November 1, 2001
by John Straube
Masonry has been used for thousands of years in many famous and durable structures, such as the Parthenon and Notre Dame. In fact, some cultures only allowed kings and priests to live in masonry buildings. Masonry includes fired clay brick, calcium silicate (lime sand), concrete, and natural stone units erected into an assembly using mortar.
The benefits of masonry walls include a moisture-resistant, fireproof, impact-resistant, sound absorbing, high thermal mass and structurally sound material. Masonry has the disadvantage of being heavy (a drawback for tall buildings and in earthquake zones), requires skilled labour and warm weather for construction, is sensitive to freeze-thaw damage, and can be expensive in some applications. Masonry walls can easily be designed to provide one of the most durable high-performance enclosures. Some basic guidelines for design and construction are provided below.
The first design decision should be the structural scheme. Historically the most common masonry walls were solid but one can also choose veneer, composite, or cavity walls. Each approach has its advantages and disadvantages. Each also requires different design, detailing, and construction.
Cavity walls have been widely used only recently even though their superior rain control characteristics were recognized by Vitruvius. Veneer walls are presently the most popular type of masonry wall. Although both contain an air space, the difference between the two types of wall is one of structural design. Cavity walls are designed to share the lateral loads between interior and exterior wythes of masonry. The loads are transmitted across the air space using stiff metal ties. Veneer walls use a non-load bearing exterior masonry wythe as cladding, laterally supported by some type of structural backup.
It is important to avoid inadvertently applying vertical load to the veneer–it must be allowed to move freely. In buildings over three or four storeys in height, soft joints, sized to accommodate building creep and shrinkage, should be provided at floor levels. Movement space below roof overhangs and windows and vertical control joints should be provided on most projects. The veneer must be supported at its base over a minimum of two-thirds of its thickness. To account for construction tolerances, a support of seven-eighths thickness should be specified.
Veneer ties should be adjustable to account for construction tolerances, flexible enough not to impose vertical loads on the veneer and strong and durable enough to resist lateral loads for the design life of the veneer. The Canadian masonry connector code (CSA A370) requires stainless steel ties for tall buildings in wet climates. If a cavity wall design is chosen, one must ensure that the ties are stiff enough to transfer the vertical shear, and that differential movement between the wythes due to moisture, temperature, and structural effects will not overstress the system or cause bowing and cracking.
The thermal mass of insulated masonry provides improved comfort and reduced energy consumption in both summer and winter. As with other types of walls, a significant amount of insulation should be placed in the cavity outside of the inner wythe or framing. The thermal bridging at floors and shelf angles can largely be eliminated by the use of exterior insulated sheathing and shelf angles that incorporate stand-offs. Small wire ties are rarely a thermal bridging problem.
An air barrier system is important and must be provided for essentially all enclosure systems. Concrete masonry is not part of an air barrier system unless treated with parging or some other coating. An air barrier system with flexible membranes at movement joints should always be provided. The vertical air flow through concrete block cores should be controlled by filling the top courses with grout or foam.
Vents should be installed at the top and bottom of the exterior veneers to encourage ventilation drying. Open head joints at 600 mm on centre or an unfilled and protected soft joint are two common approaches.
Veneer and cavity walls, if designed and built as screened and drained walls, can provide excellent rain penetration control. The magnitude of the rain load should be controlled by the appropriate use of overhangs and by directing and controlling water flow on the surface by using drips, proper window sills, and so on.
It must be assumed that all single wythe masonry veneers will leak rain water. Hence, unobstructed drainage and good flashing are critical. The most common cause of rain leakage in masonry walls is the absence, poor design, or improper installation of flashing.
Drainage can be assured by the use of wide air spaces (40 to 50 mm), large clear weep holes, and careful job-site inspection. Weep holes are best provided by open head joints at 600 mm on centre. It is simple to visually inspect drain holes to ensure they are clean.
Flashing should return up the wall at least 150 mm and must be well sealed at joints to render it waterproof. Although veneer walls are forgiving because they provide drainage, flashing must be waterproof, and hence requires detailed design, good materials, workmanship and inspection. All flashing must have end dams, and should return up behind the drainage plane within the air space. To reduce water penetration and improve mortar durability, specify tooled concave joints and inspect work to ensure that head joints are reasonably well filled. It should be noted that pressure equalization is unlikely to provide any real rain control benefits for masonry veneer walls.
Always protect the top of exposed masonry walls with cap flashing (installed in a waterproof manner) and a coping stone overhanging by at least 38 mm. Always separate masonry in contact with the ground from masonry above grade by using a capillary break–this is normally in the form of a plastic or metal flashing. Finally, the above-grade masonry must be separated from below grade contact by at least 200 mm to prevent back splashing and direct snow melting.
Masonry walls are among the most durable, attractive, and high performance enclosure systems available, but require care and attention in specification and detailing. For sample details, specifications, and design advice see the three CMHC Best Practise Guides for Masonry: Brick Veneer Concrete Masonry Unit Backing, Brick Veneer Steel Stud, and Flashings.
John Straube teaches in the Department of Civil Engineering and the School of Architecture at the University of Waterloo.