Concrete block, particularly lightweight concrete block, is not weather tight if you do not apply a water repellent coating to it. Water repellent coatings usually last between 2 and 10 years, depending upon the exposure of the building and the competence of the applicator. As a rule of thumb, penetrating water repellents last longer because they are not as prone to ultraviolet degradation as surface coatings are. I recommend Silane and Siloxane-based coatings because of this longevity.
If you want to read more about water repellent coatings for concrete block, go to the web site for the National Concrete Masonry Association at www.ncma.org. Look for an icon labeled “E-Tek”. Click on this heading and it will take you to another web site where you can download technical articles. I suggest you download:
Tek Note 8-1A - Maintenance of Concrete Masonry Walls
Tek Note 19-1 - Water Repellents for Concrete Masonry Walls
My favorite flashing/weep system for single wythe walls is called Cavity Vent. It is a strip of corrugated plastic that you install at the base of the wall, where the block meets the foundation. I like the product because it is fast and easy to install and it does not deteriorate over time. Structural engineers like it because it does not interfere with rebar placement and it does not cause shear plane weakness at the base of the wall. Go to www.masonrytechnology.com and click on the drawing of “Cavity Vent” to see more information about this system.
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