Drain the Rain on the Plane

Claudette Reichel  |  7/22/2008 11:53:20 PM

Figure 1, Window Flashing System Drains Leaks to Outside

Figure 2, Drainage Plane and Space Behind Cladding

Figure 3, Vented Drainage Plane for High Rainfall Areas

Figure 4, Two Layer Wrap for Stucco

Continuous drainage plane: A drainage plane is the layering of water-resistant materials between the cladding and the framing to drain water to the exterior and keep leaks (at windows, doors and cladding) from wetting the framing and insulation. Remember, leaks happen. Plan for them.

A drainage plane can be created by using housewrap or building paper installed shingle fashion, or by sealing the seams of foam sheathings with foil tape. Nonperforated housewraps are recommended behind wood siding and brick veneer because wood and mortar can leach surfactants that allow water to penetrate the tiny perforations. Special attention is needed to make sure the drainage plane provides complete protection at windows and doors.  (Figure 1, Window Flashing Systems Integrated with Housewrap)

Window and door flashing; All openings in wood frame walls must be properly flashed, especially window sill corners, but this is often omitted or not detailed correctly. Water should not be able to drip behind the housewrap, flashings or seep into the corners of a window or door opening. Shingle fashion installation must be maintained throughout. For example, the top of a window flange (nail-on frame) should be slipped behind the drainage plane housewrap or paper, but the bottom flange should be on top of the paper.

Recently developed window and door flashings products and techniques provide fast, easy and dependable long-term performance. Effective sill flashing systems should include a back-dam or slope to drain outward under the window unit, seamless corner protection, and shingle fashion integration with the housewrap.  (Figure 2, Drainage Plane and Space Behind Cladding) 

Vented space behind cladding: In rainy climates, all walls (except those protected by deep porches and solid masonry “reservoir” construction systems) should have a continuous drainage plane and a drainage space (a gap) behind the cladding. In high rainfall areas, the drainage space should be vented to speed drainage and reduce pressure that can drive water vapor into a wall.  (Figure 3, Vented Drainage Plane for High Rainfall Areas)

Brick veneer walls with an air space (not obstructed with mortar) and weep holes at the bottom course is a good example. Including vents at the top and bottom courses of the brick veneer with a clear 1 inch min. air space creates an especially effective system that drains rapidly and relieves vapor pressure.

Sidings: Wood or fiber cement siding should be primed on the back side and installed with spacers, on furring strips or over a rainscreen drain mat to create a drainage space behind it. Vinyl or aluminum siding has a built-in drainage space.

Portland cement stucco and synthetic stucco exterior insulating finish systems (EIFS) both must include a drainage plane and space if there are any penetrations in the wall (windows, doors, etc.). Barrier system EIFS (adhered to the sheathing) are not recommended, except under deep porches and in dry climates. A water-managed EIFS can be a good choice that provides a stucco appearance with added insulation and good drainage. If using real stucco, a drainage space is still needed. It can be created by using a crinkled housewrap (providing drainage gaps) with a second layer of building paper, or two layers of building paper so the stucco does not stick to the drainage plane.  (Figure 4, Two Layer Wrap for Stucco)

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