Table 43 Permitted deviations on structural timber sizes to BS EN 336 2003

Maximum deviations Tolerance Class T1 Tolerance Class T2 from target sizes

Thicknesses and — 1 to + 3 mm — 1 to + 1 mm widths < 100 mm

Thicknesses and — 2 to + 4 mm — 1.5 to + 1.5 mm widths >100 mm centre of the timber outwards due to the creation of a vapour-pressure gradient. The art of successful seasoning is to control the moisture loss to an appropriate rate. If the moisture loss is too rapid then the outer layers shrink while the centre is still wet and the surface sets in a distended state (case hardening) or opens up in a series of cracks or checks. In extreme cases as the centre subsequently dries out and shrinks it may honeycomb.

changes in dimension are called movement. The controlled loss of moisture from green timber to the appropriate moisture content for use is called seasoning.

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