Moisture Movement

Wood is an anisotropic material, with differing moisture movements along the three principal axes: tangential, radial and longitudinal (Fig. 4.10). The highest moisture movement is tangential to the grain, next being radial, with the least along the grain. Typical figures are given in Table 4.4. The larger the ratio between tangential and radial movement, the greater the distortion. Moisture movements are conventionally

Shrinkage

Fig. 4.10 Moisture movement and initial drying shrinkage

Table 4.4 Moisture movement of some hardwoods and softwoods normally available in the UK

Hardwoods

Table 4.4 Moisture movement of some hardwoods and softwoods normally available in the UK

Hardwoods

Small moisture

Medium moisture

Large moisture

movement

movement

movement

(less than 3.0%)

(3.0-4.5%)

(over 4.5%)

Afzelia

Ash

Beech, European

Agba

Cherry

Birch

Iroko

Elm, European

Keruing

Jelutong

Maple

Ramin

Lauan

Oak, American

Mahogany, African

Oak, European

Mahogany, American

Sapele

Meranti

Utile

Merbau

Walnut, European

Obeche

Teak

Softwoods

Small moisture

Medium moisture

Large moisture

movement

movement

movement

(less than 3.0%)

(3.0-4.5%)

(over 4.5%)

Douglas fir

European redwood

Sitka spruce

European whitewood

Western hemlock

Parana pine

Western red cedar

Moisture movement is assessed on the sum of the radial and tangential movements for a change in environmental conditions from 60 to 90% relative humidity.

Moisture movement is assessed on the sum of the radial and tangential movements for a change in environmental conditions from 60 to 90% relative humidity.

quoted for a change in relative humidity from 90% to 60% at 25°C. The BRE classifies woods into three categories according to the sum of radial and tangential movement effected by this standard change in relative humidity. Small movement is defined as less than 3%, medium between 3% and 4.5%, and large over 4.5%. Large movement timbers are not recommended for use as cladding.

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