Flying buttress, in architecture, is the term given to a structural feature used to transmit the thrust of a vault across an intervening space, such as an aisle, chapel or cloister, to a buttress built outside the latter. The employment of the flying buttress meant that the load bearing walls could contain cut-outs, such as for large windows, that would otherwise seriously weaken the vault walls.

The load is reduced on the vault wall by throwing a semi-arch across to a vertical buttress outside the building. Though employed by the Romans and in early Romanesque work, it was generally masked by other constructions or hidden under a roof, but in the 12th century it was recognized as rational construction and emphasized by the decorative accentuation of its features, as in the cathedrals of Chartres, Le Mans, Paris, Beauvais, Reims, etc.

Sometimes, owing to the great height of the vaults, two semi-arches were thrown one above the other, and there are cases where the thrust was transmitted to two or even three buttresses across intervening spaces. As a vertical buttress, placed at a distance, possesses greater power of resistance to thrust than if attached to the wall carrying the vault, vertical buttresses as at Lincoln Cathedral and Westminster Abbey were built outside the chapterhouse to receive the thrust. All vertical buttresses are, as a rule, in addition weighted with pinnacles to give them greater power of resistance.

See also:

• Gothic architecture
• Cathedral architecture