Masonry Magazine February 2001 Page. 13
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nnovation
Mortar crumbs
The project budget is sufficient to use materials of adequate and marginal quality, but not enough for anything of premium quality or durability.
Were you thinking about using masonry veneer as a major part of the enclosure system for this building? Keep in mind the following possible additional realities.
As you begin the design process the clay and shale for the brick is still in the ground at the quarry, where it has resided for millions of years, just waiting for your project to come along and provoke the manufacturer to make the brick. The groundface concrete masonry units will likewise be made only after your order is placed. For both you can expect several months lead time before delivery.
To make a profit the mason will have to maximize production, perhaps laying 750 to 1000 brick per day; at that pace it is unlikely that 2" x 2" pullboards or other prudent practices will be consistently used, nor that head joints will be full. Under these circumstances it is not uncommon to get 1" of mortar crumbs at or near the bottom of the cavity for every foot of wall height'. (Figure 1, Mortar Crumbs)
Due to pressure from the general contractor, the implicit theme on the mason's scaffolding will be Faster! Faster! Faster!, not Better! Better! Better! The mason doesn't like this any more than the architect does; both want a high quality project.
Due to budget constraints, the flashing will be a thin, unreinforced polymer, not metal nor one of the more durable synthetic flashing materials. It will be easily punctured. It will not be carried out to the exterior face of the masonry. It may be lapped at corners and joints, but it will either not be adhered or will be adhered with mastics that have a life expectancy which is a fraction of that of the building.
Metal ties and screws that secure the veneer to the steel studs will be galvanized steel, not stainless steel. Research studies show that some of the thin zinc coating on the steel will be abraded off of the screws as they are driven in, making the steel vulnerable to corrosion at the most critical location.
Power-washing the masonry veneer will introduce at least a liter of water into the cavity for every 80 square feet of wall area, due to unfilled headjoints, hairline cracks between mortar and brick, and caulk joints that are not properly executed. Acids, which cause corrosion, may be included in the power-washing solution. (Figure 2, Power Washing)
This daunting scenario is a collage of several actual projects.
Power-washing
Transparent Sheathing
Cavity Moisture
Figure 1. This brick veneer was carefully removed to reveal excessive mortar droppings in what should have been a clear cavity. The weep mechanism relied on cotton sash cord to wick moisture out of the cavity, but could not work because it was fully embedded in mortar droppings, which were the full 2" width of the cavity and up to 13" deep.
Figure 2. All openings in these brick veneer walls were sealed, then standard procedures were followed to power-wash the masonry. On average one liter of water drained out of the cavity behind each of the ten 80 square foot veneer panels. Previous research showed that less than half of the water that enters a cavity drains out of its weeps.
Figure 3. Transparent sheathing made of 1/4" lexan was used in place of conventional exterior gypsum sheathing in the lower 2' of the walls in this experimental facility. This provided an economical means of observing the condition of the cavity during construction and thereafter. The skilled masons sportingly competed with each other to see who could keep the cavity most free of mortar droppings.
Figure 4. With interior finishes removed, transparent sheathing also provided an unobtrusive means of visually inspecting the cavity from inside the building. Water presence in the cavity can readily be observed especially where drainage and air circulation in the cavity is impeded by mortar droppings. In this case an open-weave plastic fiber drainage material was used in the lower portion of the cavity.
MASONRY FEBRUARY, 2001 13