One of the building components occasionally overlooked during roof replacement and repairs is the through-wall flashing. Failure to take this component into consideration can result in leaks that are difficult to find and expensive to repair.

The through-wall flashing is a water resistant membrane or material assembly that collects water that has penetrated the exterior wall and facilitates its drainage back to the exterior. It is considered to be the most satisfactory method of preventing water that enters the exterior from getting into the building.

All types of masonry walls can contain through-wall flashings, as well as other walls with other types of facing materials. Although through-wall flashings are occasionally found in older buildings with solid brick walls (barrier type wall construction), their use gradually increased with newer wall construction methods. The most common use of these flashings is in modern buildings with cavity type wall construction. This type of construction started to become popular in the 1920s with the use of structural clay tile covered by a brick veneer. In modern buildings with cavity walls, the construction usually consists of concrete masonry units (CMUs) a.k.a. concrete block, waterproofing (membrane or coating), approximately 1 to 2 inches (occasionally more) of air space (the cavity) , and an outer veneer of face brick. The face brick is secured to the concrete block structure with metal ties embedded into the mortar joints of the face brick.

Within the walls, through-wall flashings are located under coping stones, above the counterflashing metal at the roof, at apandrels, above and below windows and doors, and at grade level. The location of most concern in roofing work is the through-wall flashing above the counterflashing metal at the roof level. Through-wall flashing materials include lead (older buildings), copper, stainless steel, bituminous membrane and PVC membrane. Although galvanized steel and aluminum have also been observed in masonry walls, they are not recommended due to corrosion from the caustic alkalies in mortar. Membranes are replacing metals in recent construction to prevent a thermal bridge from the building exterior. In addition to being sloped to the exterior of the wall, properly installed through-wall flashings are also sealed at the joints and turned up at the ends to prevent leakage around the flashing and into the building.

During the design of roof replacements and recovers, as well as during leak investigations, the existing building plans should be reviewed. If existing building plans are not available, through-wall flashings in masonry construction can also be identified by the weep holes. Weep holes are installed along the top of the through-wall flashing to facilitate drainage and ventilation of the air space, or cavity. The weep holes are usually about 2 to 3 inches apart, but the spacing varies depending on the volume of the cavity.

They are formed during wall construction with cotton cord, tubing of metal or plastic, small pieces of corrugated plastic, small metal vents, or lack of mortar in the vertical joints.

Ventilation at the top and bottom of the cavity is essential for the walls to function properly. Even well constructed brick walls with no defects are not impervious to water. When brick gets wet, it gradually absorbs the water like a dense sponge. Since water movement is from wet to dry and from hot to cool, water vapor is driven further into the wall if the outer face of the brick heats up from the sun and warm humid weather. If the wall interior is not ventilated and properly flashed, leaks and mold can develop. In addition to this, any defects in the brick veneer can substantially increase the potential for leaks.

The most obvious symptom of a wall defect is an interior leak that cannot be traced to the roof system either above or below the wall. Other symptoms include efflorescence, spalling of the face of the bricks, or mold growth on the interior side of the wall. (note: efflorescence is the deposit of soluble substances, sulfates, carbonates, and chlorides leached from the mortar and brick. The deposit appears as a white crusty substance or a white powder that disfigures the face of the wall. Its presence on new construction is usually a minor powdery substance that can easily be cleaned off. However, its appearance on existing construction indicates moisture inside the wall).

If the symptom is a leak that recently developed, possible sources are:

previous roof's baseflashing. This can occur when the roof is torn off and replaced with one containing thicker insulation (including tapered insulation systems), or when the roof is recovered with single-ply membranes, spray-applied polyurethane foam (SPF), and sloped metal retrofits. This can also occur when the existing metal counterflashing is cut off at the wall or covered with a new surface-mounted counterflashing to avoid cleaning off old

roof cement or asphalt

flashing. 

If the symptoms include long-term leaks, efflorescence, or spalling of the face of the brick, possible sources are:

at the top of the wall cavity, an insufficient amount of weep holes for the volume of the wall, or from blocked weep holes due to mortar droppings into the cavity during construction (mortar droppings several inches deep are often found in walls that can also allow water to flow over edges of through-wall flashings

Because of the wide range of possible sources noted above, a thorough investigation of the building should be done by a qualified professional, experienced in wall construction, prior to making repairs. Premature judgments may lead to ineffective repairs, and result in additional damage. Excessive and long-term moisture in the cavity can corrode the embedded structural steel components.