Masonry Magazine March 1975 Page. 22
Weep Holes
Weep holes are essential for allowing moisture to escape from the cavity. They should be located immediately above the flashing and spaced 24 to 48 inches apart. Various methods have been successfully used to form the weep holes. If the units used in the exterior wythe are brick or of brick height the weep holes can be formed by omitting whole vertical joints at intervals and inserting a piece of 1/2" fiberglass insulation or non corroding screening into the opening. Other materials include nylon rope or sash cord which can be left in place to act as wicks, and lengths of plastic tubing which can also be left in place.
Workmanship
During construction of a cavity wall it is essential that the air space between wythes be kept continuous and not be short circuited by mortar or other material that would form a "bridge" and allow water to pass across the cavity. Also, mortar droppings may plug weep holes and prevent moisture from getting back out of the wall. One of the most frequently used methods to keep the cavity clean and the wall ties free from mortar droppings is to place a wood strip (approximately 6 or 8 feet long and wide enough to bridge the cavity) on the row of wall ties.
Split-rib concrete masonry units were used for both wythes of the exterior cavity walls at Detroit Metropolitan Airport International Terminal.
Heavy string or wire is attached to each end of the wood strip. The mason then builds the wall up to where the next row of ties is located. He then lifts the strip out of the cavity, cleans off mortar droppings and, after the next row of ties is set, is ready to repeat the process. In addition to this method, it is suggested that after the proper amount of mortar is spread on the horizontal mortar bed the mason use the flat of the trowel to bevel the cavity edge of the mortar. This results in very little mortar, if any, being squeezed out of the bed joints into the cavity when units are laid. Any mortar fins which may protrude into the cavity can be trowelled smooth.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 9185, Rosslyn Station, Arlington, Virginia 22209