Know your timber sizes and tolerances

TDUK’s Knowledge Sheet ‘Timber Sizes and Tolerances’, written by Damian Clarke for TDUK, sets out what you need to know about the most common sizes of structural timber.

Timber can be any size permitted by the raw material’s dimensions, but there are customary sizes that are most widely available, most often used, and suitable for the vast majority of uses, with strict tolerances set out for the use of structural timber.

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:

  • Tolerance Class 1, with a sawn edge
  • Tolerance Class 2, with a planed or machined edge.

Tolerance Classes

Timber cut from a log has a high moisture content and must be dried for most construction applications. Timber is sawn oversize but reduces in size as moisture is lost during drying to a moisture content of 20%.

Tolerance Class 1 for sawn timber reflects the natural variance in the initial drying process, with the following deviations assuming a 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.

Tolerance Class 2 timber is further planed or milled to achieve the specified size and edges at right angles, with the following deviations:

  • ±1mm for thicknesses or width of < 100mm
  • ±1.5mm for thicknesses or width of > 100mm.

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

To meet construction demands, Tolerance Class 1 timber is milled or planed down to the smaller, but more accurate, Tolerance Class 2 sizing. This is why Tolerance Class 2 timber has smaller standard dimensions and a smoother surface – often with eased edges – to aid installation. BS EN 1309-1:1997 Round and sawn timber provides guidance on measurement of dimensions.

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%.

While the UK National Annex to BS EN 336: 2012 lists the Tolerance Class 1 sizes above (table 1) as commonly available in the UK, this increasingly requires some of the sizes to order within a few days.

The more concentrated range of Tolerance Class 2 sizes, listed above, now form the mainstay of structural timber supply in the UK, and are available off the shelf at most merchants. The sizes above for Tolerance Class 1 can be processed within a few days to meet Tolerance Class 2 tolerances where required.

Increasingly, products produced to Tolerance Class 2 will have rounded corners to aid handling and installation. The manufacture of trussed rafters in the UK utilises a specific set of sizes only available through specialist suppliers. 

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, depending on the species, and are produced to order only.

For timber machined after grading, it is considered to have maintained its strength grading – where the grading stamp is reinstated – if: 

  • For timbers with dimensions < 100mm, the machining has removed no more than 5mm from that dimension.
  • For timbers with dimensions > 100mm, the machining has removed no more than 10mm from that dimension.

Structural hardwoods

Timber sizing tolerances for structural hardwoods are defined in BS EN 336: 2012 Structural Timber. Sizes, permitted. For other hardwood sizes see BS EN 1313-2, Round and sawn timber – Permitted deviations and preferred sizes – Part 2: Hardwood sawn timber.

Structural hardwoods are produced to order, meaning there is no clearly defined set of commonly available sizes.

All the sizes listed above for Tolerance Class 1 for softwoods will be available in some form of hardwood. However, individual species will only be available in a small sub-set of these sizes.

Please contact Timber Development UK specialist hardwood suppliers for more information.

In addition to the lengths listed above for softwood, hardwood specifications also include longer lengths such as 7,800mm and 8,400mm, while shorter lengths like 1,800mm are common.

Impact of moisture content

All standard timber sizes and tolerances assume a moisture content of 20%, as this is the level which dry-graded structural timber must achieve. Additional moisture will cause the  timber to swell, and less moisture will reduce its size. This means that timber cut with a high moisture content may shrink as it dries, while timber milled to size while its moisture content is lower than 20% may expand over time.

The thickness or width of a piece of timber is likely to increase by 0.25% for every 1% increase in moisture content over 20%, up to 30%.

The thickness or width of a piece of timber is likely to decrease by 0.25% for every 1% or moisture content below 20%.

Your TDUK Knowledge Library

Timber Development UK (TDUK)’s Timber Knowledge Sheets introduce and answer common questions about every aspect of working with wood, from the difference between hardwood and softwood, through to acoustic regulations, Eurocode 5 principles, embodied carbon, and sourcing timber sustainably.

Developed with experts from Edinburgh Napier University and TDUK’s unrivalled membership network of supply, manufacture and design professionals, the knowledge sheets already published are the first batch of a planned 140 to help the industry to build better with wood – with more being added all the time. 

As part of your membership, TDUK members have access  to our entire Knowledge Library, which includes Timber Knowledge Sheets, case studies, research summaries and more. 

Some Timber Knowledge Sheets are only available to TDUK members, but anyone can access and download Timber Sizes & Tolerances, Timber and Embodied Carbon, or Construction Site Timber Best Practice with a free user account.

To get started, visit the Knowledge Library