Field Note: Covering Walls
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is a simple technique that perfectly illustrates the old adage:
“an ounce of prevention is worth a pound of cure.” Because
masonry is site-constructed, it is susceptible to the weather while
it is being built. Specifiers and designers have to write clear
specifications instructing builders to cover new construction at
the end of each workday. And builders need to recognize the importance
of this step in ensuring weather resistant walls that look attractive.
Moisture in the Wall Cavity
Walls are meant to be barriers to weather upon completion. Good
design and workmanship minimizes water penetration into the wall
cavity and keeps moisture out of the building interior, but during
construction, masonry is typically exposed on both sides and the
top of the wall is open. Water that enters the top of the wall can
saturate units and cavities. As it passes through the wall, it very
likely will carry dissolved salts to the surface of the masonry,
resulting in efflorescence.
What to Do
Quite simply, water has to be kept out of the wall cavity during
construction. A common method for achieving this is to cover the
top of the wall with a plastic sheet at the end of the workday.
The sheet should be anchored so it remains in place. In addition,
the inside scaffold boards should be tilted up against the wall
at the end of each day. If rain should occur, this prevents water
from splashing mortar and dirt back onto the wall face. It not only
keeps the face cleaner, it also keeps out excess moisture that could
lead to efflorescence.
 The
cavities also need to be kept clean of mortar. A drainage wall will
not function properly if mortar droppings are blocking the weep
holes. Beveling the mortar bed away from the cavity before placing
units on top of the fresh mortar minimizes doppings into the cavity.
To assure unobstructed openings, some masons use a board inside
the cavity to catch droppings while placing mortar and units, removing
the board every few courses to empty it. Some masons and designers
prefer a proprietary non-woven permanent cavity insert to catch
the droppings and keep them from blocking weep holes.
Other Resources
Efflorescence,
PCA IS239
Workmanship Part
II, Imparting Quality to Masonry,
PCA IS246
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