Saturday, February 23, 2013

WATERPROOFING - Water Sources.

Water likely to penetrate building envelopes is most commonly from rainwater on abo grade components and groundwater intrusion below-grade. Other sources also should considered as appropriate, such as melting snow, overspray from cooling towers, la scaping sprinklers, and redirected water from such sources as downspouts and gutters

The presence of any of these water sources alone, though, will not cause leakage; for l age to occur, three conditions must be present. First, water in any of its forms must be pres

Second, the water must be moved along by some type of force, including wind and gravity for above-grade envelope components and hydrostatic pressure or capillary action for below- grade components. Finally and most important, there must be a breach (hole, break, or some type of opening) in the envelope to facilitate the entry of water into the protected spaces.

Available water is moved into the interior of a structure by numerous forces that include

● Natural gravity
● Surface tension
● Wind/air currents
● Capillary action
● Hydrostatic pressure

The first three typically are encountered on above-grade portions of the envelope, whereas the last two are recognized at grade or on below-grade areas of buildings or structures. For above-grade envelope components, horizontal areas are very prone to gravitational forces and never should be designed completely flat. Water must be drained away from the structure as quickly as possible, and this includes walkways, balconies, and other necessary “flat” areas. In building components such as these, a minimum  1 4 in/ft of slope should be incorporated rather than the  1 8 in that is often used as a standard. The faster the water is directed off the envelope, the less chance there is for leakage.

Consider the teepee, built from materials that are hardly waterproof in themselves; the interior areas remain dry simply because the design sheds water off instantaneously. The same is true for canvass tents; the material keeps the occupants dry as long as the water is diverted off the canvass immediately, but use the same material in a horizontal or minimally sloped area, and the water will violate the canvas material. Figure 1.1 emphasizes the importance of slope to prevent unnecessary infiltration.

Sloping of envelope components maximizes drainage of water away from the enve- lope. The flat-roof design shown is often the cause for leakage problems simply because the water stands or “ponds” on the envelope surface.
FIGURE 1.1 Sloping of envelope components maximizes drainage of water away from the enve-
lope. The flat-roof design shown is often the cause for leakage problems simply because the water
stands or “ponds” on the envelope surface.

In fact, incorporating adequate slope into the design could prevent many of the common leakage problems that exist today. Simply compare residential roofs that incorporate a slope as high as 45   to commercial roofs that are designed with a minimum  1 8-in slope.

Although the materials used in the commercial application are more costly and typically have superior performance capabilities than asphalt shingles used on residential projects, the commercial roofs continue to have leakage problems at a far greater incident rate than residential roofing.

Surface tension is the momentum that occurs when water being moved by gravity approaches a change in building plane (e.g., face brick to lintel) and clings to the underside of the horizontal surface, continuing with momentum into the building by adhering to the surface through this tension. This situation frequently occurs at mortar joints, where water is draw into a structure by this tension force, as shown in Fig. 1.2.

Surface tension accelerates water infiltration.
FIGURE 1.2 Surface tension accelerates water

This is the reason that drip edges and flashings have become a standard part of any successfully building envelope. Drip edges and flashings break the surface tension and prevent water from being attracted to the inside of a building by this force. Some common drip edge and flashing details to prevent water tension infiltration are shown in Fig. 1.3.

Typical and common uses of “drip edges” to prevent tension infiltration.
FIGURE 1.3 Typical and common uses of “drip edges” to prevent tension

When wind is present in a rainstorm, envelopes become increasingly susceptible to water infiltration. Besides the water being driven directly into envelopes by the wind currents themselves, wind can create sufficient air
pressure that creates hydrostatic pressure on the facade that can force water upward and over envelope components. Again, flashing is used frequently to prevent this phenomenon from causing water penetration into a structure. This typical detailing is shown in Fig. 1.4.

 Flashing used to prevent water under pressure from entering the envelope.
FIGURE 1.4 Flashing used to prevent water under pressure from entering the

Capillary action occurs in situations where water is absorbed into an envelope substrate by a wicking action. This situation is most likely to occur with masonry or concrete portions of the envelope at or below grade levels. These materials have a high number of minute void spaces within their composition, making them susceptible to capillary water intrusion. These minute voids actually create a capillary suction force that draws water into the substrate when standing water is present. This is similar to the action of a sponge that is laid in water and begins absorbing the water.

Materials that have large voids or are very porous are not as susceptible to capillary action and in some cases are actually used to prevent this reaction on a building. For example, sand is often used as a fill material below concrete slabs placed directly on grade to prevent the concrete from drawing water from the soil through capillary action. Typical methods to prevent capillary action in envelopes are shown in Fig. 1.5.

Preventing capillary water infiltration into envelopes.
FIGURE 1.5 Preventing capillary water infiltration into envelopes.

Hydrostatic pressure most commonly affects below-grade portions of the envelope that are subject to groundwater. Hydrostatic pressure on an envelope is created by the weight of water above that point (e.g., the height of water due to its weight creates pressure on lower areas referred to as hydrostatic pressure). This pressure can be significant, particularly in below-grade areas, where the water table is near the surface or rises near the surface during heavy rainfalls. Water under this significant pressure will seek out any failures in the envelope, especially the areas of weakness—the terminations and transitions between the envelope components. This is why certain envelope substrates used below grade have to be better protected against water infiltration than those above grade. For example, concrete above grade is often only protected with a water repellent, whereas below grade the same concrete must be protected with a waterproofing material to prohibit leakage into the structure.

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