Waterprooﬁng is the combination of materials or systems that prevents
water intrusion into structural elements of a building or its ﬁnished
spaces. Basic waterprooﬁng and envelope design incorporates three steps
to ensure a watertight and environmentally sound interior:
- Understanding water sources likely to be encountered.
- Designing systems to prevent leakage from these sources.
- Finalizing the design by properly detailing each individual envelope component into adjacent components.
Water likely to penetrate building envelopes is most commonly from
rainwater on above-grade components and groundwater intIusion below-
grade. Other sources also should be considered as appropriate, such as
melting snow, overspray from cooling towers, landscaping sprinklers, and
redirected water from such sources as downspouts and gutters.
The presence of any of these water sources alone, though, will not cause
leakage; for leakage to occur, three conditions must be present First,
water in any of its forms must be present Second, the mter 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 componentsl Finally and most important, there must be a
breach (hole, break, or some type of opening) in the envelope to facilitate
the entry of mter 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 structures For above-grade envelope components,
horizontal areas are very prone to gravitational 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
necessary “ﬂat” areas. In building components such as these, a minimum
1/4 in/ﬂ of slope should be incorporated rather than the 1/9 in that is oﬂen
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 hue 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 minimally
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