Page 135 - Masonry and Concrete

Understanding Masonry
134 CHAPTER FOUR

between units and mortar. Any time that units with integral water
repellent treatments are used, a similar and chemically compatible
admixture from the same manufacturer must be used in the mortar to
assure that good bond is achieved.

4.8.2 Prevent Moisture Accumulation

Water does not hurt masonry, and a wall that gets wet suffers no harm
as long as it can dry out easily. Saturated masonry, however, is vulner-
able to freeze-thaw damage and other problems, so draining pene-
trated moisture to prevent its accumulation is important. Cavity walls
were initially conceived to provide drainage through a system of flash-
ing and weep holes. Cavity walls and anchored veneer walls should
have an open separation of at least two inches between the exterior
masonry wythe and the backing. The open cavity, when it is properly
fitted with flashing and weep holes, functions as a drainage system for
moisture which penetrates from the exterior or is condensed from
water vapor within the wall section. Single-wythe walls and multi-
wythe solid walls must also be designed with a system of flashing and
weep holes to divert moisture to the outside. Moisture protection is
maximized when the flashing membrane is of sturdy material that is
installed without gaps or voids and is turned up to form pans at hori-
zontal terminations.
One of the most critical elements in the proper drainage of masonry
walls is keeping the drainage cavity open and the weeps unobstructed.
If the cavity is clogged with mortar droppings or other debris, drainage
is ineffective and moisture will accumulate above the flashing. In cold
climates, this saturation can lead to freeze-thaw damage, and in warm
climates to the growth of mold and mildew as well as other vegetation.
In any climate, prolonged saturation of masonry can cause efflores-
cence, which is a white, powdery stain, or “lime run,” which is a hard,
crusty white streak. Recommendations on the placement of flashing
and weeps are covered in the various chapters which follow, as appro-
priate to the type of construction involved. The removal of efflores-
cence and lime run are discussed in Chapter 7.

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