EN 336 and UK customary sizes

Timber in the lab

The UK National Annex to EN336 is to be reinstated.  Is the list of commonly available sizes (see below) still current?  Send any comments to d.ridleyellis@napier.ac.uk before 15th April.

In 2013, the European Standard EN336 was updated.  There were changes that modified the upper and lower limits of width and thickness for sawn, square edged structural timber.

  • The upper size limit of 300 mm was removed, allowing larger cross-sections (and permitted deviations were added for the larger sizes)
  • The lower size limit of 22 mm was changed to 6 mm, allowing smaller cross-sections (small thicknesses intended for laminated products)

This is the Standard that EN14081-1 refers to in the scope, so now strength grading of dimensions down to 6 mm is permitted (but not necessarily possible in practice).  The permitted reduction in dimension after grading (without requiring re-grading) was changed in EN14081-1:2016, so that  further reduction is not allowed for these newly permitted small sizes:

  • Not greater than 5 mm for dimensions greater than or equal to 22 mm and up to 100 mm
  • Not greater than 10 mm for dimensions greater than 100 mm

…otherwise a 6 mm thickness, could be planed down to 1 mm thickness and retain the grade – which would obviously not be OK…especially when considering that the 1 mm remaining thickness could be reduced to zero thanks to the tolerance class, as specified in EN336.

As before, there are two tolerance classes in EN336.  They specify permitted deviation from the target size.

The “target size” is the size from which deviations (which are ideally zero) are to be related.  The reference moisture content is 20%.
A deviation is the difference between the actual size and the corresponding target size, after allowing for actual moisture content.

The changes in size, due to moisture content, also changed in the updating of EN336.  There are now different amounts for hardwoods and softwoods.  Poplar is grouped with the softwoods and there are probably other hardwoods that are more similar to softwoods in this regard.  This possibility is caught by the text “unless there is evidence to the contrary”.  The dimensional changes due to moisture content are:

  • Softwoods and poplar:
    • Increase in thickness and width of 0.25% for every 1.0% moisture content above 20%, up to 30% moisture content
    • Decrease in thickness and width of 0.25% for every 1.0% moisture content below 20% moisture content
  • Hardwoods:
    • Increase in thickness and width of 0.35% for every 1.0% moisture content above 20%, up to 30% moisture content
    • Decrease in thickness and width of 0.35% for every 1.0% moisture content below 20% moisture content

(Width and thickness change with moisture content because of radial and tangential shrinkage and swelling in the wood.  For more detailed information on this topic, see the USDA Wood Handbook chapter 4)

There are two tolerance classes, specified in EN336.  These are called, simply, Tolerance class 1 and Tolerance class 2.  Perhaps unintuitively, it is tolerance class 2 that is the one closest to the target size.  The under limit (-) and over limit (+) are after adjusting for moisture content (target being specified at the 20% reference).

Cross-section dimension    |  Tolerance class 1  |  Tolerance class 2
up to 100 mm               |  -1.0 mm & +3.0 mm  |  -1.0 mm & +1.0 mm
above 100 mm up to 300 mm  |  -2.0 mm & +4.0 mm  |  -1.5 mm & +1.5 mm
above 300 mm               |  -3.0 mm & +5.0 mm  |  -2.0 mm & +2.0 mm

The average thickness and width should (for both tolerance classes) not be less than the target size (adjusting for moisture content).  (The dimensions are measured according to EN1309-1.  This involves at least three measurements, and taking the smallest of those measurements, for each of width and thickness)

Tolerance classes apply to the the width and thickness.  For length, the requirement is simply that the actual length should be no shorter than the target length.  (If overlength timber is a problem, an upper limit can be specified as part of the purchase contract).

The term “target size” is preferred over customary terms like “basic size”, “nominal size” and “regularized size” which have very loose meanings.   The target size is the one used in the design calculations, and therefore the size required from the supplier (ie listed in the contractual documents).

This was all previously explained in the UK National Annex to EN336 (see the 2003 version), but this accidentally disappeared when EN336:2013 was published.  This National Annex will be reinstated.  It’s main helpful function was to list commonly available sizes in the UK.

The commonly available lengths (in metres) were listed as:

2.40
3.00   3.30   3.60   3.90
4.20   4.50   4.80
5.10   5.40

Sizes come in three types, depending on which (if any) faces are machined to achieve tolerance the higher tolerance class (TC2):

            | sawn | machined on width | machined on width and thickness
Thickness   | TC1  |        TC1        |        TC2
Width       | TC1  |        TC2        |        TC2

If a face is machined to tolerance class 2, the target size is typically 3 or 5 mm smaller than it would be if it were tolerance class 1 (the difference being removed during machining).  Dimensions up to 100 mm typically have 3 mm difference, and dimensions over 100 mm typically have 5 mm difference.

The customary sizes were listed as follows:

Width (mm) [TC1]
Thickness (mm) [TC1] 75 100 125 150 175 200 225 250 300
22 Y
38 Y Y Y Y Y
47 Y Y Y Y Y Y Y Y Y
63 Y Y Y Y
75 Y Y Y Y Y Y Y
100 Y Y Y Y Y Y
150 Y Y
300 Y

 

Width (mm) [TC2]
Thickness (mm) [TC1] 72 97 120 145 170 195 220 245 295
22 Y
38 Y Y Y Y Y
47 Y Y Y Y Y Y Y Y Y
63 Y Y Y Y
75 Y Y Y Y Y Y Y
100 Y Y Y Y Y Y
150 Y Y
300

 

Width (mm) [TC2]
Thickness (mm) [TC2] 72 97 120 145 170 195 220 245 295
35 Y Y Y Y Y
44 Y Y Y Y Y Y Y Y
60 Y Y Y Y
72 Y Y Y Y Y Y Y
97 Y Y Y Y Y Y
145 Y Y
300

 

In addition, there are the CLS/ALS sizes (Canadian and American lumber sizes).  These have cross-sections with rounded corners (not exceeding 3 mm radius).

 CLS/ALS Width (mm) [TC2]
Thickness (mm) [TC2] 63 89 114 140 184 235 285
38 Y Y Y Y Y Y Y

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