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Do you really know your timber size and it’s tolerance?

 Tuesday, April 23, 2024

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Timber size and tolerance

In a recent article related to the woodworking industry: ‘Timber Sizes and Tolerances’ by Damian Clarke specially written for Timber Development UK, enlightens on identifying most common sizes and the tolerance of structural timber. We have extracted some of the important facts related to this. Excerpt:

How do you know the timber sizes and tolerances?

Timber sizes are determined by the dimensions of the raw material, although there are standard sizes that are most commonly available, frequently used, and suitable for a wide range of applications, with precise tolerances established for structural timber. This has been considered one of the major factors in the woodworking industry as well.

According to the Damian Clarke, These timber sizing tolerances are defined in BS EN 336: 2012 Structural Timber. Sizes, permitted deviations. Whatever its size, structural timber is available in two dimensional tolerances:

About Tolerance Classes of a timber

Timber harvested from logs contains a high moisture content and needs to be dried before it can be used in most construction projects. Although initially sawn oversized, timber shrinks as it loses moisture during the drying process, reaching a moisture content of 20%.

The fact is that, the tolerance class 1 for sawn timber generally reflects the natural variance in the initial drying process. If you follow the below deviations, you will easily get to know that the deviations can assume 20% moisture content:

-1mm or +3mm for thicknesses or widths of < 100mm -2mm or +4mm for thicknesses or widths of > 100mm

Regardless of deviations, the average dimensions of square-edged timber may not be less than the target size.

Standard target sizes for softwood structural timber

Softwood is sawn to standard target sizes for use in construction. These sizes and tolerances assume a total moisture content of 20%.

Common strength classes for structural timber

The most common strength classes for softwoods, including CLS softwoods, are C16 and C24. TR26 is a strength grading specific to trussed rafters. Hardwoods range from strength class D18 to D70, and these are completely depend on the species.

Impact of moisture content

Standard timber sizes and tolerances are based on a moisture content of 20%, which is the required level for dry-graded structural timber. If the moisture content exceeds this level, the timber will swell, whereas if it falls below, the timber will shrink. Consequently, timber initially cut with high moisture may reduce in size as it dries, and timber milled when its moisture content is under 20% might expand over time.

For every 1% increase in moisture content above 20%, up to 30%, the thickness or width of a timber piece is likely to expand by 0.25%. Conversely, for every 1% decrease in moisture content below 20%, the thickness or width is likely to contract by 0.25%.

PS: This is a research based article and credit goes to TDUK Knowledge Library.

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