Jack arch

Unlike lintels, which are subject to bending stress, jack arches are composed of individual masonry elements cut or formed into a wedge shape that efficiently uses the compressive strength of the masonry in the same manner as a regular arch.

Jack arches have the advantage of being constructed from smaller pieces of material that can be handled by individuals, as opposed to lintels which must necessarily be monolithic and which must be oversized unless reinforced by other means.

In small-scale brick masonry projects, jack arches are typically sawn from an appropriately sized fired-clay lintel, giving a more precise and consistent joint width than field-sawn shapes.

Keystones, stepped or arched top profiles, and polychrome or contrasting colors and materials may all be used to create the desired effect.

The floor of the chapel (and the roof of the crypt) is formed from a jack arch and is said to have been constructed in the centuries BC.