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With grades and design values assigned per species combination, adjustment factors can be applied to calculate the performance of different sized members in end use. U.S. design values and adjustment factors for western species are provided in the
RIS, WCLIB and WWPA grade rule books and are published by various governmental agencies in destination countries.
Structural timber products are generally bought, sold and specified for end use according to the species combinations shown
in Table 1. Because of unique durability characteristics, California redwood and the western cedars are marketed as distinct species.
Structural Grades
General classifications for structural timber in western species are explained below. Nominal sizes are used as “names.” Metric references throughout this text are based on a soft conversion
of actual, surfaced sizes (with 1" equaling 25.4 mm).
Dimension Lumber grades, based on the
National Grading Rule, are divided into three categories for size and four for strength.
Size: The first size category includes products from
38 x 38 mm through 89 x 89 mm. (In the U.S., these sizes are referred to as 2x2 through 4x4, which reflect the nominal, unseasoned, and unsurfaced sizes of 2" x 2" and 4" x 4".) Sizes are available in
Structural Light Framing, Light Framing and Stud strength categories.
Strength: Structural Light Framing (SLF) grades are intended to fit engineering applications where the highest design values are needed in light framing sizes from 38 to 89 mm thick x 38 to
89 mm wide.
SLF grades include:
SELECT
STRUCTURAL
No.
1
No.
2
No. 3
Strength: Light Framing (LF) grades in these sizes (from 38 to 89 mm thick x from 38 to 89 mm wide) are intended for framing uses where highest strength values are not required such as for wall framing, plates, sills, cripples, blocking etc.
LF grades include:
CONSTRUCTION
STANDARD
UTILITY
Size: The second size category includes products from
38 x 38 mm through 89 x 337 mm (U.S. nominal sizes 2x2 through 4x14). Products in this size range may
be graded as Stud grade.
Strength: Stud grade is an optional all-purpose grade (in
sizes 38 to 89 mm thick x 38 to 337 mm wide) for vertical installations in load-bearing
applications, such as for wall framing.
There is only one grade:
STUD
Size: The third size category includes products from
38 x 114 mm through 89 x 438 mm (nominal 2x5 through 4x18). These sizes are categorized as
Structural Joists & Planks.
Strength: Structural Joists & Planks (SJ&P) are intended to fit engineering applications for timber 114 mm and wider, such as floor joists, rafters, headers, small beams, trusses and general framing.
SJ&P grades include:
SELECT
STRUCTURAL
No.
1
No.
2
No. 3
Additional Special Dimension structural products include
Structural Glued and Machine Stress-Rated (MSR) timber.
ALSC-certified grades for redwood with assigned design values include
DECK HEART and DECK
COMMON. These are classified as
Special Purpose structural grades, specifically intended for residential deck construction and available in 38 x 89 mm and 38 x 140 mm only. RIS can provide redwood products graded to NGR specifications upon request. Redwood structural and appearance grades are shown on
California Redwood.
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