SECTION CONTENTS
Specifying Framing Grades
Specifying Finish and Siding Grades
Specifying Natural Wood Siding

This information is available in the publication Specifying Western Lumber (A-2) which can be ordered or downloaded in Adobe Acrobat (PDF) format.

All lumber should be gradestamped by an agency certified by the Board of Review of the American Lumber Standards Committee, Inc. and manufactured in accordance with Product Standard PS 20, as published by the U.S. Department of Commerce.

The following guidelines are intended to assist the designer and specifier in establishing the most economical and efficient use of solid lumber products and to eliminate potential misunderstandings between specifier and supplier.

A specification should include all species suited to the job. Species are presented in Species & End Uses section. Specify standard grades as described in the WWPA's Western Lumber Grading Rules (G-5) and presented in tables at the end of this section. Grades are further addressed in Grades & Quality Control section. Consider all grades suitable for the intended use. For economy in construction, it is recommended that the lowest grade suited to a job be specified.

Verify availability of species and grades with local suppliers. Not all species, grades or patterns are available in all locations.

Specifying Framing Grades

Structural design values vary depending on size, grade and species. Values assigned to lumber nominal 2" to 4" in thickness are based on the dry size. Unseasoned (green or S-GRN) lumber is manufactured oversized so that when it reaches an equalibrium moisture content it will be approximately the same size as the dry (S-DRY) size. Therefore, when S-GRN lumber is shipped, the same design values assigned to and used for dry lumber will apply.

Design values assigned to lumber nominal 5" x 5" and larger are assigned to the unseasoned (S-GRN) green size.

S-DRY and S-GRN lumber should not be mixed in a horizontal framing system, such as floor joists. While both are of the same strength and both will eventually reach similar sizes, the two will acclimate at different rates before achieving equilibrium with the atmospheric moisture content level.

2" to 4" Thick by 2" to 4" Wide Framing Lumber

The most widely available grades in all of the commercial softwood species are STANDARD & BETTER (STAND & BTR) and STUD. These grades are appropriate for most light-frame applications such as general wall framing.

STUD grade is intended for vertical use; whereas, STAND & BTR and the other grades (CONSTRUCTION and UTILITY) are multi-purpose and available in a variety of lengths. Dimension lumber grades apply to both solid sawn and certified structural-glued lumber.

UTILITY, in any commercial softwood species, may be used for plates, blocking, etc. and some walls. Review building codes for acceptance. 

Framing lumber in 2" thickness is typically produced with a 1/8" eased edge. However, some mills produce lumber with a 1/4" eased edge to assist in handling. These products are identified on the grademark as "1/4" EE."

When small posts or beams (2x4, 4x4) require specific design values, refer to the Design Values section to determine appropriateness of CONSTRUCTION, STANDARD and UTILITY grades. Specify according to Base Values. Specified Base Values are then modified by adjustments for engineering analysis.

Some 2x2s through 4x4s may require higher design values than available in the light-framing grades. In this case, refer to SELECT STRUCTURAL, No. 1 & BTR, No. 1, No. 2 and No. 3 grades. Specify Base Value structural values. Adjust Base Values for engineering analysis. Or refer to the design values for Machine Stress-Rated (MSR) lumber, MSR Lumber section.

Specifying MSR lumber is very straightforward because it is generally marketed by strength and stiffness values, Fb and E. Some MSR lumber producers provide daily quality control for tension (Ft) and specific gravity (SG) in addition to the mandatory Fb and E testing. When these additional levels of quality control are provided, the Ft and SG values will appear on the gradestamp (in addition to Fb and E).

When ordering, specify machine-rated, gradestamped lumber and list the strength value (Fb) and corresponding modulus of elasticity (E) values, nominal sizes and lengths required. Species should only be specified when design criteria require a horizontal shear (Fv), compression perpendicular to grain (Fc | ) or specific gravity (SG) value associated with a specific species, or when design criteria call for higher specific gravity (and related Fc | and Fv) resulting from voluntary specific gravity quality control.

2" to 4" Thick by 5" and Wider Framing Lumber

Joists, rafters and small beams should be specified by minimum required Fb and E Base Values. Refer to the Design Values section. Whenever possible design analysis should be based on No. 2 grade values of locally available species as most material is marketed No. 2 & BTR.

No. 1 & BTR may be available in some markets in Douglas Fir, Douglas Fir-Larch or Hem-Fir. Higher values should be used only for longer spans or higher loads. Lightly-loaded structures should take advantage of the economy of No. 3 grade. MSR and certified structural-glued material of same stresses can be used interchangeably with the above. Where engineering analysis permits, 2x6 studs can be No. 3 or STUD grade.

