WOOD - Materials for Civil and Construction Engineers - page 405

dry southwestern states to 14% in the damp coastal regions. However, as it
leaves the mill, framing lumber typically has an average moisture content
of 15%.
Wood is seasoned by air and kiln drying. Air drying is inexpensive, but
slow. The green lumber is stacked in covered piles to dry. These piles of
lumber are made of successive layers of boards separated by 25 mm (1 inch)
strips so that air can flow between the layers. The time required for drying
varies with the climate and temperature of the area. Normally, three to four
months is the maximum air drying time used in the U.S. Air drying is com-
plete when the moisture content of the wood is in equilibrium with the air
humidity. The optimum moisture may not be achievable through air drying
alone.
After air drying the lumber may be kiln dried. A kiln is a large oven
where all variables can be closely monitored. Drying temperatures in a kiln
range from 20°C to 50°C (70°F to 120°F), typically requiring 4 to 10 days.
Care must be taken to slowly reduce the moisture content of wood. Drying
too rapidly can result in cracking and warping. Kiln-dried lumber will take
on moisture again if exposed to water; therefore, care must be used when
storing and transporting wood.

10.5 Lumber Grades

The final step in wood production involves grading the lumber accord-
ing to quality. Typically, lumber is graded according to the number of flaws
that affect strength, durability, or workability. The most common grade-
reducing qualities of lumber are knots, checks, pitch pockets, shakes, and
stains. Due to the high degree of natural variability within lumber, it is near-
ly impossible to develop an exact, uniform set of grading standards. As a re-
sult, grading techniques and standards can, and do, vary among organizations.
Organizations such as the National Bureau of Standards and the United States
Department of Agriculture have spent many years trying to develop a simple,
uniform method of lumber sizing, common nomenclature, and grading
standards.
The following agencies are certified by the American Lumber Standards
Committee Board of Review (Germantown, MD) for inspecting and grading
of untreated lumber:

Northeastern Timber Manufacturer Association (NELMA), Falmouth, MN

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Northern Hardwood and Pine Manufacturer Association (NHPMA),
Green Bay, WI

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Redwood Inspection Service (RIS), San Francisco, CA

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Southern Pine Inspection Bureau (SPIB), Pensacola, FL

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West Coast Lumber Inspection Bureau (WCLIB), Portland, OR

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Western Wood Products Association (WWPA), Portland, OR

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National Lumber Grader Authority (NLGA), Ganges, B.C., Canada

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