Comparing species: reading density, shrinkage and durability tables

Four questions for every table row

Species profiles quote several sources side by side, and they rarely agree. Before comparing two numbers, ask:

1. Which species and which region? European and North American species are not interchangeable.
2. Which moisture basis? Density and strength change with moisture content (MC).
3. Which sample? Small clear specimens give higher strength than graded, knotty lumber.
4. Is the origin stated? Wood Database and CIRAD rows often do not state it.

Moisture content is water mass divided by oven-dry mass, times 100, so it can exceed 100 %. The fibre saturation point is about 30 % MC: below it, wood starts to shrink.

Density

Density is mass per volume: 1 g/cm3 = 1000 kg/m3. Always attach a moisture state.

Green density. A green piece of basic specific gravity Gb at moisture content MC weighs Gb x 1000 x (1 + MC/100) kg per m3 of green volume (the Wood Handbook definitions, as set out in the portal's density article).

Worked example 1 (green logs). Spruce with Gb 0.40 and sapwood at 120 % MC: 0.40 x 1000 x 2.2 = 880 kg/m3. Ten m3 of such logs weigh 8,800 kg, which matters for truck loading.

Shrinkage: three different bases

The profiles use three bases that must never be mixed:

• Green to oven-dry (an extreme real products never reach): spruce R 3.6 / T 7.8 %, beech R 5.8 / T 11.8 % (Germany/Europe).
• Green to 12-15 % MC: spruce R 2.0 / T 4.0 % (Teknos, Europe).
• Differential shrinkage per 1 % MC change (5-20 % MC): spruce R 0.19 / T 0.39 %; beech R 0.20 / T 0.40 %; oak R 0.16 / T 0.36 %.

Tangential shrinkage is always larger than radial. The ratio T/R is about 2 for beech (11.8 / 5.8 = 2.03); the portal's anisotropy article puts the usual range at 1.5-2.5. A high ratio means more distortion; flat-sawn boards cup toward the bark side.

Worked example 2 (green to 12-15 %). A spruce board 250 mm wide when green with tangential face loses 4.0 %: 250 x (1 - 0.04) = 240 mm. Using the oven-dry figure (7.8 %) would wrongly give 230.5 mm.

Worked example 3 (differential). A beech board 150 mm wide across the tangential direction dries from 20 % to 12 %. That is 8 percentage points, so 8 x 0.40 = 3.2 % of width, or 150 x 0.032 = 4.8 mm. A spruce board 200 mm wide over the same range loses 8 x 0.39 = 3.12 %, or 200 x 0.0312 = 6.24 mm. These are simple linear estimates (a percentage of the width at the start), not measured values; individual pieces vary.

Durability classes

EN 350:2016 describes five classes for the resistance of heartwood to fungi, from 1 (most durable) to 5 (not durable). Sapwood is treated as non-durable. Use classes (EN 335) describe exposure: class 1 fully sheltered, class 2 sheltered with occasional wetting, class 3 outdoors above ground, class 4 in ground or fresh-water contact. The profiles give:

Treatability matters too: spruce is hard to impregnate (class 3-4) and larch heartwood is difficult (class 4).

Checklist: before you quote a figure

• Species, region, source and moisture basis written next to every number.
• Clear-wood strength not used for graded lumber.
• Shrinkage allowances calculated on one basis only.
• Where sources disagree, quote a range.

Key points

• Attach species, region, MC basis and sample type to every figure.
• Green density is Gb x 1000 x (1 + MC/100); wet logs can weigh nearly twice as much as dry wood.
• Never mix shrinkage bases.
• Durability classes refer to heartwood only; class 5 means interior use.