Question: How can I flash a masonry wall that rests on a balcony slab? I am worried about water collecting on the balcony and running into the inside. Should I caulk the bottom edge of the masonry wall to prevent this? Or will that simply trap water within the wall?

Answer: This is a common problem. Flashing at the base of balcony walls is often incorrectly installed or omitted altogether. The balcony slab should be sloped away from the masonry wall. Sometimes, however, if the slope is not sufficient, deflection of the interior portion of the concrete slab can cause the balcony slab to rotate. This can eliminate the slope and, in some cases, direct water back toward the building. If the finished surface of the balcony slab is the same as the interior floor slab, water collecting on the balcony can simply run underneath a flashing installed on it. Applying sealant in this location can prevent this from happening. However, if flashing is installed in the base of the wall, the sealant will trap the water reaching the flashing.Perhaps the best way of avoiding this problem is to design the finished surface of the balcony slab 2 or more inches lower than the interior floor slab. This offset helps prevent water that collects on the slab from entering the interior. It also helps prevent water from backing around the edge of the flashing and entering the interior. To be most effective, flashing installed in this location should be fully bonded to the offset in the concrete slab.If the concrete slab has already been poured, one alternative is to position the flashing one course above the slab. The lowest course of masonry is bonded to the concrete with mortar. Sealant is applied between the brick and the concrete slab to prevent any water that collects on the slab from entering underneath this unit. The flashing, which is installed one course above the balcony, projects beyond the face of the wall. Sealant is applied between the underside of the flashing and the brick mortared to the balcony slab. The success of this detail depends on good bond between the sealant and the concrete slab.