As a masonry contractor, I have built several buildings with parapet walls. Most of these have not had problems. However, the owner of one building has reported that interior water leaks occur during every hard rain. Looking at the building, I noticed a horizontal crack at the location of concealed through-wall flashing just below the roof slab. Why has this crack occurred? Is it causing the leakage? Can it be repaired?
The cracking may be the result of differential moisture or thermal expansion. Parapets move more than the remainder of the wall. The concealed flashing forms a bond break within the wall that is subject to cracking. Once the wall is cracked, water can enter and, depending on the details, may then make its way into the interior. The cracking, however, may be only part of the problem. Because the flashing does not extend beyond the face of the masonry, water that enters the wall above the flashing may simply travel around the edge of the flashing and re-enter the wall below. But why does water that enters this crack cause interior leakage? If the cavity drains freely, water should move down to the lower levels of flashing and drain from the wall. There are other common causes for leakage at parapet walls. Water may be entering the coping at the top of the parapet or entering the roof side of the parapet. Water from either of these locations may bypass flashing and enter the backup wall directly. Mortar bridges below the through-wall flashing may be allowing water to cross the cavity into the backup wall. I recommend investigating thoroughly to determine the sources of water entering the wall. If water is entering primarily at the crack, replace the concealed flashing with one that projects beyond the face of the masonry.