There actually is a connection between the most commonly used weep screeds and Halloween — they both can involve some scary outcomes! The purpose of a weep screed in thin veneer wall is to provide drainage at the bottom of the thin veneer wall. Unfortunately, the manufacturer’s literature often states that this drainage space will come from the formation of a shrinkage crack. This is neither a predictable nor adequate solution for providing the necessary drainage of moisture at the bottom of a thin veneer wall.
A paradigm shift is needed when it comes to draining thin veneer walls. Most architects and builders now recognize the need for a drainage plane (commonly referred to in the industry as a drainage mat) behind thin veneers. These products create a space behind the veneer that acts as a capillary break and a transport mechanism to move water that gets into the building envelope down to the bottom of the wall. The problem arises when that moisture reaches the base of the wall; the most commonly employed weep screeds restrict the flow out of the wall because of that pesky shrinkage crack! And no, those tiny little holes are not for drainage. Again citing the manufacturer’s literature, they are for “attachment purposes (not weeping).
Moisture-related failure of walls is not as much about volume of water in walls as it is about the time that some moisture is allowed to remain in that wall. If an adequate opening isn’t available at the bottom of the wall for moisture to exit and adequate airflow to occur, the wall is ripe for failure.
I prefer to remain optimistic about the building industry, but I recently read an article in a national building trades publication on the correct installation of adhered masonry veneers that set my optimism back a bit. As many of you know, adhered concrete masonry veneers have experienced a large number of moisture-related issues when not installed properly. Many articles have been written about this issue including “Best Practices: Adhered Concrete Masonry Veneer” by Mark Parlee that appeared in the Journal of Light Construction. Parlee, who is a builder as well as a forensics expert, notes in his article that ACMV walls have now become a trigger for inspections because of the large number of moisture-related failures they have experienced. He goes on to state that the solution is “drainage.”
Back to the article I read. It implied that drainage was necessary, but it referenced the use of two layers of WRB to create that drainage. That is a very old technique, proven by testing to retard the rapid transport of moisture out of walls. It does not provide the minimum drainage space specified by respected scientists like Dr. Joseph Lstiburek. In his article Mind the Gap, Eh! a minimum 1/8” gap or void needs to be present to create a capillary break and an effective drainage mechanism.
The building trades magazine article also mentioned the use of a weep screed, but it did not indicate what to look for in an effective product. Remember, the holes are for attachment, and a shrinkage crack isn’t predictable. I am disappointed that while we seem to embrace the goal of better buildings, we still employ old, outdated techniques even though many of the new materials we build with don’t work with the old way of doing things. We need that paradigm shift!
You need to look for a weep screed that provides frequent, large openings for drainage. They do exist. Without “real” drainage space behind the veneer and “real” openings at the bottom of walls, you will wind up with a moisture “trick” rather than the “treat” of a beautiful, long-lasting building. Happy Halloween, hope you get a treat!
*For more information about weep screeds and weeps at the bottom of walls, download the MTI article “MTI-014: Getting to the Bottom of Moisture Management” in the MTI Library at http://www.MTIdry.com.