If through-wall flashing is properly installed and if cotton wicks extend 8 inches up into the cavity, why are mortar droppings a problem?
Excessive mortar droppings in a cavity can, at times, do more than simply block water from exiting weep holes. If excessive mortar droppings are present, there are also likely to be excessive mortar bridges across the cavity. With a concrete block backup, water can bridge the cavity and enter in the joints of the concrete block wall. Water could then travel across the thickness of the concrete block wall and enter the interior without ever reaching the masonry flashing. Mortar droppings can also allow water to collect at flashing defects or poorly sealed flashing laps, causing leakage.