5x5 and Larger, Beams & Stringers and Posts & Timbers

The grades are SELECT STRUCTURAL, No. 1 or No. 2. Grade and species should be determined by required design values. Refer to the Western Lumber Dimension, Timbers and Structural Decking Grades table, along with appropriate adjustments for conditions of use. Where maximum dimensional stability is a requirement, specify Free of Heart Center (FOHC), realizing costs may be increased and availability limited.

Structural Decking

The grades are SELECTED DECKING and COMMERCIAL DECKING. Decking is manufactured either at 19% (S-DRY) or 15% (MC-15) moisture content. MC-15 will minimize shrinkage for exposed applications and may be available at increased cost. Check with local suppliers.

Some tongued-and-grooved decking is manufactured to pattern from No. 2 & BTR or No. 3 2x6 or 2x8 framing lumber. It is generally used for concealed subfloors in post and girder construction.

Appearance Framing

It is important to recognize structural grades of lumber are graded primarily for strength and not for appearance. Even in the highest structural grades, visual imperfections are not addressed by the grading rules.

The basic grade descriptions for dimension lumber provide good structural capacity, serviceability and general good appearance. However, sometimes structural grades with a higher appearance value are desired.

An Appearance classification is available that provides standard specifications for such uses. The Appearance category provides the same level of structural performance as the applicable NGR grade, but with the added restrictions on manufacture, wane, skip and warp.

Any NGR grade may be graded or ordered with Appearance limitations. "Appearance" shall be designated by either including the term or the abbreviation "APP" on the grade stamp or by specifically identifying "Appearance" in the purchase and shipping documents.

Board Lumber Used for Rough Carpentry

Boards are typically 1'' nominal thickness. Board grades used for rough carpentry are generally controlled by building code requirements, and the grades are selected from the Common or Alternate Board grades.

As an example, model building codes recognize No. 3 COMMON or STANDARD grades as equal minimum grades for spaced roof sheathing even though there are differences in grading characteristics. Verify local dealer availability prior to specifying.

Specifying Finish and Siding Grades

A specification for an appearance lumber grade should include a reference to the section number, title and edition of the grading rules from which it is written (refer to the Tables section below) In other words, if you are specifying from Section 21.11, special Western Red Cedar Rules, WWPA Western Lumber Grading Rules, so state.

Grain patterns, when desired, can also be specified for Select, Finish and Special Western Red Cedar grades. Three categories are available: vertical grain (VG), flat grain (FG) or a shipment of both VG and FG, generally referred to as mixed grain (MG). The most readily available and least costly is mixed grain. Unless otherwise specified, siding, paneling and finish boards are shipped with mixed grain. Stair treads, stepping, etc. should be vertical grain as it is more durable.

Interior and Exterior Trim and Finish Board Materials

Select from appearance grades shown in the Tables section below or as described in the WWPA Western Lumber Grading Rules.

Refer to the WWPA publication, Vol. 2 Western Wood Species Book for color photographs of Select, Finish, Common and Alternate Board grades in many Western lumber species. (Order from the WWPA Online Literature Store.)

Moisture Content of Appearance Lumber

Moisture content (MC) designations on the grade stamps indicate the condition of seasoning at the time of surfacing:

• MC15 or KD15 - 15% maximum MC moisture content

• S-DRY, KD or KD-HT - 19% maximum MC moisture content

• S-GRN or HT - over 19% moisture content (unseasoned).

References to air dried or kiln dried indicate the method of drying or seasoning, not specific moisture content levels.

However, regardless of whether Finish and Select grades are marketed as MC15 or as S-DRY, these grades are limited to a maximum of 15% MC as defined in the WWPA Western Lumber Grading Rules.

WWPA Finish and Select grades, as well as the highest quality Special Western Red Cedar grades, are shipped seasoned as follows: S-DRY or MC15 with at least 85 percent of items not exceeding 12% in moisture content and no portion exceeding 15% moisture content.

The Common grades in the general purpose category are typically marketed S-DRY, with a maximum of 19% moisture content. However, these grades may by specified to MC15 by special arrangements with the manufacturer.

Alternate Board grades are manufactured and shipped S-DRY and S-GRN so it is important to specify the desired moisture content when ordering these general purpose boards.

The Select Knotty and Quality Knotty Special Western Red Cedar grades are also manufactured and shipped S-GRN or S-DRY.

Specifying Natural Wood Siding

This information is available in the publication Natural Wood Siding Technical Guide (TG-8) which can be downloaded in Adobe Acrobat (PDF) format.

A siding specification should include specific grade names, along with paragraph numbers from ALS-approved grading rules. Refer to Western Lumber Siding Grades in the Tables section below.

