Avoid Litigation Due to Masonry Wall Failure
Recognizing difficulties with exterior walls and coping with them during planning can help builders/designers avoid problems later
Environmental moisture, interior humidity, gravity, pressure differentials, wind, joints, faulty materials and shoddy workmanship are all responsible for wall failures. But, if designers and builders take the time, they can control these aspects, which make walls the second most common cause for litigation on buildings. (Roofs hold the number one spot.)
"Designs for exterior walls have usually not been developed in a systematic, rational way," says Lynn Lauersdorf, the deputy director of the Bureau of Architecture and Project Management with the Wisconsin Division of State Facilities Management. He has researched wall and masonry extensively and is the chair of the ASTM (American Society for Testing and Materials) C-12 committee, which sets standards for mortars and grout for unit masonry. He has also served on the ASTM committee that sets standards for masonry wall materials.
Wall designs have evolved slowly, keeping pace with gradual changes in social and economic patterns as well as environmental requirements, says Lauersdorf. In Wisconsin with its harsh winters and humid summers, the state developed guidelines for the exterior building envelope, but other states have yet to follow suit.
Designers and builders in any state can minimize wall failure if they recognize the difficulties with exterior walls and know how to cope with them.
Moisture
The most common difficulty with wall systems most often relates to moisture, Lauersdorf states. Moisture from rain, snow or other environmental conditions penetrates the wall leading to cracks, efflorescence, rust staining, wood rotting, paint peeling, darkening of masonry and spalling.
It would seem that the solution is to develop a watertight skin for the building. "This is often attempted, more often than not unattained, or at best attained only on a short-term basis," Lauersdorf says.
The perfect sealing of a masonry wall surface is almost impossible since fine cracks and joints will allow the passage of water into the wall. "Typical brick masonry, as an example, has 6.5 linear feet of joint for every square foot. For a building, 100 by 200 by 20 feet, this equates to more than 15 miles of joint. Leakage starts with a joint opening in the minute width of one five-thousandths of an inch," notes Lauersdorf.
Moisture from the outside environment is not the only concern. Most buildings are humidified internally during the winter. Air leakage and vapor pressure tend to draw moisture into the exterior envelope to reach the outside.
"If the dew point is reached, the moisture will condense," Lauersdorf says. "Hence, the use of air and vapor barriers, which should be located on the warm side of the exterior envelope to minimize condensation problems. Without this, any heated, occupied building with even moderate internal humidity will allow condensation to occur in or on exterior walls."
With a solid wall, condensation occurs on the surface or in the body of the masonry. In a hollow wall, "it will probably occur on the face of the cavity and be readily drained or sublimated if frozen," indicates Lauersdorf.
He says symptoms of condensation include efflorescence, cracks, paint peeling, frost or moisture in the walls, rusting of support structure and spalling materials.
Movement
Another problem causing wall failure is inadequate compensation for movement. Loading, which produces strains and deformation, is only part of the problem, Lauersdorf states. "Creep or plastic flow, a deformation with time under load and thermal expansion and contraction, might be neglected in the design of the wall. Dimensional changes also might result from aging and degrading material or from the material's change in moisture content."
Consider the concrete building frame in conjunction with a clay brick exterior wall system. "All concrete products are subject to creep under sustained load, such as their own dead load," Lauersdorf explains. "This is an irreversible deformation, proportional to load intensity and time. Concrete deforms with time at a diminishing rate, but with no apparent limit."
Then, all fired clay products contract about 15 percent in the drying and burning process. The finished product will physically absorb vaporous moisture from air and, subsequently, begin to expand irreversibly. "Typical long-term expansion of such products of high quality is equivalent to an unrestrained thermal change of 200 degrees F," says Lauersdorf.
If this differential movement between the interior and exterior portions of the frame is not taken into account, several problems occur, he says.
"Many building elements are assembled with a minimum of restraint to allow for changes in dimension," Lauersdorf admits. "But failures occur when clearances are insufficient, fasteners don't allow movement, or deformities are greater than sealants or gaskets can accommodate.
"Cyclic movements in one plane often cause movements perpendicular to that plane when partially restrained, which are many times the movement within the plane itself. An example of this is a wall bulging. Cyclic movements also may not allow return of a system to its original position."
Other symptoms are cracks near the corners, displacement of shelf angles, and the deformation of the wall near the corners.
Pressure Differentials
Neglecting to consider gravity and pressure differentials in the design and construction of walls can also cause failures.
"When the designer or builder fails to take into account the natural forces that drive moisture inward, rain penetration can result," says Lauersdorf. "During a rain storm, forces acting on an exterior wall typically include wind, internal building pressure, gravity, rain drop momentum and capillary suction."
Fine capillaries draw and hold small volumes of water with high suction so they seldom contribute to rain penetration. But greater amounts of water are held in large capillaries such as cracks and unbonded interfaces. "These are major contributors to the rain penetration problem when wind, gravity or pressure differentials exert their influence," Lauersdorf says.
"Under the influence of wind, raindrops approach the wall with enough velocity to carry them through large openings. Gravity pulls the water downward. Water running down the sides of vertical cracks or joints can be diverted inward by surface irregularities.
"A pressure drop through a wall is produced by wind pressure on the face of the building. At a point where a high rate of inward airflow occurs as a result of an opening and an air pressure drop, water can be dragged along the wall of the opening and cause rain penetration," Lauersdorf says. "Then, if there is a large amount of water at the surface, openings up to three-eighths inch or more are bridged with water, which is readily forced through passages by even small differences in air pressure."
Creating air space in the wall system can control water entry from air pressure differences, but the air pressure in the space must be equal to that of the wall face.
"This can be accomplished by providing sufficient free area of opening to the exterior to allow the wind pressure to maintain equalization. When the air pressures both outside and inside a wetted plane are equal, obviously there is no air pressure difference to move the water inward. The primary air barrier of the building must be located inward of the vented air space," explains Lauersdorf.
For More Information:
Studies (links exit this site):
"TEK" information series, National Concrete Masonry Association, 2302 Horse Pen Road, Herndon, VA 20171-3499. (703) 713-1900.
Web site: ncma.org*
"Technical Notes on Brick Construction," Brick Industries Association, 11490 Commerce Park Drive, Suite 300, Reston, VA 20191-1525 (703) 620-0010.
Web site: brickinfo.org*
Seminar
"How to Avoid Masonry Problems," University of Wisconsin–Madison, Department of Engineering Professional Development, Madison, WI
(608) 263-3372. Annual winter program since 1985.
Edited by Lisa Schuetz
This article is based upon work supported by the University of Wisconsin–Madison Department of Engineering Professional Development. It is for general information and distribution. It is not intended to provide specific solutions or advice for specific circumstances, which should be sought from appropriate professionals.