Masonry Magazine October 2006 Page. 36
DRAINAGE
When a mason builds a brick wall with a two-inch cavity, the greatest concern is the accumulation of mortar within the wall.
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Cavity Wall Space
The cavity size in commercial construction is pretty straightforward-almost everyone provides the two-inch cavity recommended by the Brick Industry Association.
When a mason builds a brick wall with a two-inch cavity, the greatest concern is the accumulation of mortar within the wall. As the wall is built, the mortar is not struck off in the back of the wall. When the brick is pressed into place, the excess mortar clings to the back of the brick and can fall to the bottom of the wall, clogging weep holes. In effect, the most important aspect of the solution is to handle the mortar and assure the droppings do not cause a clog hence a mortar collection device. This is a very straightforward solution for our typical commercial construction.
Unfortunately, that two-inch cavity is rarely used in residential housing construction often diminished to one-inch thick-or, when it is used, the two inches of airspace are filled with an inch of insulation. Worse yet, many residential buildings have no sizable cavity and a half-inch space between the brick and the interior weather barrier is standard. Also, residential jobs typically have no flashing or weep holes.
In cavities of one inch or less, the mortar often does not fall to the bottom of the cavity, but instead can become stuck between the masonry veneer and the inner wall, causing a bridge. Although clogging of the weep vent is less likely, it does present other problems in the form of blockages, which impedes ventilation and draining. The mortar that is oozing out the back and clinging to the interior wall creates a shelf for moisture to sit on and places surfactant-laden mortar in direct contact with surfactant vulnerable weather barriers.
Depending on the length of the wall the mason is building and the set time of the mortar, he or she may unknowingly create a series of dam-like mortar buildups at different points above the flashing line. A mason installing a 20-foot section may return to the next course to find a hardened mortar dam on the course below. Any mortar that does not cling to the inner wall behind the newly laid brick will ultimately cling to the already hardened mortar on the previous courses, creating even bigger blockages.
34 Masonry
October 2006
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