Storage of Unseasoned Framing Lumber
Solid-stacked, unseasoned lumber may be stored outdoors without protection in cool weather for short periods of time, although some of the top boards in a stack may experience some dimensional stability-related problems if exposed to sun. A light cover (over the top only) will help prevent potential problems. Rain is not particularly detrimental to unseasoned lumber, as short term wetting and drying has little effect on the moisture content of the wood; however, no lumber should be allowed to sit in puddles of water or mud. Grading rules used across North America regulate the amount of deviation from true or straight that is allowed in lumber at the time of manufacture. Any deviations from straight or flat across the length and width is checked by the lumber grader at the time of surfacing. Caution should be exercised during lumber transit, storage and handling to ensure additional deviations do not occur as a result of inappropriate practices.

For extended storage, lumber should be separated with lath (stickers) or other thin strips of wood to allow air to circulate through the pile. See Figure 2. Proper storage will keep the lumber straighter as it seasons and the air circulation will eliminate the opportunity for mold or mildew to develop. (Refer to WWPA’s Lumber Storage TIP Sheet for additional information.)

Mold and Mildew
It is not uncommon for unseasoned framing lumber to develop surface mold and mildew during transit or storage prior to use. This is especially true during the warm summer months. Surface molds and mildews produce a fuzzy surface growth which may range in color from yellow to black. These organisms feed off of the sugars and starches readily available in the wood. As the moisture content of the wood drops to below 20%, the growth of these organisms stops. Normally, they do not break down the wood cell wall material and, therefore, cause only cosmetic damage.

Stain-causing fungi occur primarily in the sapwood (fiber from the outer region of the tree). Like mold, stain fungi can affect the appearance of lumber but do not cause structural damage to the wood.

Blue Stain is an example of a cosmetic stain that can readily occur in Ponderosa Pine. Although Blue Stain can be aesthetically pleasing in certain types of applications, lumber manufacturers dry most prevent its development in their products. While mold, mildew and stain do not affect the strength of the wood, their presence does indicate a higher than 20% moisture content level (as in unseasoned wood). This condition can lead to decay if the affected framing lumber is not allowed to dry out. The easiest way to eliminate this possibility is to sticker and stack the lumber or frame the building. Air circulation around the affected pieces lowers the surface moisture content level and stops the growth of fungal spores. Lumber seasoned to below the 20% moisture content range rarely, if ever, supports fungal growth. (Source: USDA Agriculture Handbook 72.)

Surface discoloration from mold and mildew on framing lumber can be left on the wood when it is to be covered by a finished wall surface. Proper construction practices will prevent moisture from entering the wall cavity and elevating the moisture content level of the wood.

For exposed conditions, any discolored wood can be cleaned by using a mixture of one part household bleach to three parts water, sprayed on the wood or applied with a non-metal brush. The wood should be flushed clean afterwards. Commercial mildew removers are also available.

As with any cleaning product, wear protective gloves and clothing, avoid skin contact, protect vegetation, and follow manufacturer’s safety instructions.

People with severe respiratory conditions or specific allergies may be sensitive to the presence of mold and associated spores. While this is exceptionally uncommon, if allergies are known to be problematic, dry framing lumber may be preferred.

Heavy Timbers (5" and larger)
Although the word “timber” is sometimes used inappropriately to describe various large-size wood members, its meaning in lumber terminology is specifically limited to the size classification of members 5x5 and larger (which includes Beams & Stringers and Posts & Timbers). Timbers are always manufactured and sold unseasoned.

Depending on species, size, surrounding temperature, air circulation and humidity levels, the amount of time required for acclimatization of timbers is variable.

Lumber dries more quickly on its surfaces and at the ends than in its interior. As mentioned earlier, softwood lumber begins to shrink as it dries below the fiber saturation point, a 28 to 30% moisture content level. As this happens, the outside fiber becomes smaller than the interior fiber and “seasoning checks” (fissures or cracks on the surface of the wood) or end splits may occur. Checking and end splitting are accounted for in the development of design values so no reductions in strength properties are necessary; however, these natural developments do have cosmetic impact and dimensional change in timbers must be addressed in the design of fastenings and application of finishes. (Refer to WWPA’s Western Woods Use Book for information on designing with heavy timbers.)

Several things can be done to reduce the amount of checking and end splitting to be expected in heavy timbers. Timbers may be specified “Free of Heart Center” (FOHC). Such timbers typically demand price premiums and are limited in availability. Check with local suppliers. In addition, the ends of timbers can be protected with end waxes or other coatings containing paraffin. End coating is available from selected WWPA Member mills upon request (through buyer/seller agreements on large orders), or may be undertaken at the job site if seasoning checks have not yet begun to develop. End coatings help timbers to season more evenly and are beneficial in extending the shelf life of timbers during storage and transport prior to installation.

Some degree of checking is virtually unavoidable in large timbers. Prior to specifying for a project, designers and property owners are advised to visit several old buildings that contain large timbers in order to evaluate the degree of checking to be anticipated over time. The rustic quality timbers naturally gain with time may or may not meet design criteria and owner expectations. If a large cross section is required, but seasoning checks are to be minimized, designers may want to fasten together a number of 2x dry pieces to make up a larger cross section or Investigate the availability of glu-lam timbers.

However, when the heavy scale and rustic aesthetic of Western timbers are central to a design, the special considerations these heavy members require are worth the effort.