Jack arch

A jack arch, also known as a flat arch or French arch, is a structural element used to span an opening in a wall. Unlike a true arch, a jack arch is constructed with a nearly flat intrados (the underside or inner curve of an arch). The individual voussoirs (wedge-shaped stones or bricks) are typically inclined at a slight angle to each other, creating a gentle upward thrust. This thrust is resisted by the abutments on either side of the opening.

Jack arches are often used where a more decorative arch is not desired or where headroom is limited. They are frequently seen above windows and doors in brick or masonry buildings. While appearing relatively simple, the construction of a jack arch requires careful consideration of the materials and the forces involved to ensure stability and prevent failure. The slight angle of the voussoirs is critical for distributing the load effectively. The joints between the voussoirs are typically thin and filled with mortar. While they appear flat, a slight camber (convexity) is often built into the arch to compensate for settlement over time.