There is a water leakage problem between the gym and the remainder of a recently completed school building. The flashings were installed improperly in this area with the front edge held back from the face of the wall and the unsealed splice joints. There were many other locations in the building where the flashings were also installed improperly.
Why is this the only location that is leaking? The masonry contractor reported that he had installed flashings in this manner in many buildings over the last 20 years without a leakage problem of this magnitude.
How can this be the case?
Not every flashing deficiency results in noticeable water leakage problems. Much depends on the location of the flashing.
Many masonry walls contain flashing systems that are completely ineffective due to problems of design or construction. When the through-wall flashing is held back from the face of the wall, has poorly sealed splice joints, or has tears or other defects, water bypasses this flashing and flows down to the construction below. In many cases, this water flows to an area where it does not result in an immediate leakage or moisture problem.
One such case occurs at the base of a wall in a single-story building. If the masonry walls rest on a foundation with a brick ledge, this ledge serves as the flashing. Even without a brick ledge, the water may not result in an immediate problem.
When there is a crawl space or a slab-on-grade, water that bypasses the flashing can flow into the crawl space or into the joint between the foundation and the interior slab-on-grade. Although this water leakage may go unnoticed for many years, increased moisture beneath the floor slab or in the crawl space may lead to problems such as corrosion or mold growth.
Even flashing leakage problems in foundation walls of basements without brick ledges may not result in noticeable interior leakage. Water in finished basements can be absorbed by the surface of the concrete or concrete masonry foundation walls. As in other locations, unnoticed leakage may result in future mold problems or corrosion. Interior leakage problems may actually develop in the future as the masonry veneer cracks or weathers, which increases the rate of water penetration into and through masonry walls.
Some buildings with flashing defects do not experience noticeable leakage problems because features on the wall greatly limit the exposure of the masonry to wind-driven rains. Large overhangs or buildings that are protected from heavy wind-driven rains as a result of adjacent structures may not develop leakage problems because the volume of water is too small for leaks to be noticed.
Leakage problems in multi-story buildings with shelf angles and flashing at each floor also may go unnoticed for years. Water leakage may soak into the concrete masonry backup walls or into sheathing in steel stud backup walls. This water leakage increases the humidity inside the wall space, leading to mold growth or corrosion of ferrous metal elements within the wall. Even if water is prevented from reaching the backup because of a membrane or other barrier on the exterior surface of the backup wall, flashing leakage can lead to accelerated corrosion and premature deterioration of the angles in some cases.
Because many flashing problems do not develop into immediate concerns, it is possible for a masonry contractor to make the same mistakes for decades without knowing it. The contractor may not understand why it occurred when a problem does develop.
Flashing problems are more likely to result in noticeable interior leakage in masonry above an occupied space or above ribbon windows or other large areas of windows. These leakage problems are more noticeable because water that penetrates the wall and bypasses the flashing system naturally flows downward. At the location where the masonry wall terminates at a roof level, the wall was likely supported by a steel beam and the space below is either a concrete masonry wall or is open. Water entering the wall system at this location travels downward and reaches the interior spaces where it shows up as leakage.
Whenever leakage problems are noted in a building, a decision must be made about which flashings need to be repaired. Carefully consider what the short- and long-term consequences of continued leakage will be on the building. In some cases, repairs are not necessary to address flashing deficiencies, whereas in other cases future problems may be likely.
Keep in mind that added moisture in walls and floors can sometimes result in mold problems that affect the air quality of the building or deterioration problems that influence the durability of the structure.