Requirements For Rain Resistant Masonry Construction

1) General
2) Material Considerations
3) Suitable Rain Resistant Wall Systems
4) Coating Recommendations
5) Workmanship
6) Check List For Rain Resistant Construction

1. General

The evolution of masonry construction throughout the centuries has resulted in a multitude of masonry materials, shapes and wall systems. The selection of the system or systems best suited to local climates, economics and traditions is often quite complex, requiring consideration of several factors. The populated coastal area of the Northwest is located along active earthquake faults and in a climate of mild winters with high rainfall. The combination of high rainfall and seismic activity make the Pacific Northwest conditions unique, thus requiring special considerations. The selection of economical masonry wall systems, that are trouble free from water penetration, is of foremost concern to the Masonry Industry.

The causes of rain penetration are many. They are divided between extreme weather exposure, material failure, detailing, workmanship and inspection.
1.1. Three conditions are essential for rain to penetrate a wall:
- there must be water on the wall,
- there must be entry paths in the wall to provide a passage, and
- there must be forces to drive the water through the wall.
All of these conditions must exist simultaneously; if any one of the three conditions is eliminated, water will not penetrate the wall.

a) water
The prolonged and frequent rainfall of the Northwest coastal climate provides a more than adequate supply of water to wet the exposed areas of the masonry wall surface.

b) entry paths
Paths for passage of water through a masonry wall can be the result of:
- porosity of the masonry units
- porosity of the mortar
- lack of bond at the interface of mortar and unit incompletely filled joints
- stress cracks due to settlement, shrinkage etc.

Calculations by the Canadian Bureau of Standards on the rate of flow through pores show that a small number of large pores may allow the passage of more water than a much larger number of small pores. Therefore cracks or imperfections in the units or mortar may give a disproportionately large contribution to the flow of water through a wall.

c) forces
Forces are necessary to drive the water through the wall if rain penetration is to occur. These forces can be wind pressures, capillary suction and gravity forces. The wind pressures are the most significant forces causing rain penetration. Wall areas with positive pressures are subjected to leakage, whereas negative pressures (suction) cause exfiltration.

Data on wall penetration, based on wind speeds and directional effect show that many times rain is accompanied by winds. Severity of exposure thus varies considerably due to site location and orientation.

Rain penetration in a single wythe wall.

2. Material Considerations

Although most of the related publications will emphasize that rain penetration mainly occurs through the joints, it should be noted that in areas of high rainfall, water penetration through the unit must not be neglected. The prolonged heavy rain conditions give a high moisture load on the total wall area, and can cause rain penetration through the joints, as well as through the units, even to saturation level of the unit. This will not be evidenced immediately in masonry facing wall systems, however it makes through-the-wall masonry vulnerable to rain penetration when constructed without adequate protection.

This means that the Northwest coastal area requires a different approach regarding rain resistance in comparison with the average inland climate. The exterior face of masonry units should be protected against water penetration. The degree of protection is directly related to the wall type, the porosity of the unit and to the shape or texture of the unit.

2.1. "Building brick or "used brick" should not be used for exterior applications. Face brick should be Grade SW. For hollow structural brick, Grade I is recommended.

2.2. Concrete masonry units should conform to ASTM C-90. The typical Northwest manufactured unit is a Grade "N" block, moisture controlled.

2.3. Mortar. Use cement-lime mortar to provide a higher degree of rain resistance. Type N mortar is recommended for exterior use above grade. It is medium strength mortar and suitable for general use in exposed masonry. Type S mortar can be used where higher strength is required. Engineered masonry has led to a trend in masonry specifications toward higher compressive strength mortars, while the most important aim in mortar selection is to obtain the right combination of both strength and workability. The importance of mortar compressive strength tends to be overemphasized, while bond strength, which is more important, is often ignored. Good workability and water retentivity are essential for maximum bond and rain resistance.

2.4. Flashings. Some important properties flashing materials should comply with are:
- Impervious to moisture penetration
- durable in the environment: resistant to corrosion caused by exposure to the atmosphere and/or to the caustic alkalies present in mortar
- the soluble constituents of the flashing should be low enough to prevent discoloration of the masonry surface
- sufficiently tough to resist puncture, tearing, or other damage during construction
- easily formed to the desired shape, and capable of retaining this shape throughout the life of the structure.

In selecting a flashing material, both installation method and material cost should be considered. Repair or replacement at a later date will be difficult and expensive. It is advisable, therefore, to select a flashing material of sound durability.

Copper combinations and plastics are popular flashing materials that meet most requirements. Metal copings and sills provide the highest degree of impermeability and continuity.

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The NW Masonry Guide Table of Contents

Masonry Institute of Washington
Washington State Conference of Mason Contractors