The two basic drainage walls are the cavity wall and the veneer wall. Each has a masonry outer wythe and is separated from a structural backup by 2- to 4 1/2-inch air space. Cavity walls have a backup wythe of masonry—usually concrete masonry—and the exterior wythe is designed to resist lateral loads. In veneer walls, the outer wythe is backed by a masonry wall, a concrete wall, or a wall of steel or wood framing covered by sheathing.
Keeping the air space between the backup wall and outer wythe clean of mortar droppings is critical to the proper functioning of the drainage wall. Here are some recommendations on preventing mortar droppings: Bevel the bed joints or "roll" the brick into place; Catch droppings with mortar collectors or drainage mats; Make the flashing continuous; Extend the flashing past the face of the wall; At the backup wall, seal or cover the top edge of the flashing; Protect the flashing from punctures.
Flashing is made of sheet metal or a flexible material. Metal flashing is made of stainless steel, galvanized steel, or copper. Flexible flashing is made from plastic (usually PVC), rubberized asphalt, EPDM, or copper laminates.
Weep holes drain away water collected on the flashing and ventilate the air space. They may seem like a small part of masonry wall construction, but they are critical to the durability and performance of the drainage wall.