Flashing and factors that influence water flow

Flashings must deal with the forces that drive the flow of water, because water can move upward and sideways as well as downward. These forces include gravity, surface tension, capillary action, kinetic energy and differential pressure.


The building should have the following features to deflect water that flows downward from the pull of gravity:

Surface Tension

Surface tension allows water to flow along the underside of a surface horizontally, and even upward, in narrow spaces such as crevices. In confined spaces, spacing horizontal surfaces more than 9 mm (0.38 in.) apart will prevent the adherence of water to the two surfaces, thus allowing the water to drain away. A “drip” edge is placed at points of discharge to break that surface tension and allow water to drop by gravity.

Capillary Action

In porous materials such as concrete and brick, water can be drawn into small-diameter openings of less than 5 mm (0.20 in.) by capillary action or “wicking.” The flashing joints should stop this from occurring. The design of joints and upturns must address this particular issue.

Kinetic Energy

Rain is often directed at flashings with high velocity and significant horizontal motion. On the upper locations of buildings, the wind actually carries rain drops upward. The momentum of wind forces can be strong enough to carry the rain drops through even small unsealed joints or openings. To prevent this, it is important to overlap and seal all joints in flashings as well as the joints between the flashing and the moisture barrier.

Air Pressure and Pressure Differentials

The combined effect of a positive air pressure on the outside of a building and a negative pressure inside, which is called a “pressure differential,” can drive water through unsealed or poorly sealed joints. Wind can drive water through even small holes and gaps. To prevent this, care must be taken when incorporating a flashing into the building air barrier. Alternatively, the joint may be sealed. Flashings at the top of buildings such as at parapets are subject to uplift. They should be anchored to the wall securely and sealed to prevent water penetration.

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