After a general pattern type has been selected, the pattern number should be specified from the WWPA publication, Standard Patterns (refer to the Literature section to order or download).

When a saw-textured face is desired, the face to be textured and the type of texture (band sawn, rough sawn, circular sawn, etc.) should be specified.

A siding specification should also include WWPA's industry recommendations for acclimatization, backpriming, nailing and finishing.

Moisture Content of Siding Products

The premium siding grades are manufactured and shipped MC15. The Alternate Board grades (WCLIB Rules) and the Select Knotty and Quality Knotty Special Western Red Cedar grades (WCLIB Rules) are manufactured and shipped at S-DRY or S-GRN.

The moisture content level should be included in the specification along with the guidelines for acclimating the specified product to the climate of the end designation. Keep in mind that DRY for the clear grades means 15% MC, whereas DRY for knotty grades means 19% MC.

The Moisture Content Guidelines table suggests the approximate moisture content level of the air in various regions of the country. When unseasoned (S-GRN) siding products are brought into dry climates, extra precautions should be written into the specification for the acclimatization process.

For more information on moisture content in lumber, go to the Shrinkage section

Acclimatization

Wood is a natural material and will adjust its moisture content level to surrounding atmospheric conditions once it is in place. Consequently, preconditioning (acclimating) is an essential step in the installation process.

In a covered structure, lumber will stabilize at approximately 6 to 12% moisture content. Outside, lumber will stabilize at the levels shown in Moisture Content Guidelines table. Size will vary approximately 1% for each 4% change in moisture content.

A written specification should include acclimatization guidelines to ensure that material is correctly handled, before it is installed, so that it will perform satisfactorily over time.

Acclimating Interior Finish Materials and Paneling

Interior construction contains moisture-laden materials such as plaster or textured drywall which should be dry before finish lumber is delivered or installed. Masonry or concrete walls should be waterproofed prior to finish lumber installation.

Any grade may pick up moisture in transit or during storage and will need some acclimatization. It is a good practice to stack all paneling and/or finish materials in the room where wood will be installed, with separators between the layers, for a week to 10 days prior to installation. If preconditioning is not feasible, use as narrow a width as possible in order to minimize dimensional change.

"Knotty" grades of paneling generally have a higher moisture content than premium grades. When installed in a heated room, without preconditioning, paneling may be expected to shrink. This shrinkage will be minimized by acclimating the paneling prior to installation.

Once the wood product has become dimensionally stable, it should be sanded and primed or prefinished prior to installation.

Wood in kitchens, bathrooms, basements or other high moisture areas, should be protected. A suitable installation in these areas would include an application of sealer to front, back, ends and edges of each piece.

Acclimating Natural Wood Siding

Any wood siding product may pick up or lose moisture content in transit or storage so it is important to allow it to acclimate to the surrounding air of its final site prior to application.

Stack the siding, with spacers between the pieces in an area where there will be good air flow through the stack. An open garage is ideal; if completely outdoors, then loosely cover the top (only) of the stack. If stack is over wet ground or damp concrete, then lay down a vapor barrier beneath the stack.

Allow air to flow through and around the stack for a week to 10 days for DRY siding. Once siding has reached climatic balance, seal, prime or prefinish both sides, edges and ends before application.

If unseasoned S-GRN siding is used, then the following additional precautions should be taken:

• Allow materials to acclimate, as described, over a longer period – at least 30 days or longer in damp or humid conditions – before installation.
  

• Use patterns which allow for some shrinkage, such as bevel, channel or board-and-batten. These patterns have a profile which includes a gap that can more easily accommodate dimensional change.
  

• Use as narrow a width as possible. Dimensional change is proportional; the wider the width, the greater the change.

Additional Information

The following titles referenced in this section can be ordered from the WWPA Online Literature Store:

Western Lumber Grading Rules (G5)
Vol. 1 Species Book: Dimension Lumber (10)
Vol. 2 Species Book: Selects-Finish/Commons-Boards (11)

Western Lumber Product Use Manual (A)
Lumber Storage TIP Sheet (TG-5)
Natural Wood Siding Technical Guide (TG-8) 
WWPA Standard Patterns (G-16)

Western Lumber Dimension, Timber and Structural Decking Grades (struct.pdf)

Western Lumber Standard Sizes - Structural Lumber (strsize.pdf)

Western Lumber Appearance Grades (appear.pdf)

Western Lumber Standard Sizes - Appearance Lumber (appsize.pdf)

Western Lumber Standard Sizes - Patio Decking (dsize.pdf)

Western Lumber Standard Stress-Rated Boards (srbsize.pdf)

Western Lumber Siding Grades (siding.pdf)

Regional Moisture Content Guidelines, Siding/Paneling Material Specification (sidespec.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