BUILDING - 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 hydrostatic 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 horizontal 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.

Today, however, it is not uncommon for high-rise structures to have an envelope skin thickness of  1 8 in. Such newer construction techniques have developed from the need for 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 necessary above-grade waterproofing systems, one that must be carefully tied into other building 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.

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