Page 112 - Construction Waterproofing Handbook

CHAPTER 3

ABOVE-GRADE WATERPROOFING

INTRODUCTION

Waterproofing of surfaces above grade is the prevention of water intrusion into exposed
elements of a structure or its components. Above-grade materials are not subject to hydro-
static pressure but are exposed to detrimental weathering effects such as ultraviolet light.
Water that penetrates above-grade envelopes does so in five distinct methods:
● Natural gravity forces
● Capillary action
● Surface tension
● Air pressure differential
● Wind loads
The force of water entering by gravity is greatest on horizontal or slightly inclined enve-
lope portions. Those areas subject to ponding or standing water must be adequately sloped
to provide drainage away from envelope surfaces.
Capillary action is the natural upward wicking motion that can draw water from ground
sources up into above-grade envelope areas. Likewise, walls resting on exposed horizon-
tal portions of an envelope (e.g., balcony decks) can be affected by capillary action of any
ponding or standing water on these decks.
The molecular surface tension of water allows it to adhere to and travel along the under-
side of envelope portions such as joints. This water can be drawn into the building by gravity
or unequal air pressures.
If air pressures are lower inside a structure than on exterior areas, water can be literally
sucked into a building. Wind loading during heavy rainstorms can force water into interi-
or areas if an envelope is not structurally resistant to this loading. For example, curtain
walls and glass can actually bend and flex away from gaskets and sealant joints, causing
direct access for water.
The above-grade envelope must be resistant to all these natural water forces to be water-
tight. Waterproofing the building envelope can be accomplished by the facade material itself
(brick, glass, curtain wall) or by applying waterproof materials to these substrates. Channeling
water that passes through substrates back out to the exterior using flashing, weeps, and damp-
proofing is another method. Most envelopes include combinations of all these methods.
Older construction techniques often included masonry construction with exterior load
bearing walls up to 3-ft thick. This type of envelope required virtually no attention to
waterproofing or weathering due to the shear impregnability of the masonry wall.

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