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EXTERIOR FINISHES AND ROOF COVERINGS

Wood properties and dimensions change with moisture content (MC). Living wood contains a considerable amount of free and bound water. Free water is contained between the wood cells and is the first water to be driven off in the drying process. Its loss affects neither volume nor structural properties. Bound water is contained within the wood cells and accounts for most of the moisture under 30 percent; its loss results in changes in both volume (i.e., shrinkage) and structural properties. The strength of wood peaks at about 10 to 15 percent MC. Given that wood generally has an MC of more than 30 percent when cut and may dry to an equilibrium moisture content (EMC) of 8 to 10 percent in protected environment, it should be sufficiently dried or seasoned before installation. Proper drying and storage of lumber minimizes problems associated with lumber shrinkage and warping. A minimum recommendation calls for using “surface dry” lumber with a maximum 19 percent MC. In uses where shrinkage is critical, specifications may call for “KD-15,” which is kiln-dried lumber with a maximum moisture content of 15 percent. The tabulated design values in the NDS are based on a moisture content of 19 percent for dimension lumber. The designer should plan for the vertical movement that may occur in a structure as a result of shrinkage. For more complicated structural details that call for various types of materials and systems, the designer might have to account for differential shrinkage by isolating members that will shrink from those that will maintain dimensional stability. The designer should also detail the structure such that shrinkage is as uniform as possible, thereby minimizing shrinkage effects on finish surfaces. When practical, details that minimize the amount of wood transferring loads perpendicular-to-grain are preferable. Shrink and swell can be estimated in accordance with Section 5.3.2 for the width and thickness of wood members (i.e., tangentially and radially with respect to annual rings). Shrinkage in the longitudinal direction of a wood member (i.e., parallel to grain) is negligible. Durability Moisture is a primary factor affecting the durability of lumber. Fungi, which feed on wood cells, require moisture, air, and favorable temperatures to survive. When wood is subject to moisture levels above 20 percent and other favorable conditions, decay begins to set in. Therefore, it is important to protect wood materials from moisture, by: • limiting end use (e.g., specifying interior applications or isolating lumber from ground contact); • using a weather barrier (e.g., siding, roofing, building wrap, flashing, etc.); • applying a protective coating (e.g., paint, water repellent, etc.); 5-6 Residential Structural Design Guide Chapter 5 – Design of Light-Wood Framing • installing roof overhangs and gutters; and • specifying preservative-treated or naturally decay-resistant wood.