Visual strength grading of timber can be done to a wide range of grading rules – many of which are national standards. However, in Europe, no matter how it is done it must fit under the requirements of EN14081-1 – because this is the harmonised/designated standard for structural timber of rectangular cross-section (if the graded timber is being placed on the market).
EN14081-1 has certain requirements for visual grading standards and they are not all totally obvious. This post highlights the main points. The requirements most easily missed are in relation to distortion, wane, fissures and dote – and in particular where there are different requirements for higher strength classes (above C18, D18, T11). Visual grading rules that were in line with EN14081-1 might become out of line if there is a new, higher, strength class assignment (perhaps a new species or growth area).
The following information is correct as of May 2023. Future versions of standards may change requirements – indeed hopefully they fix the non-ideal restrictions.
Cross-section size and tolerance on cross-section dimension
The scope of EN14081-1 says it applies to timber with cross-sectional dimensions complying with EN336. That standard imposes certain requirements on cross-section size and tolerance.
In 2013 the minimum width and thickness allowed by EN336 was reduced substantially from 22 mm to 6 mm (in order to cover lamella sizes). The upper size limit of 300 mm was also removed. Visual grading rules may apply tighter restrictions on sizes – in fact that it is almost certainly necessary to do so in order for the visual grading and strength class assignment to work correctly.
Nevertheless, the visual grading rule must refer to EN336 or have compatible requirements in relation to:
- The reference moisture content of 20% for “target size” (the size from which deviations are measured)
- The measurement of sizes according to EN1309-1 and moisture content to EN13183 (parts 1, 2 or 3).
- The change in size due to change in moisture content (unless there is evidence to the contrary), which are given as:
- 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
- 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
- Permitted deviations in cross-section dimension, handled by two tolerance classes:
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
Since EN336 is restricted to those tolerance classes, visual grading rules must refer to EN336 or echo the same requirements on cross-section tolerance. The visual grading rule could have tighter requirements, but once it is placed on the market EN336 means it can only be described with reference to tolerance class 1 and/or tolerance class 2. Certainly it is not possible to have more relaxed requirements on the cross-section dimension.
Note: EN14081-1:2005+A1:2011 mentions EN336 in relation to the definition of timber size (clause 3.12) and EN14081-1:2016+A1:2019 covers it in clause 5.6 (geometrical data).
Cross-section reduction due to planing
Clause 5.1 of EN14081-1 gives a limit on how much processing reduction is allowed on graded timber.
“If the grading has been carried out before processing, provided that the processing reduction is not greater than 5 mm for dimensions greater than or equal to 22 mm but less than or equal to 100 mm, or not greater than 10 mm for dimensions greater than 100 mm, the grade shall be considered not to have changed.”
Visual grading rules must be in line with this – both anticipating the effect of the processing reduction (and drying shrinkage) and not allowing any greater reductions (including ones that may occur in more than one step) while keeping the grade.
Note: EN14081-1:2005+A1:2011 (clause 5.1.2) does not have a different limit for the smaller size because when it was written EN336 had a minimum cross-section of 22 mm. This was fixed in the EN14081-1:2016+A1:2019 (clause 5.1.1) to avoid the possibility of timber of zero dimension (6 mm with 5 mm taken off and -1 mm from the tolerance class = 0 mm).
Marking of the timber
Clause 7 of EN14081-1 allows visually graded timber to be individually piece marked or package marked. If piece marked, the required elements (clause 7.2) are:
- Name or identifying mark of the manufacturer
- Symbol “DG ” when dry-graded (or compatible alternative symbol)
- Identification code, which identifies product from the accompanying documents
- Strength class (or other means of stating the key properties)
Visual grading rules cannot contradict the EN14081-1 marking requirements, but they can require additional information marked on the timber – such as the name of the grading rule and grade.
EN14081-1 also defines what dry-grading means – that the grading has been completed at a mean moisture content of 20% or below (with nothing more than 24%). Specifically this refers to the restrictions on things affected by drying – bow, spring, twist, fissures etc. More on that below. The rest of the grading process can be done at higher moisture contents, as long as the effect of moisture content is properly accounted for.
Note: EN14081-1:2005+A1:2011 says what the moisture content requirements for dry grading are in clause 3.4 and EN14081-1:2016+A1:2019 covers it in clause 5.1.1.
Clause A.1.1 of EN14081-1 gives requirements for rules about knots. The grading rule must specify the method of measurement and the maximum dimensions of knots (and knot holes) must be given in one (or more) of the following ways:
- In relation to the width and/or thickness of timber on the basis of linear values
- In relation to the cross-sectional area of timber on the basis of cross-sectional values
- In relation to absolute values for a given range of timber sizes
The clause requires that knot groupings must also be taken into account – and of course visual grading rules may also impose other kinds of restrictions on knots.
