Density and strength: what the number tells you

One number, many meanings

Density is mass divided by volume. Because wood takes up and gives off water, a density figure means nothing without its moisture state. The Wood Handbook (North America) uses these bases:

• Basic specific gravity (Gb): oven-dry mass over green volume. The steadiest reference.
• Specific gravity at 12 % (G12): oven-dry mass over volume at 12 % MC.
• Density proper uses the actual mass at the stated MC.

Worked examples (arithmetic from the definitions). White ash with G12 about 0.605 has a density at 12 % of 0.605 x 1000 x 1.12 = 678 kg/m3 (the Handbook's own example). A spruce with Gb 0.40 and sapwood at 120 % MC weighs 0.40 x 1000 x 2.2 = 880 kg per m3 of green volume, the number that matters for truck loading.

Typical values in Europe

Czech studies from Mendel University in Brno measured density at 12 % MC (Gryc and others): Norway spruce 410 kg/m3 near the pith and 516 in outer mature wood; Scots pine 391 and 552; European larch 573 and 652; European beech about 752; sessile oak 717 and English oak 669.

Where density varies

• Inside the ring. In 85 Norway spruce from Finland (Jyske and others, 2008), 49 to 80 % of density variation lay within individual rings, 11 to 27 % between rings and only 3 to 6 % between heights in the stem.
• Pith to bark. In the Czech conifers density rises outward by about 106 kg/m3 in spruce, 161 in pine and 79 in larch (own subtraction).
• Between boards. In one Polish stand, 27 Scots pine butt boards ranged from 469 to 686 kg/m3, a ratio of about 1.46.

Density and strength

Across species, higher density usually means higher strength. In the Handbook's clear-wood table at 12 % MC (North America): Sitka spruce (G12 0.40) has bending strength (MOR) about 70 MPa and stiffness (MOE) 10.8 GPa; coast Douglas-fir (0.48) 85 MPa and 13.4 GPa; white oak (0.68) 105 MPa and 12.3 GPa. From spruce to oak density rises 70 % and MOR 50 %, but stiffness only about 14 %, and Douglas-fir is stiffer than oak at lower density. Rank species by density for strength and hardness, not for stiffness.

Within one species the scatter is larger. The Polish pine boards had stiffness from 7.25 to 16.15 GPa (EN 408 bending), a factor of about 2.2 for a density ratio of 1.46. Knots and grain matter too, which is why machine grading combines density with other measurements.

Moisture and size

Below the fibre saturation point (about 30 % MC) clear wood gets stronger and stiffer as it dries. A VTT note (Finland, EN 384 practice) gives roughly 1.5 to 2 % of MOE per percentage point between 8 and 20 % MC (the source's table labels are garbled); 8 points is 12 to 16 % more stiffness (own arithmetic). In sawn timber bending and tension strengths are nearly unaffected, because knots and grain deviations control failure. Tabulated clear-wood MOR is an upper bound, not a grading value: design values are 5th percentiles of structural-size tests (Canadian lecture, North America).

Growth rate and density

Fast growth tends to lower density in spruce through a higher earlywood share. In Swedish fertilised trials (Lundgren, 2004), more than doubled ring width went with about 20 % lower density. In a German case study (Höwler and others, 2022; 100 Norway spruce in Hesse and Lower Saxony), ring width correlated with density at rho = minus 0.754. The plot with 1.78 mm rings had 511 kg/m3, bending strength 51.8 N/mm2 and median MOE about 15,900 N/mm2; the plot with 3.74 mm rings had 415 kg/m3 (19 % lower), 22.0 N/mm2 and about 7,830 N/mm2 (half). The authors stress that age, site and thinning were confounded, and ring width alone is a poor quality indicator.

What it means at the saw and in the kiln

• At the saw. The portal's density article gives only experience-based statements: light species need less feed power and wear tools less.
• In the kiln. Water to remove per m3 of green wood is Gb x 1000 x MC/100. Dense hardwoods are said to dry more slowly and check more (general experience, not a figure), so set schedules by species and thickness.

Key points

• Always attach a moisture state to a density figure; Gb is the most repeatable.
• European conifers run about 390 to 650 kg/m3 at 12 % MC from pith to outer wood; beech and oak about 670 to 750.
• Most density variation in spruce is inside the ring.
• Density ranks strength across species but predicts stiffness poorly; within a species, knots and grain matter as much.
• Faster growth lowers spruce density, but site, age and thinning are confounded.