WATER-RESISTANCE OF CEMENT PLASTER
Some people have developed the misconception that portland cement plaster is porous and allows water to readily pass through a plaster wall. That is not true, and the purpose of this treatise is to refute that misconception. A typical portland cement plaster membrane (often called "stucco") applied to walls of a building effectively resists passage of rain water through the protective layer of plaster. It is recognized that a small amount of moisture may ooze through cracks in a plaster wall, or that moisture may enter a building system through openings at windows, doors, electrical fixtures or other fenestrations, but not through the plaster membrane, per se. Moisture would enter a building around windows, doors or other breaches through a wall only if the openings were not fully and properly flashed and sealed by the builder, in accordance with good industry practice.
An example of resistance of portland cement mortar or plaster to passage of water will be provided to the reader. Several decades ago, during the 1960's, a plastering contractor named Jay Goold performed the following extreme test in San Jose, California. The test was monitored regularly during the test period by Tom Geary.
Mr. Goold formed a cement plaster bucket of typical (unaltered) cement plaster taken from his mixer on a job site. The cement mortar bucket had walls that ranged from three-quarters inch to seven-eighths inch in thickness. The height of the cementitious pail was about eight inches, and the diameter was about six inches. The cement plaster bucket was cured in the manner that a cement plaster wall would be cured, then was filled with water. At no time during the test period of weeks of exposure to water under approximately seven to eight inches of hydrostatic pressure was a drop of water ever evident on the underneath surface of the exterior of the cement plaster bucket. That was an extreme test, but is exemplary of water resistance of portland cement plaster.
Years later, other tests of resistance of cement plaster to passage of water have been performed in a laboratory, under carefully controlled lab conditions, by skilled technicians. In preparation for the laboratory determinations, specimens of cement plaster were removed from walls of buildings which were in dispute concerning water intrusion into the buildings. The specimens were placed flat on a laboratory table, and a plastic cylinder, frequently about eighteen inches high, was cemented in vertical alignment to the face of the sample of removed plaster. The base of the test cylinder was sealed effectively against leakage to the exterior, and approximately eighteen inches of water was poured into the cylinder. (Note that the result of that test is that the specimen of removed plaster was subjected to approximately eighteen inches of hydrostatic pressure, which is greatly in excess of exposure of a plaster wall to water during the most violent of storms!)
An indicator, to tint the water to a pink color, was added to the water in the cylinder to aid in identification of depth of penetration of water into the specimen. That tinted water was allowed to reside on the surface of the plaster sample for a period of seven days and seven nights, continuously, to exert eighteen inches of hydrostatic pressure against the plaster. At the end of the test period of seven continuous days and nights of extreme hydrostatic pressure, the water was removed from the cylinder, and the cylinder was carefully removed from the specimen of plaster. The specimen then was sawed in half, across the center of the area of exposure to water pressure. The typical depth of penetration of tinted water into the plaster, during that extended period of exposure, was approximately five millimeters, or about three-sixteenths of an inch.
A detailed description of procedure for testing specimens of cement plaster for resistance to passage of water is provided in a separate memorandum.
All of the data provided above substantiates the contention of the writer that portland cement plaster does not allow rain water to soak through the plaster membrane during violent rain storms. Water can enter only at cracks and voids, just as in the case of walls constructed of wood siding or brick veneer. The ultimate assurance against water intrusion is provided by the weather-barrier of asphalt-treated building paper or asphalt-saturated felt, installed shingle fashion behind the plaster, wood or brick veneer membrane.