Load Bearing vs. Cavity Walls
By: Rebecca Reilly
The construction of masonry walls has developed over time. The 2 most common types of masonry systems are load bearing and cavity walls, and it’s important to be aware of the type of wall system at a building when designing restoration repairs.
The earlier form of masonry wall construction is the load bearing masonry wall. In this system, the exterior wall serves as structural support where floor joists, often timber, rest on the masonry wall and distribute the loads from the floors to the wall. The wall then distributes the load from the building to the building’s footings. Wythes of brick masonry are held together by header courses, which are brick units installed perpendicular to the length of the brick. Since the load bearing walls support the loads from the rest of the building, there are often additional wythes of brick at the walls at the lower building floors.
In the load bearing walls of older buildings, multiple wythes of brick masonry served as the primary waterproofing system. When the exterior of a building gets wet, the brick walls absorb and shed water. If the bricks and mortar are not deteriorated or cracked, the water will not infiltrate the building interior.
During the 1920’s, the cavity wall design became popular. Steel and concrete structural systems had become more cost effective, and allowed the construction of taller buildings. As a result, brick masonry was more often used as a veneer rather than a structural component. The cavity wall system has an opening between the exterior brick masonry and the backup wall. The back-up walls are typically constructed of CMU. This cavity allows water and air to travel between the two walls. The exterior veneer masonry is typically supported by a relieving angle at each floor line. A relieving angle is a steel angle which is fastened to the structure of the building. Additionally, the exterior wythe of masonry is also fastened to the backup masonry with brick ties. The cavity wall system introduced the construction practice of waterproofing masonry. At minimum, waterproofing is usually installed at the relieving angles. Sometimes, waterproofing is installed over the entire backup masonry wall. Early cavity wall systems were designed without expansion joints, but today, expansion joints are preemptively installed. These joints allow for the expansion and contraction of the masonry caused by freeze-thaw cycles. When chronic vertical cracks are observed in exterior masonry, it’s typically a sign that an expansion and contraction issue exists.
Although waterproofing was rarely installed as part of load bearing masonry wall construction, waterproofing installation to prevent water infiltration is common practice in building envelope restoration. Additionally, brick ties are used to fasten new brick to backup masonry. Since they are crucial for supporting the exterior wythe of brick masonry, steel relieving angles should be inspected during the restoration of a cavity wall system to ensure that they are in good condition. Additionally, waterproofing should be installed to protect the relieving angle steel, and the relieving angles should be painted to prevent corrosion.