Masonry Cavity Wall Construction Manages Water by Draining It Rather Than by Keeping It Out Completely
Modern masonry construction uses a cavity wall system - two parallel wythes (layers) of masonry separated by a continuous air space (the drainage cavity) - rather than a single solid masonry wall. The cavity serves a critical function: any water that penetrates the outer wythe (which will inevitably happen through minor cracking, imperfect mortar joints, or through the mortar-brick bond itself) falls to the bottom of the cavity and is drained back to the exterior before it can reach the interior. This drainage-plane approach to moisture management makes cavity wall masonry far more reliable than face-sealed masonry systems, which depend on a perfect seal at every joint - an impossible standard for materials that move with temperature changes and cure with some shrinkage. The ARE PDD exam tests masonry cavity wall construction as a building envelope detailing and performance topic.
The Two Wythes
A typical masonry cavity wall for a commercial building consists of: the outer wythe - typically 4-inch brick veneer; the drainage cavity - minimum 2 inches wide (ACI 530 requires 2 inches minimum for cavity walls with insulation in the cavity, to maintain a clear drainage path); the backup wythe - commonly 8-inch CMU (concrete masonry unit), concrete, or steel stud framing with rigid insulation in the cavity; and the air barrier/waterproofing on the backup wythe face. The outer brick wythe is a veneer - it carries no structural load (the structure is carried by the backup wythe or the frame) - but it does carry its own weight in gravity through shelf angles at each floor, and it carries out-of-plane wind loads through masonry veneer ties to the backup system.
Through-Wall Flashing
Through-wall flashing is the waterproofing component that collects water from the drainage cavity and diverts it to the exterior. Flashing must be installed at: the base of the wall (sill); at every shelf angle (the shelf angle supports the brick veneer at each floor and interrupts the drainage cavity - water must be redirected out at this point); above all window and door heads; and at any other horizontal interruption of the drainage cavity. Flashing extends through the full depth of the outer wythe and turns up against the backup wythe - any water that reaches the bottom of the cavity lands on the flashing and is directed out through weep holes in the mortar joint immediately above the flashing.
Weep Holes
Weep holes are small openings in the face of the mortar joint immediately above the through-wall flashing. They allow water that has collected on the flashing to drain back to the exterior. Without weep holes, water would pond on the flashing and eventually overflow to the interior. Weep holes are typically spaced at 24 inches on center - every third or fourth vertical mortar head joint is left open (an open head joint weep hole). Alternatively, small plastic or metal inserts are used to maintain a drainage opening while preventing insects from entering the cavity.
Shelf Angles
At each floor level, a horizontal steel angle (the shelf angle) is attached to the structural frame and supports the weight of the brick veneer above. The shelf angle is the gravity support for the veneer at each floor - it prevents the full height of brick from acting as a single loaded element and it accommodates differential movement between the brick and the structure. The joint at the shelf angle (between the top of one course of brick below and the bottom of the shelf angle) is an expansion joint filled with backer rod and sealant, not mortar - because the brick and the structure experience different amounts of thermal expansion and this joint must accommodate that movement.
Key Exam Points
- Cavity wall: outer wythe (brick veneer), drainage cavity (min. 2 inches), backup wythe (CMU, concrete, or stud).
- Drainage cavity: manages water by drainage rather than by sealing - more reliable than face-sealed systems.
- Through-wall flashing: at base, shelf angles, window/door heads; redirects water to exterior at weep holes.
- Weep holes: at 24 inches o.c. in mortar joint above flashing; allows cavity drainage to escape.
- Shelf angle: gravity support at each floor; expansion joint below (not mortar) to accommodate differential movement.
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