Question: When using through-wall we flashing at brick and block cavity walls, should the top edge of the flashing in the backup wythe pass straight over an 8inch block or should two 4-inch blocks be used? Some say it makes the wall stronger to use two 4-inch units; some say it doesn't matter.
- Answer: Structurally, it doesn't really matter. In both cases the flashing creates a bond break that needs to be considered in the design of the backup. For resistance to water leakage, however, I recommend the second method. Extending the flashing up between two courses of 4-inch block will work to collect water that may bridge the cavity and bypass the face shell mortar (figure B, right).