Copyright Goodheart-Willcox Co., Inc. Chapter 14 Wall Systems 299 Conversely, the voussoirs of a semicircular arch are all the same size and shape. The arch span is a consideration when selecting the arch brick. For short spans, tapered brick is recommended to avoid excessively wide mortar joints at the extrados (the curve that bounds the upper edge of the arch). Larger span arches require less taper of the voussoirs, so they can be formed with rectangular brick and tapered mortar joints. The thickness of mortar joints between arch bricks should be minimum of 1/8″ and a maximum of 3/4″. Depth is the dimension of an arch at the skew- back that is perpendicular to the arch axis, except in jack arches. Arch depth depends on the size and orientation of the brick used to form the arch. The depth is usually a multiple of the brick’s width. For structural arches, a minimum arch depth is deter- mined from the structural requirements. If an arch is supported by a lintel, any arch depth can be used. The depth of an arch is also based on the scale of the arch in relation to the scale of the building and surrounding brickwork. For proper scale and visual balance, arch depth should increase with increasing arch span. As a rule-of-thumb, segmental and semicir- cular arches should have an arch depth that equals or exceeds 1″ for every foot of arch span or 4″, whichever is greater. For example, the minimum arch depth for an 8′ span should be 8″ for segmental arches. The depth of a jack arch is the vertical dimension of the arch at the springing. The arch depth of jack arches should equal or exceed 4″ plus 1″ for every foot of arch span or 8″, whichever is greater. For example, the minimum arch depth for an 8′ span should be 12″. Typically, the depth of a jack arch is equal to the height of 3, 4, or 5 courses of the surrounding brickwork, depending upon the course height. Arch Construction Two general methods are used in the construc- tion of brick masonry arches. One method entails the use of special shapes that provide for uniform joint thickness. The second method uses regular units of uniform thickness while the joint thickness is varied to obtain the desired curvature. The arch dimensions and desired appearance determine the method to be used. Because an arch is a structural member, it is important that all mortar joints be completely filled. The crown of the arch is generally laid in a soldier bond or rowlock header bond, and it is frequently difficult to completely fill the joints. Brick Industry Association Figure 14-47. A typical method of supporting a brick masonry arch. Continuous flashing along arch span Curved steel angle as required Weld Weep hole at end Exterior sheathing and building paper Overlap flashing and seal with mastic Structural steel member as required Anchor bolt Flashing with end dam Horizontal steel angle as required Aneese/iStock/Thinkstock Figure 14-48. The keystone is located in the center of a brick arch to enhance the appearance of the window. Keystone Tapered or rectangular brick may be selected based on the arch type, arch dimensions, and desired appearance. Some arch types require unique shapes and sizes of brick if uniform mortar joint thickness is desired. For example, the bricks in a traditional jack arch or elliptical arch are different sizes and shapes from the abutment to the keystone.