Page 21 - Construction Waterproofing Handbook

WATERPROOFING PRINCIPLES—THE BUILDING ENVELOPE 1.3
Second, the water must be moved along by some type of force, including wind and gravity for
above-grade envelope components and hydrostatic pressure or capillary action for below-
grade components. Finally and most important, there must be a breach (hole, break, or some
type of opening) in the envelope to facilitate the entry of water into the protected spaces.
Available water is moved into the interior of a structure by numerous forces that include

● Natural gravity
● Surface tension
● Wind/air currents
● Capillary action
● Hydrostatic pressure

The first three typically are encountered on above-grade portions of the envelope,
whereas the last two are recognized at grade or on below-grade areas of buildings or struc-
tures. For above-grade envelope components, horizontal areas are very prone to gravita-
tional forces and never should be designed completely flat. Water must be drained away
from the structure as quickly as possible, and this includes walkways, balconies, and other
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necessary “flat” areas. In building components such as these, a minimum 4 in/ft of slope
1
should be incorporated rather than the 8 in that is often used as a standard. The faster the
water is directed off the envelope, the less chance there is for leakage.
Consider the teepee, built from materials that are hardly waterproof in themselves; the
interior areas remain dry simply because the design sheds water off instantaneously. The
same is true for canvass tents; the material keeps the occupants dry as long as the water is
diverted off the canvass immediately, but use the same material in a horizontal or mini-
mally sloped area, and the water will violate the canvas material. Figure 1.1 emphasizes
the importance of slope to prevent unnecessary infiltration.
In fact, incorporating adequate slope into the design could prevent many of the common
leakage problems that exist today. Simply compare residential roofs that incorporate a slope
1
as high as 45 to commercial roofs that are designed with a minimum 8-in slope.
Although the materials used in the commercial application are more costly and typically
have superior performance capabilities than asphalt shingles used on residential projects,
the commercial roofs continue to have leakage problems at a far greater incident rate than
residential roofing.
Surface tension is the momentum that occurs when water being moved by gravity
approaches a change in building plane (e.g., face brick to lintel) and clings to the underside
of the horizontal surface, continuing with momentum into the building by adhering to the
surface through this tension. This situation frequently occurs at mortar joints, where water
is draw into a structure by this tension force, as shown in Fig. 1.2.
This is the reason that drip edges and flashings have become a standard part of any
successfully building envelope. Drip edges and flashings break the surface tension and
prevent water from being attracted to the inside of a building by this force. Some com-
mon drip edge and flashing details to prevent water tension infiltration are shown in
Fig. 1.3.
When wind is present in a rainstorm, envelopes become increasingly susceptible to water
infiltration. Besides the water being driven directly into envelopes by the wind currents