Copyright Goodheart-Willcox Co., Inc. 280 Section 4 Construction Details A cavity wall is built of masonry units arranged to provide a continuous air space of 2″ to 3″ wide. See Figure 14-5. The facing and backing wythes or tiers are connected with rigid metal ties. The exte- rior wythe is usually a nominal 4″ thick. The inte- rior wythe may be 4″, 6″, or 8″, depending on the load to be supported and the height and length of wall. The exterior masonry wythe is generally solid or hollow brick, and the interior masonry wythe can be brick, structural clay tile, or hollow or solid concrete masonry units depending on the properties required. When a cavity of nominal 2″ is maintained, the overall thickness will be 10″, 12″, or 14″. The cavity provides two advantages: • The air space has insulation value, and it can be filled with insulation material for added reduction of heat transfer. • The cavity acts as a barrier to moisture. However, to be effective, the cavity must be kept free of mortar droppings during construction. A board may be used to collect the droppings as shown in Figure 14-6. When weep holes are required at the bottom of a cavity wall, flashing should be used. A weep hole is a mortar opening, usually a joint left out, which creates a void where moisture inside the wall cavity can exit to the outside. The weep hole usually is located immediately above flashing. See Figure 14-7. Flashing is a thin, impervious material, usually made of metal, which is placed in mortar joints to prevent water penetration and to direct the drainage of water. The flashing keeps any moisture that might collect in the cavity away from the inner wall. See Figure 14-8. One of the major functions of an exterior wall is to resist rain penetration. A properly designed and built masonry cavity wall is totally resistant to rain penetration through the wall assembly. Goodheart-Willcox Publisher Figure 14-5. A method of cavity wall construction that uses metal ties and joint reinforcement. Metal tie Joint reinforcement Portland Cement Association Figure 14-6. A recommended method of keeping a cavity free of mortar droppings is to rest a board on the joint reinforcement (wire ties). The board is moved up when the next reinforcement level is reached. Goodheart-Willcox Publisher Figure 14-7. Weep holes provide an outlet for moisture that has collected in the space between wythes of masonry.