Is it necessary to seal properly lapped splice joints in flashing? In a properly designed and constructed wall, doesn't water simply flow out the weep holes? If the flashing is lapped 6 inches or more at a splice, what are the chances that it will leak? What is the purpose for installing sealant or mastic in lap joints?

Splices in through-wall masonry flashing must be sealed to prevent leakage. Even in the best conditions, some water will reach these joints. During a wind-driven rain, water will penetrate the outer wythe of the masonry wall. Some water will penetrate regardless of how well the wall is built. Water flowing down the back face of the masonry will reach the flashing. At this point, some water will flow through the weep holes and reach the exterior. A significant percentage of the water, however, will likely flow laterally. The direction of the flow depends on the slope of the flashing. Water reaching poorly sealed joints will bypass the flashing.

During many rainstorms, especially wind-driven rains, some water will build up in the cavities. This will depend on the amount of water penetration, the velocity of the wind, the air-pressure differential across the masonry, and the effectiveness of the weep holes. The amount of buildup will also depend on the amount of mortar droppings on the base of the cavity. When water builds up on flashing at the base of the cavity, it will easily flow through unsealed joints.