Strainer arch

A strainer arch is an internal structural arch built to relieve the inward pressure off the spanned vertical supports (providing a "buttress", thus also called buttressing arches), usually as an afterthought to prevent the supports from imploding due to miscalculation. In the past they were frequently adorned with decoration, with one of the best examples provided by the Wells Cathedral. Strainer arches can be "inverted" (upside-down) while remaining structural.

The typical construction of Romanesque and Gothic churches includes east-to-west arcades, where each arch is buttressed by its neighbors. The issue at the east end is resolved using the buttressing of an apse to the choir, while at the west a massive "westwork" is used. In a large church a similar problem occurs at the crossing, where the arcades of the nave and choir have to terminate. A "spectacular" solution for the crossing buttressing issue at the Wells Cathedral was found by William Joy (1338, 17 years after completion). A similar arrangement was added to the both crossings of the Salisbury Cathedral more than 100 years after the completion of most of the building, but shortly after the addition of a tower (1380). The straining arches were added in the east-west direction, thus being unobtrusive when viewed from the nave. The arrangement at the east crossing (built in 1388) is similar to the one at Wells, while the great crossing uses a single-arch strainer design. In the Bristol Cathedral the strainer arches are used to carry the thrust from the central vault over the aisles, as in this hall church building the width of the aisles is half that of the nave, therefore the transverse forces cannot be balanced in an arcade-like fashion.