Aside from its visual appeal, brick (as an exterior building material) is durable. Over time, however, its deterioration is inevitable. Because bricks are porous – they expand or contract according to moisture levels and thermal influences – water is a constant threat and the principal cause of deterioration in brick at the building envelope. So is the restriction of movement in brick building envelope systems.
Types of Wall Construction
Brick exterior walls can be classified as either barrier walls or drainage walls. Barrier walls are constructed of solid masonry without drainage cavities. They can be constructed of single or multiple wythes, entirely of brick, or with concrete masonry unit or terra cotta back-up. Multiple wythe brick barrier walls (three wythes or more) are designed to prevent water infiltration to interior spaces through mass. Ideally, the amount of water absorbed by a wall over a given period of time is less than can be dissipated in the same time period. In a barrier wall constructed with two wythes of brick (or in composite walls), a collar joint (grouted solid with mortar) joins face brick with a masonry back-up. Water that penetrates the face brick follows the collar joint down to flashing where it is either expelled through the bed joint and/or at weeps, or it dissipates through the face of the wall.
Drainage walls are designed with cavities between outer wythes of face brick and back-up walls (brick, concrete masonry units, metal or wood stud framing). Ideally, water that penetrates the face brick or enters the cavity is collected at flashing where it is expelled through a bed joint and/or at weeps.
When Brick Exteriors Fail
Symptoms of deterioration in brick exterior walls are generally attributable to water infiltration and include staining and efflorescence, cracking/spalling/displacement, and deterioration in mortar joints, among other things.
Efflorescence occurs when water washes soluble salts out of mortar and onto the surface of brick. It is apparent in the form of white crystalline particles that develop on brick surfaces as water evaporates.
Cracks and spalls in brick can result when water absorbed/retained by brick freezes. The expansion of steel (embedded reinforcing or lintels) from rust in brick wall systems can also cause cracking/displacement.
Mortar, used to bond bricks together, must be softer than the brick it binds (so bricks don’t crack during expansion), and must be tooled in a way (concave/rodded) that discourages the collection of water in the joint. Re-pointing is required when the bond between the brick and the mortar fails.
The Roles of Relieving (Shelf) Angles and Soft Joints
Brick expands and contracts with changes in temperature and moisture content. Relieving (shelf) angles are necessary to ensure that movement is accommodated between face brick and back-up wall systems, and that cracks and displacement attributable to restraint in the system are alleviated. Soft joints installed at horizontal (shelf) angles, and at vertical control and expansion joints, will accommodate movement and create relief for the brick’s expansion.