What keeps brick caps from popping off a freestanding exterior masonry wall if full flashing is used beneath the cap?
Rowlock brick copings are used on many freestanding walls and parapets. Because they have many mortar joints, these copings can let a considerable amount of water into the walls. Water in the walls can cause efflorescence and, in northern climates, freeze/thaw deterioration of the masonry. To prevent water from entering the walls, flashing that extends across the full width of the wall should be installed underneath the coping. As you pointed out, however, such flashing reduces bond between the coping and the masonry walls. Two methods commonly are used to ensure a connection between the coping and the wall below. One is to install vertical dowels at regular intervals, embedded in the mortar joints between the rowlock bricks. Seal the flashing around dowels to prevent water from leaking into the masonry below. A second method is to use specially formed metal flashing that has interlocking dovetails or ridges. The flashing is set in mortar. Then the coping is laid in a mortar bed on top of the flashing. The folds allow mortar to key into the flashing, creating a mechanical bond. The mortar beneath the flashing should be raked back and filled with backer rod and sealant. Flashings installed under rowlock copings protect only the masonry beneath the flashing. The rowlock coping itself remains susceptible to efflorescence and freeze/thaw deterioration. For this reason, I generally do not recommend using rowlock masonry copings in northern climates. If used, they may need to be replaced many times during the life of the masonry wall.