A parapet is a low wall constructed above the roofline that usually spans around the perimeter of a building. Parapets may be purely aesthetic or primarily functional, such as hiding mechanical equipment or performing as a firewall.
The top of a parapet wall is vulnerable to moisture penetration problems. Choosing an appropriate cap is an effective way to eliminate this condition. A variety of materials are available to cap off the wall, with limestone, terra cotta, hard-fired clay, or precast concrete preferred. These materials have thermal properties similar to those of brick and concrete masonry.
Metal caps, on the other hand, have a coefficient of thermal expansion nearly three times greater than masonry, which causes a significant amount of differential movement between the parapet cap and the wall below. Metal versions should be provided with tightly sealed slippage joints where the cap section laps.
Caps can be manufactured in a variety of shapes, but they must possess adequate pitch, sufficient projections (1-in. minimum), and continuous drips. These elements protect the parapet from ponding water and prevent moisture from running down the outer wall surface.
Most caps are susceptible to moisture penetration at the head (vertical) joints. One way to minimize penetration at these locations is to rake out the mortar head joints, bed joints in the cap to a depth of ½ in., and fill the joint with an elastomeric sealant.
To protect moisture from entering through the top of the wall, a continuous flashing membrane should be installed directly beneath the mortar joint below the cap. This flashing membrane can be manufactured from sheet metal, combination sheet metal/asphaltic, or rubberized asphalt. The flashing should be placed completely through the wall, which creates a slippage plane between the cap and wall below. The cap must be anchored to the wall below.
A variety of stainless steel anchorage systems can be used to attach the coping to the wall. An adjustable system should be installed to allow the mason to properly align the dowel anchor with the pre-drilled holes in the cap. A generous amount of mastic should be applied to the flashing where anchor punctures occur.
When the back side of the parapet is completely exposed, both outer wythes should be constructed of the same material. Brick on the backside of the parapet should be coated with a breathable water repellant.
The bottom flange of the steel beam should be attached to the concrete masonry below with a debonded shear anchor. The anchor can be mechanically fastened to the bottom of the beam at designated intervals that align with the head joints in the concrete masonry wythe. A debonded shear anchor resists out-of-plane (but not in-plane) shear forces and permits structural movement of the steel beam (deflection). The anchor must be fully embedded in mortar, within the head joint of the concrete masonry unit, for this connection to be effective.