You can't depend on the exterior finish of a home to prevent water intrusion through the exterior walls. The exterior finish is the first line of defense against water intrusion, but it isn't entirely waterproof. Stucco can crack, brick is porous, and courses of siding have gaps between them. Making the exterior surface as weather-resistant as possible is only part of the drainage solution. To prevent water intrusion, you have to think of walls as a water management system, and apply a second line of defense, consisting of the drainage plane and the proper flashings, to drain any water that penetrates the wall cladding back out to the exterior.
As all builders know, flashing is a thin sheet or strip of water-resistant material that's installed at roof intersections and projections, around windows and doors, and along the tops of foundation walls to direct water flow away from the home. Flashing guides water out from the drainage plane to the exterior. It's layered shingle fashion with the drainage plane to provide positive drainage.
The home's exterior isn't a continuous surface. It has intersections, penetrations and abrupt terminations. Roofs have chimneys, dormers, ridges and valleys. Exterior walls have windows, doors and adjoining roofs. Porches and decks interrupt the flow of rain and melted snow as they move down the home. Wherever water moving down the exterior or the drainage plane has to change direction, it needs the help of flashing to keep it moving down and away from the home.
To direct water away from the home, the flashing must be high enough and wide enough to guide water out of vulnerable areas. Imagine water that's flowing across an open valley on the roof. If that water can flow across the valley, up the flashing and then down behind the flashing, the flashing hasn't done its job. The flashing needs to be high enough that water, driven by wind or its own momentum, will move back down again without moving up and behind the flashing. The following are two water management solutions for a home:
Flash and seal all penetrations. Make sure that the flashing is the proper size and fits tightly to the penetration. The flashing should be able to protect the opening from water intrusion before any caulk is applied. The flashing must be large enough to counteract wind-driven rain. If the flashing is large enough, wind-driven rain will move up the flashing piece and then back down, instead of behind, the flashing.
Check the flashing at intersecting roofs. If a porch or bay roof intersects the wall, flashing must guide water from the wall's drainage plane, where it can drain from the home. The wall's drainage plane material should overlap the flashing at this intersection, guiding water from the walls to the flashing, and finally, to the roof.
If you think of the home as a water management system and use flashing as one of your basic drainage strategies, you'll help protect the home from water intrusion and mold growth.
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