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Flying buttresses at Bath Abbey, Bath, England. Of the six seen here the left hand five are supporting the nave, and the right hand one is suporting the transept. Notice their cast shadows on the windows

In architecture, a flying buttress, or arc-boutant, is usually on a religious building, used to transmit the thrust of a vault across an intervening space (which might be an aisle, chapel or cloister), to a buttress outside the building. The employment of the flying buttress means that the load bearing walls can 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.

Close-up of two flying buttresses at Bath Abbey, Bath, England. These are the right hand two buttresses of the picture above

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 butts 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. Vertical buttresses are usually weighted with pinnacles to give greater power of resistance.

This technique has also been used by Canadian architect William P. Anderson to build lighthouses at the beginning of the 20th century.

Villard de Honnecourt's drawing of a flying buttress at Reims, ca. AD 1320–1335 (Bibliothèque nationale)

Information on the constructon of flying buttresses from the ThinkQuest library: "To build the flying buttress, it was first necessary to construct temporary wooden frames which are called centering. The centering would support the weight of the stones and help maintain the shape of the arch until the mortar was dry. The centering were first built on the ground by the carpenters. Once that was done, they would be hoisted into place and fastened to the piers at the end of one buttress and at the other. These acted as temporary flying buttresses until the actual stone butt was complete." [1]

See also:

• Gothic architecture
• Cathedral architecture

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.