Saturday, February 23, 2013

WATERPROOFING - Designing to Prevent Leakage.

Once a complete understanding of the potential sources of water and forces that can move this water into an envelope is gained for a particular structure, the design must incorporate effective systems to prevent such intrusion. Expected conditions for a particular geographic area that will affect the above-grade envelope are available from the national weather service at www.nws. Below-grade water tables are determined by testing actual site conditions.

It also should be understood that substrate water penetration and absorption do not nec- essarily cause leakage to interior spaces. Water absorption occurs regularly in masonry facades, but the masonry is either large enough to absorb the penetrating water without passing it on to interior finishes or this water is collected and redirected back to the exteri- or by the use of dampproofing systems. Water penetration also occurs at the microscopic and larger voids in the masonry mortar joints, but again, the masonry absorbs it or the water is redirected back out through the dampproofing system.

For definition purposes, water infiltration and leakage are used interchangeably in this book because each is not an expected outcome in envelope design. All envelope compo- nents are designed to prevent leakage or infiltration by one of three systems:

1. Barrier
2. Drainage
3. Diversions

Barrier systems are, as their name implies, effective and complete barriers to water infiltration. They include actual waterproofing systems such as below-grade urethane membranes and other envelope components such as glass. They completely repel water under all expected conditions, including gravity and hydrostatic pressure. Refer to Fig. 1.6.

Barrier waterproofing system.
FIGURE 1.6 Barrier waterproofing system.

Drainage systems are envelope components that might permit water absorption and some infiltration through the substrate but provide a means to collect this water and divert it back out to the exterior before it causes leakage. Examples include masonry walls with cavity-wall dampproofing and flashing to divert penetrating water and water vapor back to the exterior. Refer to Fig. 1.7.

Drainage waterproofing system.
FIGURE 1.7 Drainage waterproofing system.

A new term in construction design is being used, namely, rain-screen system. This is simply another term for drainage system. Rain-screen systems use cavity-wall systems in curtain-wall and similar construction techniques, where the air space in the cavity wall is used to prevent air pressure from permitting water to enter the initial barrier facade com- ponents into the interior portions of the building.

Diversions actually redirect water being forced against envelope components and divert it elsewhere before it infiltrates or absorbs into the substrate. These might include sloping of roof decks and balconies, vertical drainage mats applied to below-grade walls, gutters and downspouts, flashings, and wind screens. Refer to Fig. 1.8 for typical examples of diversion systems.

Diversion systems.
FIGURE 1.8 Diversion systems.

Building facades usually contain combinations of these systems, each preventing water infiltration at their location on the envelope. However, regardless of how well the individual systems function, if they are not properly transitioned into other envelope components or terminated sufficiently, leakage will occur. These situations become the major issues preventing effective building envelope and waterproofing functioning and are the cause of most leakage that occurs in all structures.

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