Page 113 - Construction Waterproofing Handbook

3.2 CHAPTER THREE
Today, however, it is not uncommon for high-rise structures to have an envelope skin
thickness of 8 in. Such newer construction techniques have developed from the need for
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lighter-weight systems to allow for simpler structural requirements and lower building costs.
These systems, in turn, create problems in maintaining an effective weatherproof envelope.
Waterproof building surfaces are required at vertical portions as well as horizontal
applications such as balconies and pedestrian plaza areas. Roofing is only a part of neces-
sary above-grade waterproofing systems, one that must be carefully tied into other build-
ing envelope components.
Today roofing systems take many different forms of design and detailing. Plaza decks
or balcony areas covering enclosed spaces and parking garage floors covering an occupied
space all constitute individual parts of a total roofing system. Buildings can have exposed
roofs as well as unexposed membranes acting as roofing and waterproofing systems for
preventing water infiltration into occupied areas.

DIFFERENCES FROM BELOW-GRADE SYSTEMS

Most above-grade materials are breathable in that they allow for negative vapor transmission.
This is similar to human skin; it is waterproof, allowing you to swim and bathe but also to per-
spire, which is negative moisture transmission. Most below-grade materials will not allow neg-
ative transmission and, if present, it will cause the material to blister or become unbonded.
Breathable coatings are necessary on all above-grade wall surfaces to allow moisture
condensation from interior surfaces to pass through wall structures to the exterior. The sun
causes this natural effect by drawing vapors to the exterior. Pressure differentials that
might exist between exterior and interior areas create this same condition.
Vapor barrier (nonbreathable) products installed above grade cause spalling during
freeze–thaw cycles. Vapor pressure buildup behind a nonbreathable coating will also cause
the coating to disbond from substrates. This effect is similar to window or glass areas that
are vapor barriers and cause formation of condensation on one side that cannot pass to
exterior areas.
Similarly, condensation passes through porous wall areas back out to the exterior when
a breathable coating is used, but condenses on the back of nonbreathable coatings. This
buildup of moisture, if not allowed to escape, will deteriorate structural reinforcing steel
and other internal wall components.
Below-grade products are neither ultraviolet-resistant nor capable of withstanding ther-
mal movement experienced in above-grade structures. Whereas below-grade materials are
not subject to wear, above-grade materials can be exposed to wear such as foot traffic.
Below-grade products withstand hydrostatic pressure, whereas above-grade materials do
not. Waterproofing systems properties are summarized in Table 3.1.
Since many waterproofing materials are not aesthetically acceptable to architects or engi-
neers, some trade-off of complete watertightness versus aesthetics is used or specified. For
instance, masonry structures using common face brick are not completely waterproof due to
water infiltration at mortar joints. Rather than change the aesthetics of brick by applying a
waterproof coating, the designer chooses a dampproofing and flashing system. This damp-
proofing system diverts water that enters through the brick wall back out to the exterior.