Controlling Moisture: Deflection Checklist

Deflection is the first principle and main priority of water management. Some of the primary strategies that have proven effective at reducing the amount of rainwater on exterior walls include the following: • Place the building so it is sheltered from prevailing winds by other buildings, trees, etc. • Use a pitched roof where appropriate. • Provide sizable roof overhangs and water collection systems at the roof perimeters. • Provide architectural detailing including flashing and caulking to direct rainwater away from the building. • Provide an approved drainage system around the foundation perimeter to accept roof water run-off. • Install a weather barrier within the wall in an appropriate location and with vapor permeability appropriate for the climate and moisture management approach. • Install permeable bulk vapor diffusion retarders on the exterior of walls and floors, including below a concrete slab or on top of the bare soil within a crawl space. • Separate wood elements from moisture sources including soil and concrete, using impermeable membranes. Images courtesy of Canadian Wood Council and Canada Wood Source: Design of Wood Frame Structures for Permanence, American Wood Council.

 

Drying: Drying is the mechanism by which building envelope assemblies remove accumulated moisture by venting (air movement) and vapor diffusion. If, due to construction or maintenance errors, water penetrates the water-resistant membrane, the wood sheathing, studs, roof truss, and other wood elements in the building envelope can get wet. These elements must be allowed to dry. In properly designed building envelope assemblies, water will evaporate and the resulting vapor will go through the assembly's outer layers, providing vapor permeability has been designed into the building envelope assemblies.

Exterior wall assemblies must be designed to allow sufficient drying both to the exterior and interior depending on the climate. The permeability of cladding, moisture barrier, vapor barrier, insulation (exterior insulation in particular), and interior finish materials will greatly affect the wall's overall drying potential. Rainscreen cavities may also dry the wall and cladding if vented.

Experts caution that the drying ability of wall systems should not be relied on to compensate for serious flaws in other moisture management mechanisms, since only minimal amounts of water can be dissipated through drying. The bulk of the moisture protection in a wall stems from deflection and drainage.

Durable materials: Designers should not discount the value of preservative-treated wood or naturally decay-resistant wood for applications such as cladding, shingles, sill plates, and exposed timbers or glulam beams, where moisture tolerance is necessary or where termite infestation is likely. Approved durable or preservative-treated wood is usually recommended when location of the member in question cannot be maintained at a safe moisture content and when climatic or site conditions may not permit control of decay or termites by construction practices alone. The role, characteristics, and availability of naturally durable and preservative-treated wood species are discussed later in this course.

Designed by tvsdesign, the Branson Convention Center in Branson, Missouri features a large overhanging roof. By sheltering the wood underneath, the roof deflects precipitation and helps to ensure long-term durability. Photo courtesy of Brian Gassel, tvsdesign