Q: I am working on a brick building that has masonry parapets with a face brick veneer that extends past the concrete roof slab and two common brick wythes supported on the roof slab. The masonry parapets are being repaired due to deterioration.

In conjunction with the repairs, we want to increase the height of the parapets around 1 foot to 16 inches to better conceal some of the equipment on the roof. How far can the brick parapet be increased without bracing or reinforcing it?

A: You should not increase the height of a parapet wall without involving a structural engineer. By increasing the height of the parapet, you increase the exposure to wind. Since the parapet resists wind by acting as a cantilever, the stresses at the base of the parapet (the level of the roof structure) are proportional to the square of the height of the parapet.

In other words, if you double the height of the parapet, you will quadruple the wind-induced bending stress at the base of the parapet. For instance, adding an extra foot of height to an existing 2-foot parapet roughly doubles the stresses.

How far you can extend the parapet without reinforcing or bracing will depend on several factors. The wind load capacity of the parapet is a function of the height, width, bond strength, and condition of the masonry parapet. Short thick parapets have lower tensile bending stresses induced by wind than tall slender parapets. In fact, if the parapet is thick enough, it may not develop any tensile stresses at all.

For parapets that are tall and slender enough to develop tensile stresses during strong winds, the bond strength and condition of the masonry will determine the capacity. In many bearing walls where roof framing members are set into beam pockets, the parapet is a continuation of the masonry wall below. In this case, the capacity of the parapet depends on the bond strength condition of the masonry at the point where the roof framing members engage the wall.

However, in your situation, the strength of the bond between the concrete roof slab and the parapet become critical as well. But in many cases, the brick may not be bonded to a structural concrete slab at all. This is because differential movements between the parapet masonry and the concrete structure (moisture expansion of the brick masonry and shrinkage of the concrete slab) create a crack at this bond interface. Clearly, a cracked joint has no bond and therefore can only withstand wind loads based on its weight and the resistance to overturning.

There is one relatively simple method of bracing a parapet. Install a continuous channel along the top of the wall that in turn is supported by steel angle kickers installed at a 45-degree angle that are attached to the channel and to the roof structure, as shown in the diagram.

Generally, the attachment to the roof will consist of a structural T or fabricated member with a vertical leg that is bolted to the roof deck and sealed at the roofing using appropriate roofing details. The continuous channel along the top allows greater spacing for the kickers. Obviously, the bracing system should be designed by a qualified architect or engineer to meet the required design wind loads for the parapet.