Masonry Magazine October 1995 Page. 35
Flashing Arches
Installation of flashing with other arch types, such as segmental and semicircular arches, can be more difficult. This is because most rigid flashing materials are hard to bend around an arch with tight curvature. If the arch span is less than about 3 ft (0.9 m), one section of tray flashing can be placed in the first horizontal mortar joint above the keystone, as illustrated in Fig. 5b. For arch spans greater than 3 ft (0.9 m), flashing can be bent along the curve of the arch with overlapping sections, as illustrated in Fig. 4. Alternately, a combination of stepped and tray flashing can be used, as shown in Fig. 5c. To form a step, the end nearest the arch should be turned up to form an end dam, while the opposite end is laid flat. A minimum of No. 15 building paper or equivalent moisture resistant protection should be installed on the exterior face of the backing over the full height of the arch and abutments. The building paper or equivalent should overlap the arch flashing.
The design of a structural masonry arch should include consideration of the effect of flashing on the strength of the arch. Flashing acts as a bond break. If flashing is installed above the arch, the loading on the arch will likely be increased, and the structural resistance of the arch will be reduced. Installation of flashing at the abutments will affect their structural resistance and should also be considered. Consult Technical Notes 31A for a more extensive discussion of arch loads and structural resistance of brick masonry arches.
DETAILING CONSIDERATIONS
The brick masonry arch should serve its structural purpose and also provide an attractive architectural element to complement its surrounding structure. Careful consideration should be given to the options available for the arch, soffit and skewback. Proper configuration of the abutments and location of expansion joints should be considered for any arch design.
Arch
Arches can be configured in a variety of arch depths, brick sizes and shapes and bonding patterns. The arch is normally composed of an odd number of units for aesthetic purposes. Some of the more common arch configurations are illustrated in Fig. 6. Arch voussoirs are typically laid in radial orientation and are most often of similar size and color to the surrounding brickwork. However, the arch can be formed with brick which are thinner or wider than the surrounding brickwork and of a different color for variation. Another variation is to project or recess rings of multiple-ring arches to provide shadow lines or a label course.
Brick masonry arches are constructed with two different types of units. The first is tapered or wedge-shaped brick. These brick are tapered in the appropriate manner to obtain mortar joints of uniform thickness along the arch depth. The second is uncut, rectangular brick. When rectangular brick are used, the mortar joints are tapered to obtain the desired arch curvature. In some cases, a combination of these is used. For example, a slanted arch is formed with a tapered keystone and rectangular brick. This arch is similar to a jack