Through-Wall Flashing

Through-wall flashing needs to be designed and installed to collect the water within the air space, and to allow it to drain to the exterior. Through-wall flashing is not optional; it is required by most codes. The CABO code requires through-wall flashing at the base of a wall, as well as at window and door lintels. It also requires flashing when brick veneer is installed above a roof - for example, where a two-story brick veneer house rises above the roof of an attached one-story garage (Figure 4).

Figure 4.

Where a brick veneer wall extends above a roof, through-wall flashing is required above the roof flashing. While the roof flashing prevents water from traveling between the roofing and the brick, the through-wall flashing stops any water that has entered the brickwork from above. Is flashing necessary at window sills? CABO requires through-wall flashing at window sills only when the windows do not include a self-flashing flange. Regardless of window design, installing through-wall flashing under window sills is always good practice, since any water hitting a window travels down the window and over the sill (Figure 5).

Figure 5.

Flashing is required above the steel lintel at each window and door. Wherever flashing is installed, weepholes should be provided. It is a good practice to install flashing under a brick window sill, since any rain water hitting the window travels over the sill. In fact, self-flashing window flanges do not serve the same function as under-sill flashing. Flashing below the sill prevents moisture from entering the wall system, while the self-flashing window flanges simply prevent moisture from entering the interface between the window and the backup wall. Attach the flashing to the backup wall and install it carefully, so water can't find a route around it. For instance, if the backup wall behind the air space is concrete block, the flashing should be tucked into a mortar joint in the block wall to prevent water from getting behind the flashing. Where this is not possible, a reglet, pressure bar, continuous nailer, or self-adhesive type of flashing (such as rubberized asphalt) may be attached to the block wall. When self-adhesive flashing is used in wood-frame construction, it must be attached directly to the sheathing. Don't make the mistake of installing the flashing on top of the housewrap or building paper. If the housewrap is installed before the brick masonry, it should be detached along the bottom or slit horizontally with a knife, and the flashing tucked under it. If the flashing is not one of the self-adhering types, and there is no housewrap or felt, then either the flashing must be installed before the sheathing, or the flashing must be attached to the sheathing with a continuous nailer. Both of these alternatives are awkward, however, and they are more likely to create discontinuities, holes, or tears in the flashing.