MANUAL Published: 01 January 2009

Moisture-Related Properties of Wood and the Effects of Moisture on Wood and Wood Products


MOISTURE IS ARGUABLY THE MOST IMPORTANT factor affecting the performance and service life of wood and wood products. Moisture affects the dimensional movement of wood and wood products; under certain conditions, moisture change can result in major dimensional change. The integrity and strength of adhered (bonded) wood products can be compromised by swelling-induced stresses that accompany wetting. Progressive deflection over time of wood members under load is influenced by moisture conditions, particularly by large repetitive fluctuations in moisture content. Mechanical connections between wood members can be compromised by exposure to elevated moisture conditions or by significant moisture cycling. It is widely recognized that the structural integrity of wood can be irreversibly degraded by biological attack. In some cases biological infestation does not influence structural integrity but nevertheless influences serviceability. For many insect pests and all fungi, moisture conditions higher than the preferred in-service conditions are either required for infestation, or increase the likelihood of infestation. Although wood, wood products, and wood construction can be degraded by elevated moisture levels or by greatly fluctuating moisture conditions, the vast majority of residential structures built in North America over the past three centuries were constructed primarily of wood, and most of these have performed reliably. Wood and wood products dried to an appropriate level, and maintained within a reasonable range of fluctuating moisture conditions will perform nearly indefinitely. In contrast, wooden buildings constructed without consideration of moisture control may rapidly suffer moisture-induced damage, leading to excessive repair and maintenance costs; in extreme cases the damage may even justify premature demolition. The central topic of this chapter is how moisture affects the properties and behaviors of wood and wood-based products used in building construction. Physical properties and behaviors are discussed, as are structural behaviors, and what may be considered biological behaviors (specifically, the likelihood of biological infestation by microbes and insects). The order of discussion is that outlined in the previous sentence, namely physical first, then structural, then biological. Before the chapter delves into its central topic, it provides background information on wood and on adhered (bonded) wood products. The information presented on wood includes discussion of wood structure, composition and basic characteristics, with an emphasis on structure. The information presented on bonded wood products follows a similar discussion path, but inasmuch as these are manufactured products, the emphasis is on product classification, composition, fabrication, and characteristics. Information concerning contemporary wood products is presented, as is information on wood products produced in past decades. The intent is to provide information applicable to buildings of various ages, not just to recently-constructed buildings.

Author Information

Carll, Charles
USDA Forest Service, Madison, WI
Wiedenhoeft, Alex
USDA Forest Service, Madison, WI
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Developed by Committee: E06
Pages: 54–79
DOI: 10.1520/MNL11544M
ISBN-EB: 978-0-8031-8400-8
ISBN-13: 978-0-8031-7004-9