Brick Arch Voussoir Spacing & Timber Centering Guide

A true masonry arch is an engineering marvel: it supports compressive loads across open spans without steel lintels through wedge-to-wedge friction and arching action.

In this guide, we will examine how to calculate the exact number of voussoir bricks, wedge mortar joints, and construct temporary timber centering forms.

---

The Golden Rule: The Apex Keystone Must Be Centered

In traditional brick and stone masonry, the total number of voussoirs must always be an ODD number.

• An odd count ensures that one single, symmetrical keystone brick sits directly on the center apex line.
• An even count creates a vertical mortar joint running straight through the highest point of compression, making the arch prone to centerline cracking.

---

Calculating Voussoir Count & Mortar Taper

To find the number of bricks required across the intrados curve:

N = round(Arc_Length / (Brick_Width + Mortar_Joint))

If N is even, increment by 1 to ensure an odd count for the keystone.

Mortar Joint Taper

Because the arch curve is concave, mortar joints cannot be a uniform thickness:

• Intrados Joint (Bottom): typically 1/4” to 3/8” (6 mm - 10 mm).
• Extrados Joint (Top): expands outward based on the arch depth.