Flashing: Its Vital Role in Preventing Water Infiltration Part III – Masonry Wall Flashing

By: Sean McPartland

In a May 2018 newsletter article, we discussed the importance of roof flashing and its vital role in preventing water infiltration through the roof assembly. Equally important is the role that wall flashing plays in protecting the building envelope from water infiltration. Masonry walls are often constructed of moisture absorbent porous materials including: brick, concrete masonry units (CMU), and cast stone. Water can enter the wall system at numerous locations, including the masonry units, mortar joints, tops of walls, and cracks in the masonry. Wall flashings are typically required at locations where the flow of water inside the wall system is interrupted, such as the roof surface, windows, and door openings.

Historically, nothing has been more damaging to masonry walls than moisture; 90% of all masonry issues are related to moisture. Water breaks down masonry joints over time, and prolonged exposure to moisture in a wall system can lead to mold growth and can negatively impact the structural integrity of a building.

Building envelope consultants, engineers, and architects understand that water penetration through masonry wall systems will occur. To effectively manage water penetration, these professionals often design a combination of systems to either prevent water from entering the wall system or to divert the water that does enter the wall system. Through wall flashing and cap flashing are two of the more common types of wall flashing for managing water penetration.

Through wall flashing is used to divert moisture that has entered the wall system back to the exterior before it can saturate the masonry and cause water damage to interior spaces. Through wall flashing is horizontally embedded into a masonry wall, and when properly installed, acts as an impervious barrier that stops the vertical flow of water. Water can travel laterally in either direction along the through wall flashing to the interior or exterior of the building, or water may build up in the wall system. To ensure that trapped water does not take an unpredictable or undesirable flow path, designers implement additional measures to prevent water infiltration issues. Firstly, waterproofing membrane is installed on the backup masonry above the through wall flashing to prevent water from infiltrating the building interior. The waterproofing membrane is then wrapped onto the sides of the existing masonry, and all seams and terminations are sealed with mastic, creating end dams to prevent water from traveling laterally. Finally, weep vents, or openings in the masonry, are installed on top of the through wall flashing to direct water to the exterior and prevent water from accumulating inside the wall system. The most common types of weep vents are prefabricated plastic tubes that are installed at the bottoms of the vertical mortar joints. Through wall flashing is typically installed above horizontal surfaces in masonry systems, such as: roofs, parapet walls, and bulkheads. The recommended height of through wall flashing is 8” above the roof surface.

While through wall flashing is used to divert water that is trapped in the wall system to the exterior, cap flashing is used to prevent water from entering the wall system. There are many different types of cap flashing; cap flashing is often a metal product that can be shaped to fit the building’s needs. One common type of cap flashing is a metal coping cover. The surface at the top of any masonry or parapet wall is highly susceptible to water infiltration; for example, coping units, which are constructed of stone, precast concrete, or masonry, are not watertight. Water can penetrate through the coping unit and at the transverse joints. Design professionals require that wall flashing be installed beneath the coping stone. Alternatively, coping covers are installed on top of parapet walls and bulkhead chimneys to provide an impervious barrier that prevents water from entering the wall system. Coping covers typically come in 8’ to 10’ lengths. The lap joints in the coping covers must be soldered or sealed to prevent water from penetrating through the joints in the metal coping. A drip edge is required to divert water away from the wall surface and prevent water from running down the face of the wall.

When it comes to preventing water infiltration, flashings are critical components of masonry wall systems. All involved parties, including owners, contractors, and design professionals, must be aware of the critical nature of wall flashings. It is important that the various flashing options, including the advantages and disadvantages of each system, are thoroughly reviewed and discussed during the design phase of any building envelope restoration project.

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