I have heard several people recommend that the vertical leg of a through-wall flashing extend up 8 inches within the cavity. Can water build up to a level of 8 inches within the wall? If full head joint weep holes are used, how much wind pressure would it take to maintain 8 inches of water within the cavity?
It is unlikely that 8 inches of water could collect in an open cavity wall anywhere in the country, unless there is some unusual source of water into the cavity wall. If weep holes are working and if pressure equalization is not occurring, it would take a sustained wind of 125 mph to create enough pressure to build up 8 inches of water in the cavity. Extending the flashing up 8 inches within the cavity is, however, a good recommendation. Mortar droppings can occupy space in the bottom of the cavity, reducing the available volume. Droppings also can permit water to bridge the cavity above the level of flashing that is not high enough. Several references give recommendations for the height of flashings for cavity walls. In the book Bricklaying: Brick and Block Masonry published in 1988 by the Brick Institute of America (BIA), the recommended minimum rise for flashings is 8 inches. In BIA's book Principles of Brick Masonry published in 1989, a 6-inch flashing rise is recommended. The Masonry Institute of British Columbia recommends in its 1985 publication, "A Guide to Rain Resistant Masonry Construction for the British Columbia Coastal Climate," that flashings be extended up 6 inches.