Tempo:
Gravity The building should have the following features to deflect water that flows downward from the pull of gravity:
Surfaces should slope (minimum 6% slope) in the desired direction of flow, after any settlement or shrinkage has taken place. Points of discharge should project out beyond materials below the flashing, which may deteriorate when in contact with water or ice. Terminations on sloped surfaces should be lapped in shingle fashion so that the natural direction of the water is over and onto the next water-shedding surface.
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.
TYPES OF FLASHING Many types of flashing are available, as each location vulnerable to rain penetration has different flashing design needs. The most common types are described below and have been named to describe how they function and where they are located.
Base Flashing When a roof intersects with a wall or another roof penetration, such as a plumbing vent, the roofing system should be turned up to make the junction watertight. The part of the roofing that is turned up is generally known as a “base flashing.” It may be made
Surfaces should slope (minimum 6% slope) in the desired direction of flow, after any settlement or shrinkage has taken place. Points of discharge should project out beyond materials below the flashing, which may deteriorate when in contact with water or ice. Terminations on sloped surfaces should be lapped in shingle fashion so that the natural direction of the water is over and onto the next water-shedding surface.
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.
TYPES OF FLASHING Many types of flashing are available, as each location vulnerable to rain penetration has different flashing design needs. The most common types are described below and have been named to describe how they function and where they are located.
Base Flashing When a roof intersects with a wall or another roof penetration, such as a plumbing vent, the roofing system should be turned up to make the junction watertight. The part of the roofing that is turned up is generally known as a “base flashing.” It may be made
©
2026
Lehal.net