Timber Catalogue - NDS & Eurocode 5 Design Values

What this catalogue covers

This is a reference table of structural timber design values under two codes at once: the American NDS (National Design Specification for Wood Construction) and Eurocode 5, which takes its strength classes from EN 338 for sawn timber and EN 14080 for glued-laminated timber. It covers 34 grades across four families, and each grade has its own reference page with the full property set.

Holding both codes in one place matters because the two describe timber differently. NDS publishes allowable stresses in psi that you adjust with C-factors; Eurocode 5 publishes characteristic strengths in N/mm² that you divide by a partial factor and multiply by k_mod. A number from one system is not interchangeable with a number from the other, so the catalogue keeps them in separate columns and never converts between them.

Dimensional lumber

17 grades - Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir, Southern Pine (NDS) plus C16 to C30 (EN 338)

Glued-laminated timber

9 grades - 24F-V4, 24F-V8, 20F-V3 (NDS) plus GL24h to GL32h and GL24c/GL28c (EN 14080)

Engineered wood

4 grades - LVL 1.9E and 2.0E, PSL 2.0E, and a generic I-joist

Cross-laminated timber

4 grades - E1, E2, V1 layups plus CLT with C24 laminations (EN 16351)

Total

34 grades, each with its own reference page

Lumber and glulam values are the published tabulated figures. The engineered-wood and CLT entries are representative of the product class rather than any one manufacturer, because LVL, PSL and CLT are proprietary products whose values vary between suppliers. Use them for scheme design and take the manufacturer's own evaluation report for a final check.

Reading the NDS columns

NDS reference design values are allowable stresses in pounds per square inch, before any adjustment. The catalogue lists the six that govern most member design, plus the two stiffness moduli.

Fb

Bending, psi - the value that usually sizes a joist or rafter

Ft

Tension parallel to grain, psi

Fv

Shear parallel to grain, psi - often governs short heavily loaded spans

Fc

Compression parallel to grain, psi - stud and post design

Fc perp

Compression perpendicular to grain, psi - bearing at supports

E and Emin

Modulus of elasticity for deflection, and the reduced modulus for stability calculations

Every one of these is a reference value that must be multiplied by the applicable C-factors before use: load duration (CD), wet service (CM), temperature (Ct), size (CF), repetitive member (Cr), and for bending the beam stability factor (CL). The catalogue does not apply them, because they depend on the member and its environment rather than on the grade.

Reading the Eurocode 5 columns

Eurocode 5 works from characteristic strengths in N/mm², identified by a strength class rather than a species and grade. A C24 board is any softwood that meets the C24 profile, whichever species it was cut from - which is why EN 338 classes and NDS species grades do not map one to one.

fm,k

Characteristic bending strength, N/mm² - the number in the class name (C24 = 24 N/mm²)

ft,0,k and ft,90,k

Tension parallel and perpendicular to grain, N/mm²

fc,0,k and fc,90,k

Compression parallel and perpendicular to grain, N/mm²

fv,k

Characteristic shear strength, N/mm²

E0,mean and E0,05

Mean modulus for deflection and 5th-percentile modulus for stability, kN/mm²

rho,k and rho,mean

Characteristic and mean density, kg/m³ - drives fastener capacity

To get a design strength, divide the characteristic value by the partial factor gamma_M (1.3 for solid timber, 1.25 for glulam and LVL under the recommended values) and multiply by k_mod, which depends on service class and the shortest-duration load in the combination. Two members of the same class can therefore have quite different design strengths.

Choosing between grades

Within dimensional lumber, the ordering is usually by species group then grade: Southern Pine Select Structural is the strongest entry in the table at Fb 2550 psi, Douglas Fir-Larch No. 1 sits at 1000 psi, Douglas Fir-Larch No. 2 at 900 psi, and Hem-Fir No. 2 at 850 psi. Stiffness follows a similar order, and for floor joists stiffness rather than strength normally decides the size, so compare E before Fb when deflection controls.

For glulam, the EN 14080 classes split into homogeneous (h) and combined (c) layups. GL32h reaches fm,k 32 N/mm² with E0,mean 14.2 kN/mm², while GL24c uses lower-grade laminations in the middle of the section where bending stress is small. A combined layup is the cheaper choice when bending governs and the section is deep.

Strongest lumber in the table

Southern Pine Select Structural - Fb 2550 psi, E 1.8M psi

Most-searched lumber grade

Douglas Fir-Larch No. 2 - Fb 900 psi, E 1.6M psi

Common EN class

C24 - fm,k 24 N/mm², E0,mean 11 kN/mm²

Strongest glulam in the table

GL32h - fm,k 32 N/mm², E0,mean 14.2 kN/mm²

Common questions