Masonry Magazine March 2012 Page. 28
Masonry Wall
Flashings
BY DOUGLAS R. STIEVE, RRC, AIA
Preventing water leakage through masonry façades in contemporary construction is primarily accomplished by the use of well-placed flashings. Masonry walls, which are largely composed of brick, concrete masonry units (CMU), cast stone, and other related units, typically absorb moisture and rain. The masonry industry understands that water penetration through the outer wythe of veneers will occur. This penetration has to be managed by flashings, which divert the water back to the exterior. Water can enter the wall system at numerous locations, including individual masonry units, mortar joints, the interface between the mortar and the masonry units, at the tops of walls, at cracks, and at penetration details such as windows, doors, and pipes. The purpose of this article is to discuss the design and installation of masonry wall flashings at several typical locations throughout a building.
Wall flashings are typically required at locations where the downward flow of water inside the wall would be obstructed or interrupted. Flashings are also used under masonry copings, sills, and other horizontal surfaces. In order to understand how masonry wall flashings work, it is important to first understand the different types of masonry walls.
Load-bearing walls
Older masonry buildings were constructed with thicker, solid, load-bearing walls. Water infiltration in load-bearing walls was resisted by the mass of the multiple-wythe wall and a few well-placed flashings. Load-bearing walls performed well as long as it stopped raining before water was absorbed all the way through the mass of the wall. Flashings in these walls typically occurred at junctures of roofs and walls and at parapet locations. Often, the flashings extended completely through the wall. These flashing materials were typically made of soft metals such as copper or lead. Sometimes, masonry ledges or projecting elements (often referred to as water tables) deflected water off the façade as it flowed down the face of the building.
Drainage cavity walls
Masonry veneer construction consisting of a single-wythe of brick/masonry over a back-up wall system, which gained popularity in the 1950s, is a departure from thicker load-bearing wall construction. These walls accommodate inevitable water penetration through the masonry veneer with a drainage cavity. This cavity allows water to flow down the backside of the veneer. The water is then expelled from the cavity back out of the building by the flashings. For this type of wall to function properly, the drainage cavities have to be kept clear of mortar droppings and other blockage. Since water that penetrates the masonry veneer can flow unobstructed down inside the wall, the proper installation of the flashings is more critical than in a thicker masonry wall system.
Single-wythe walls
Single-wythe masonry walls (one vertical row of masonry one unit in thickness) are predominantly constructed with CMU but can be constructed with brick. These walls do not include a drainage cavity.