Most of the existing metal weep screeds were designed prior to the existence of any real rainscreen drainage plane technology and have a somewhat less than consistent performance history. They were, and are, a focus stress-point where cementitious materials and metal meet. The resulting shrinkage leaves voids/unpredictable openings in the rainscreen system that allows moisture to enter. In many cases the design of the weep screed itself would hold moisture through freeze-thaw cycles. Unfortunately, it seems that the design history of these devices has little foundation in moisture management.
To perform effectively, weep screeds must have drainage capacity equal to the rainscreen drainage planes they terminate. With the advent of real rainscreen drainage plane technology that effectively directs more moisture from a high entrance point to a lower point in the exterior building envelope, inadequate weep screed designs will become even more ineffectual.
As mentioned above, MTI’s L+R Weep Screed™ does not present this issue. Its overall design, which includes 1″ drainage holes, permits all of the water traveling through the drainage plane to escape from the building envelope in a timely manner. See L+R’s product specifications at MTIdry.com for more information.