I have seen several different methods of installing through-wall flashing at the base of walls above a lower roof area. Sometimes the through-wall masonry flashing is installed one course above the termination bar for the roof base flashings. Other times, the through-wall flashing extends down and covers the top edge of the roof. I have also seen a two-part flashing with a counterflashing that locks into the through-wall flashing. Which is the best way and when should each be used?
I recommend using a through-wall flashing with an integral counterflashing to cover the roof base flashing (see figure, below). Figure 1The masonry and flashing typically are installed before the roof. The counterflashing is inserted later to cover the roof.The two-piece arrangement also is beneficial when the roof is replaced in 10 to 20 years. At that time, the counterflashing is simply removed, the roofing work is done, and the counterflashing is reinstalled. Placing through-wall flashing one course above the counterflashing sometimes can result in leakage. If the flashing is held back, water may run around the end and remain in the wall. Even if the flashing projects from the face of the wall and forms a drip, water may enter through mortar joint interfaces in the course below the flashing and bypass the counterflashing. During reroofing, this through-wall flashing may be covered. The new termination bar and roof base flashing often are raised slightly to provide a clean surface to adhere new sealant. If the old through-wall flashing is covered, water reaching it will remain in the wall or flow behind the roof base flashings. A one-piece flashing will be damaged when it is bent up to install roofing. When bent back into position, it forms a gutter that collects water. This will allow water to reach laps damaged in this process and leak to the interior.