Most brick veneer and concrete block backup walls perform as they should. But there are certain details-avoidable details-that can sabotage a wall's performance. Watch out for them: AIR SPACES THAT ARE TOO SMALL The Brick Institute of America (BIA) and the National Concrete Masonry Association (NCMA) both recommend a minimum 2-inch air space. Air spaces smaller than 2 inches are difficult to keep clean. If excess mortar isn't removed from the back of the brick veneer, a bridge can form to the block backup. This bridge can trap water that penetrates the veneer. SLUSHED COLLAR JOINTS ACI/ASCE 530, Building Code Requirements for Masonry Structures, permits filling collar joints of composite walls with mortar or grout. According to BIA, slushing collar joints with mortar isn't effective because it's difficult to completely fill all voids in the joints. RECESSED BRICK COURSES When a wall differs from normal bonding it must still perform properly. For example, a recessed brick course, commonly detailed to have a 3/4-inch offset, can block the cavity. If moisture enters the wall above the recess, it can't drain to the flashing and weep holes.