The Brick Industry Association recommends a cavity of 2 inches for commercial masonry walls. However, the cavity in residential construction is only required to be 1 inch by "Technical Note 28."

What is the reason for this difference?

A 2-inch cavity is much better for preventing mortar from bridging the thickness of the wall. Mortar bridges that extend across the cavity are more likely to occur in a 1-inch cavity than a 2-inch cavity. A 1-inch cavity in residential construction has been the norm of the industry for many years and would be difficult to change.

These systems have performed satisfactorily, in part, because the exposure to frequent rainwater penetration is less than most commercial masonry structures. BIA Tech Note 28 states: "Brick veneer with wood frame backing has historically been built with a 1-inch (25-mm) minimum air space. The protection provided by roof overhangs and the relatively low wall heights aid in reducing water penetration."

Residential structures are often positioned in close proximity to one another and have less exposure to wind-driven rain. They are typically two stories or less in height and the brick veneer often occurs only on the first floor. In most residential construction, a medium to large roof overhang helps prevent water from reaching the face of the wall during rains accompanied by light winds.

Consideration, however, should be given to increasing the cavity width to 2 inches in residential construction.