Buttress (from the O. Fr. bouteret, that which bears a thrust, from bouter, to push, cf. Eng. "butt" and "abutment"), masonry projecting from a wall, provided to give additional strength to the same, and also to resist the thrust of the roof or wall, especially when concentrated at any one point. In Roman architecture the plans of the building, where the vaults were of considerable span and the thrust therefore very great, were so arranged as to provide cross-walls, dividing the aisles, as in the case of the Basilica of Maxentius, and, in the Thermae of Rome, the subdivisions of the less important halls, so that there were no visible buttresses. In the baths of Diocletian, however, these cross-walls rose to the height of the great vaulted hall, the tepidarium, and their upper portions were decorated with niches and pilasters. In a palace at Shuka in Syria, attributed to the end of the 2nd century A.D., where, in consequence of the absence of timber, it was necessary to cover over the building with slabs of stones, these latter were carried on arches thrown across the great hall, and this necessitated two precautions, viz. the provision of an abutment inside the building, and of buttresses outside, the earliest example in which the feature was frankly accepted.
In Byzantine work there were no external buttresses, the plans being arranged to include them in cross-walls or interior abutments. The buttresses of the early Romanesque churches were only pilaster strips employed to break up the wall surface and decorate the exterior. At a slightly later period a greater depth was given to the lower portion of the buttresses, which was then capped with a deep sloping weathering. The introduction of ribbed vaulting, extended to the nave in the 12th century, and the concentration of thrusts on definite points of the structure, rendered the buttress an absolute necessity, and from the first this would seem to have been recognized, and the architectural treatment already given to the Romanesque buttress received a remarkable development. The buttresses of the early English period have considerable projection with two or three sets-off sloped at an acute angle dividing the stages and crowned by triangular heads; and slender columns ("buttress shafts") are used at the angle.
In later work pinnacles and niches are usually employed to decorate the summits of the buttresses, and in the still later Perpendicular work the vertical faces are all richly decorated with panelling.