A parapet is the portion of a wall that extends above the roofline. Because parapets have both sides exposed to the environment, they are particularly vulnerable to problems. Parapet walls are susceptible to displacement, cracking, efflorescence, and spalling. COPINGS OR CAPS The top of the parapet wall is the area most vulnerable to moisture-related problems. Choosing the appropriate cap is an effective way of preventing such problems. Although a variety of copings are available, it's best to use a coping material with thermal properties similar to those of brick and concrete masonry, such as limestone, terra cotta, hard-fired clay, or precast concrete. FLASHING PARAPETS To prevent moisture from entering the wall from the top, install a continuous through-wall flashing membrane beneath the mortar bed directly under the coping. MOVEMENT JOINTS Because parapet walls are completely exposed to extremes of temperature and moisture, they can move at different rates than the walls below. To accommodate movement due to thermal expansion, provide vertical expansion joints that pass completely through the parapet wall.