A cavity wall consists of two wythes of masonry separated by at least 2 inches of air space. This air space serves as insulation and as a trap for water so moisture doesn't get inside the building. Water that penetrates the outer wythe of masonry runs down the cavity, hits flashing, and drains back out the wall through weep holes in the outer wythe. If mortar accumulates in the cavity, it can block the weep holes or act as a bridge that allows water to cross into the inner wythe. Thus, to keep water from penetrating the inside wythe of masonry, masons must keep cavities clean during construction. One method of keeping cavities clean is to catch mortar droppings on a rigid board. Another common practice is to place the mortar bed so it bevels away from the inside of the wall. Then place the brick with a rolling motion: set the cavity side down first, then roll the brick down from back to front. The beveled joint and the rolling motion squeezes most of the mortar toward the outside of the wall.