Shrinkage Allowance of Some Malaysian Timbers

The shrinkage of wood normally occurs when the moisture content (mc) dips below the fibre-saturation point (FSP), which in most cases, ranges from 25% to 30%. The magnitude of shrinkage of a piece of timber is not equal in all directions, but is significantly different across the grain in both the radial and tangential directions. While the relation of radial to tangential shrinkage varies considerably in different woods, the average tangential shrinkage for all woods is approximately twice the radial shrinkage. For the purpose of comparison, the usual method of measuring shrinkage is to determine the total contraction undergone in shrinking from the fully 'expanded' condition at saturation to the oven-dry condition. Grewal (1979) reported that the average radial shrinkage of Malaysian woods (from green to oven-dry) ranges from 1.6% in meranti bakau (Shorea uliginosa) to 7.3% in penaga (Mesua ferrea). The average tangential shrinkage ranges from 4.4% in meranti bakau (Shorea uliginosa) to 13.5% in keruing merah (Dipterocarpus verrucosus). The slight longitudinal shrinkage may be disregarded in practice, as its total amount is minimal when compared to the total length of the board, except for woods which contain a large amount of juvenile or tension wood e.g. rubberwood (Hevea brasiliensis) (Lim & Ani 1979).

This article attempts to present to the timber processors a relatively quick and practical way of estimating the shrinkage allowances at 3 common mc levels of 10%, 15% and 20% which approximately correspond to , ½  and respectively of the total shrinkage at oven-dry condition.