Page 425 - Construction Waterproofing Handbook

THE BUILDING ENVELOPE: PUTTING IT ALL TOGETHER 10.27
Walls and floor areas below grade are waterproofed to form a complete monolithic
enclosure. Structures are subjected to water not only from groundwater conditions but
also from runoff from surface collected water. Buildings built at or below-grade level are
also subject to a head of water. They are also exposed to water from capillary rise from being
in contact with the ground and water percolating downward from above-grade sources.
Below-grade surfaces typically have the most severe conditions with which to contend,
with the exception of wind-driven rain that presents water pressures higher than a head of
water. Particular attention to all of these conditions is necessary to ensure integrity of
below-grade envelopes.
Existing soil conditions also require consideration. Below-grade horizontal surfaces
should be placed on top of coarse granular soil (e.g., sand, gravel) to allow positive
drainage away from foundations. Granular soils also resist capillary water rise that occurs
in dense soil materials such as clay or silts. Areas adjacent to wall surfaces below grade
should consist of coarse granular material to promote drainage.
Proper drainage of subsurface water is a necessity for adequate protection against water
infiltration and longer life-cycling of building components below grade. Foundation drains
are installed as shown in Fig. 10.5. Drains are placed at foundation level or slightly above,
to prevent washout of soil beneath foundation structures. Drains are usually perforated pip-
ing with holes facing downward so as not to fill the pipe with soil, and are set in a coarse
granular bed. They should be sloped away from building structures, with water collected
at a natural out-face or sump area such as a retention pond.
It is also recommended that drainage mats be installed on vertical surfaces of water-
proofing membranes below-grade. Several synthetic compositions are available and com-
patible with waterproofing membrane systems. These mats promote quick drainage of
water off below-grade walls into available drainage systems.
At grade level, grading should be sloped away from structures to provide positive
drainage of surface waters away from buildings. Slope ratios differ depending on the type
of soil and adjacent exposed finishes at surface level. For planted or grassed areas, slopes
should be 5 percent minimum. For paved areas, 1 percent minimum slope is acceptable.
Draining water is collected and properly diverted, to prevent excess water from percolat-
ing into soil adjacent to structures.
Walls above-grade are subjected to water from rain, snow, and capillary action of soil
at grade level. Water conditions can become especially severe when rain is present in high-
wind conditions, forcing water through minute cracks and openings in above-grade
envelopes. Wall areas must also withstand weathering conditions to which a below-grade
envelope is not subjected. These conditions include ultraviolet degradation, air pollutants,
acid rain, chloride attack, freeze–thaw cycling, and thermal shock. Therefore to be com-
pletely effective, exposed portions of envelopes must not only be watertight but also
weather-resistant. This ensures longevity of these systems and protection of interior areas.
Above-grade envelopes must also be provided with provisions to drain water away from
a structure adequately, not allowing it to percolate down to below-grade areas. All build-
ing horizontal portions should be sloped to shed water. This includes not only roof areas
but also coping caps, sills, overhangs, ledges, balconies, decks, and walkways.
To allow areas of standing or ponding water not only makes an envelope subject to
water infiltration but also intensifies weathering from such sources as acid rain, chlorides,