A flying buttress, or arc-boutant, is a specific type of buttress usually found on a religious building such as a cathedral. They are used to transmit the horizontal 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 them. Flying buttresses are often found in Gothic architecture.
The purpose of a buttress is to provide horizontal strength to a wall. The majority of the load is carried by the upper part of the buttress, so making the buttress as a semi-arch provides almost the same load bearing capability, yet in a much lighter and cheaper structure. As a result, the buttress seemingly flies through the air, rather than resting on the ground and hence is known as a flying buttress.
Though employed by the Romans in early Romanesque work, it was generally masked by other constructions or hidden under a roof. However, in the 12th century it was recognized as rational construction and emphasized by the decorative accentuation of its features, such as in the cathedrals of Chartres, Le Mans, Paris, Beauvais, and Reims.
Sometimes, for 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. Normal buttresses would add significantly to the weight of the overall structure, so the flying buttress is an essential aspect of the architecture. Because 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 like those 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.