Masonry Magazine June 1993 Page. 19
An Information series from National Concrete Masonry Association
USE OF FLASHING IN
CONCRETE MASONRY WALLS
NCMA-TEK
TEK 134
Keywords: aluminum, composite flashing, condensation, copper, dams, flashing, plastics, polyethylene, polyvinyl chloride, urea-formaldehyde, stainless steel, vapor barrier, vents, water penetration
INTRODUCTION
The primary role of flashing is to intercept the flow of water through masonry, direct it to the exterior of the structure, and to prevent upward migration of water from below the ground level.
Most masonry buildings require flashings. The type of flashing is governed by both natural and design/build considerations. Natural considerations include such factors as the presence of moisture in the form of a liquid, solid, or vapor, air movement; and temperature. Design/build considerations include selection of the proper type of flashing, location, and installation details.
Although flashings are the primary focus of this TEK, it should be understood that the role of vapor barriers, air seals, and insulation are also important elements to consider for any masonry wall design.
EFFECT OF MOISTURE ON MASONRY
Water in any of its three physical states may contribute to various forms of damage to a masonry structure.
In the liquid state, water penetrating to the interior of a building may cause considerable damage to its contents. Water in the solid state, ice, can induce spalling and cracking of the masonry. Water in the vapor state frequently leads to the formation of condensation inside the cores and on the surfaces of masonry when the dew point temperature is reached. During cold weather, below 28°F, water as vapor can migrate to a cold surface and form hoarfrost on the surface or increase the quantity of ice within the masonry. Masonry able to withstand the destructive force of ice can hold it for later disruptive influences, such as thawing. Thawing converts the ice to water which moves as influenced by external forces. It is also possible for the humidification of interior air to cause water damage to the exterior of a structure. This damage may appear in the form of water stains, ravelled mortar joints, spalled surfaces, or efflorescence.
TEK 134A. © 1993 National Concrete Masonry Association
DESIGN CONSIDERATIONS
# Water Movement
In the design of concrete masonry structures, the presence and movement of water in any of its three forms needs to be considered. Factors influencing water movement include wind pressure and direction, gravity, and absorption. Wind pressure on a wall is intermittent and acts normally or at a slight angle to the exterior surface of the wall; gravity is present at all times and acts vertically downward; while absorption is omni-directional.
For example, wind-driven rain may enter masonry through cracks at the interface between mortar and units and migrate downward through the wall due to the force of gravity; or it may be transferred horizontally through the wall either by pressure or by bridging action. Wind-driven rain may be absorbed also by masonry units and carried from the exterior surface to the interior surface by capillary action. Additionally, ground water may be drawn upward by the wicking action of units placed on porous and wet foundations or by contact with moist soil.
Designers should never assume that any material is capable of rendering walls totally impervious to water penetration. Surface treatments, designed to lessen the quantity of water entering a masonry structure, are helpful in this regard, but should not be considered as a primary means of protection. Available as clear and opaque compounds with improved compatibility and vapor transmissive characteristics, the effectiveness of surface treatments depends on their composition and ability to resist caustic environments.
The recent use of integral water resisting admixtures in concrete masonry units and mortars should not influence the designer to eliminate flashings. The success of these materials is dependent on the use of good construction procedures. Proper selection and application of surface treatments and integral waterproofers can greatly enhance the water resistant properties of masonry, but they should not be considered as direct substitutes for flashings.
# Location
Proper design of masonry for resistance to water penetration includes consideration of the various types of wall construction such as single wythe, cavity, curtain, infill, veneer, etc. During the design phase it should be understood that all exterior masonry walls may be subjected to some