall stone vaulting: a heavy ribbed vault pushes outward at its springing points as well as downward, and earlier Romanesque builders had generally managed that outward thrust by building very thick, heavy walls or by concealing internal buttressing within the structure of the aisles, an approach that limited how tall a vault, and how large a window, a builder could safely attempt.

By moving the buttressing entirely outside the building and expressing it openly as a visible arched strut leaping across the side aisle roof from wall to pier, Gothic builders could redirect that same structural load while leaving the nave wall itself largely free for the glazed clerestory openings that define the mature Gothic interior, and the flying buttress's distinctive external silhouette, ranked rows of stone arches stepping down from the high nave wall to the ground, became one of the most immediately recognizable exterior features of a Gothic cathedral.