On the inside face of a parapet or wherever a masonry wall intersects a lower roof, I have seen cases where the masonry flashing exits the wall one or more courses above the top of the retaining bar for the roof base flashing. Will this cause any problems?
There can be problems whenever the roof base flashing and the masonry through-wall flashing are not properly integrated. This is especially true if the masonry flashing stops or is held back slightly at the face of the masonry. If the flashing does not extend beyond the face of the wall, there is a chance the water may run around the end of the flashing and re-enter the wall below. Another good reason for projecting the through-wall flashing beyond the face of the wall is to prevent the outlet edge of the masonry flashing from being inadvertently covered when the roof membrane and roof base flashing are replaced. This would seal off the masonry flashing and route the water under the roofing. (DonÆt laugh; IÆve seen it happen.)
But there can be problems even when the masonry flashing does project beyond the face of the wall if it exits more than one or two courses above the roof base flashing. When water flows across the face of the wall or out from the weep holes, it will flow over the masonry between the bottom of the masonry flashing and the top of the roof base flashing. Water penetrating this masonry through any cracks or separations will remain in the wall and can contribute to leakage.
To avoid these problems, I recommend using a masonry through-wall flashing with a removable counter flashing that extends over the top of the roof base flashing. Using masonry flashing with a removable counter flashing will help prevent problems that can occur when the roof membrane is replaced. The counter flashing piece can be removed during re-roofing to allow the roof membrane to be installed behind it. After installing the roof, the counter flashing is replaced. Many people use a single-piece metal flashing and counter flashing that extends out of the wall and bends down several inches over the edge of the roof membrane.
To replace the roof membrane, the down-turned leg of the counter flashing must be bent up during construction, then back down over the edge of the new roofing. This forms a crease in the flashing that can collect water. This practice can also damage the lap joints in the flashing. And because it is difficult to bend the counter flashing back to its original position, the roofers may cut it off. For these reasons, I recommend using a through-wall flashing with a removable metal counter flashing to cover the top edge of the roof base flashing at intersections between roofs and masonry walls.