SECTION CONTENTS
Weight of Western Lumber
Flame-Spread Rating
Thermal Conductivity
Thermal Expansion
Chemical Resistance

Weight of Western Lumber

Solid wood substance is heavier than water with its specific gravity being about 1.5, regardless of the species. Despite this fact, dry wood of most species floats in water because a portion of its volume is occupied by air-filled cavities.

Variations in the size of cell cavities and in the thickness of the cell walls cause some species to have more wood substance per unit volume than others, and therefore a higher specific gravity. Thus, specific gravity provides an index to one species’ density in relation to other species. The higher the number, the higher the specific gravity or density. The average specific gravities of Western softwood species are provided in the Tables section.

To calculate the weight per linear foot for a particular size and species or species grouping, multiply the cross-sectional area of the member by the species weight factor provided in the Tables

Flame-Spread Ratings

This information is available in the publication Flame-Spread Ratings (A-4) which can be ordered or downloaded in Adobe Acrobat (PDF) format.

Species of wood differ in their burning rates. By measuring these rates, a standard can be established to compare different species of wood with regard to fire safety.

Flame-spread classifications have been developed by Underwriters Laboratories, Inc. The UL Standard Test Method has established a numerical scale based on a noncombustible, asbestos-cement board as 0 (zero) and a combustible red oak as 91. (Prior to 1979, red oak was assigned a value of 100.) The Steiner Tunnel test (ASTM E84), conducted in a 25-foot long tunnel furnace, is used to develop the actual burning and flame-spread data. Flame-spread ratings and smoke-developed indices for Western softwood species, along with references to the facilities that conducted the tests, are listed in the Tables section.

Designers should consult their locally-applicable codes for flame-spread and smoke-developed requirements for specific use, areas, and occupancies. Pertinent code information is covered in the model building codes. When a species does not carry a flame-spread classification appropriate to a desired application, designers may be able to use an intumescent finish or fire-retardant treatment to improve the flame-spread classification and satisfy local building codes.

Thermal Conductivity

The relatively low thermal conductivity or "k" of Western softwoods provides a significant amount of insulation. The factor k represents the amount of heat (BTUs) transferred in one hour through one square foot of material one inch thick with a difference in temperature of 1°F.

The thermal conductivity of wood increases with increased moisture content and with increased density. The "k" values for the Western Woods are shown in the Tables section.

Thermal Expansion

In most structural design, the coefficient of thermal expansion (the increase in dimension per unit of length, thickness or width for a temperature rise of 1°F) of wood can be ignored since it is very small.

Longitudinally, the coefficient of thermal expansion is independent of specific gravity and varies from 0.0000017 to 0.0000025 for different species. Across the grain, the values vary directly with the specific gravity from 0.000014 to 0.000022 for the Western Woods.

The coefficient of thermal expansion varies slightly with temperature, but for all ordinary uses may be considered constant. In the longitudinal direction, thermal expansion of wood is from 1/10 to 1/3 as great as the expansion of metals, concrete, and glass.

Chemical Resistance

Western Woods are highly resistant to a number of chemicals. Consequently, they are used for various types of tanks, containers, and equipment in which chemicals are used and for structures near such equipment.

Wood owes its extensive use in chemical equipment largely due to its superiority over cast iron and steel in resistance to mild acids and solutions of acidic salts. Iron is superior to wood in resistance to alkalies.

The spread between strength properties of the species after exposure to chemicals is not great. The percentage of original (wet-breaking strength) in tests after exposure to different chemical solutions for the duration of exposure indicated are shown in the Tables section. In designing for chemical resistance, the wet-use design values are applicable when there is exposure to aqueous solutions.

Specific Gravity of Western Softwood Species (sg.pdf)

Weight Factor (15% Moisture Content) (weight.pdf)

Flame-Spread Ratings (fsr.pdf)

Thermal Conductivity of Western Species (therm.pdf)

While every effort has been made to ensure the accuracy of the Western lumber information included in this Online Technical Guide, WWPA accepts no responsibility for errors or omissions in the information presented herein, or for errors which may occur in downloading and printing any of the files, nor any liability resulting from the use of this information for design and construction applications.

The WWPA Online Technical Guide is copyrighted by Western Wood Products Association in Portland, Oregon. © 1997 WWPA