Slope of grain
Clause A.1.2 of EN14081-1 requires that the visual grading rule has a definition of slope of grain in accordance with EN844 and refer to EN1310 for its method of measurement. All grades must have a limitation on slope of grain.
Density and ring width
Clause A.1.2 of EN14081-1 requires that grading rules for softwoods and temperate hardwoods contain a requirement for either
density or ring width.
If density is specified, it shall be linked to a stated moisture content (preferably 20%).
If ring width is specified, the standard shall include limits for ring width and its method of measurement.
A density limit might arguably be applied by a requirement such as “reject pieces that are of unusually low density” – especially if the density is not a critical property in the strength class assignment (much higher than the strength class requires).
As the range of species being graded expands, it may be necessary to adapt the grading rules to give more specific density limits of certain species and species groups, and deal with temperate hardwoods with barely visible growth rings. This is one reason why people involved in new visual grading assignments should be in communication with the people responsible for maintaining the visual grading rule. For national standards, and assignments in EN1912, the mirror committees of national standard bodies can get some information on what is being done via the minutes and other documents coming from CEN TC124 WG2 TG1 via the WG2 document database.
Clause A.1.4 of EN14081-1 requires that the visual grading rule limits fissures when they “have a significant effect on strength, e.g. shear strength of a beam”. This is usually the case for general strength graded timber placed on the market, but for certain special uses the size of fissures can be disregarded.
The fissures must be measured in accordance with EN1310 and there are different limits depending on the strength class – with more stringent requirements for strength class assignments above C18, D18 and T11 (non-EN338 strength classes are checked according to the corresponding strength value limits). Note that there are also stricter limits for the higher strength classes for distortion and dote (see below).
Fissures with depth less than half the thickness of the piece may be ignored, otherwise fissures that do not go completely through the thickness are limited to:
- For C18, D18, T11 and below: not greater than 1.5 m or 1/2 the length of the piece, whichever is the lesser
- Above C18, D18, T11: not greater than 1 m or 1/4 the length of the piece, whichever is the lesser
Fissures that do go through the thickness are limited to:
- For C18, D18, T11 and below: not greater than 1 m or 1/4 the length of the piece, whichever is the lesser. If at the ends, length not greater than twice the width of the piece
- Above C18, D18, T11: only permitted at the ends with a length not greater than the width of the piece
The permitted limits for both the depth and length of fissures refer to the cumulative sum of fissures in one plane in a piece
The fissures are, of course, dependent on moisture content, and cutting timber to length (which does not change the strength class) and can therefore affect how a piece compares to these rules – but these rules only apply at the time of grading.
A visual grading rule can have stricter limits, but it must not be more relaxed than these requirements.
Clause A.2.1 of EN14081-1 requires that wane limitations are given with reference to the width, thickness and length of the piece of timber, and that the method of measurement is described. Maximum wane permitted shall not reduce the edge and face dimensions to less than 2/3 of the basic dimensions of the piece. EN14081-1 does not say anything more about the length of wane, so it is reasonable to assume a limit of “unlimited” would satisfy the requirement regarding length along the timber.
Clause A.2.2 of EN14081-1 requires that limits for bow, spring and twist are imposed by the visual grading rule, and that these must be measured in accordance with EN1310. For dry-graded timber, there are maximum limits, which like the rules for fissures and dote are different for strength classes above C18, D18 and T11.
For dry-graded timber the maximum bow and spring over 2 m of length is limited to:
- For C18, D18, T11 and below: bow 20 mm, spring 12 mm
- Above C18, D18, T11: bow 10 mm, spring 8 mm
In both cases the twist must be no more than 2 mm per 25 mm width over a 2 m length (EN14081-1:2016+A1:2019)
EN14081-1 does not impose a requirement for cup, but a visual grading rule may do so.
Note: EN14081-1:2005+A1:2011 has a maximum twist limit of no more than 1 mm per 25 mm width over a 2 m length for strength classes above C18, D18, T11, but this was relaxed in the EN14081-1:2016+A1:2019 as it is hard to achieve.
Fungal and insect damage – with a key requirement for dote
Clause A.3 of EN14081-1 requires that visual grading rules include limits on fungal and insect damage, and prohibit live insect attack. Soft rot shall not be allowed in any grade, and dote shall only be permitted in grades to a level of EN338 strength class C18, D18, T11 and below. (For non-EN338 strength classes, dote is checked with corresponding strength value limits.) Grading rules must conform to EN844 for definitions of terms relating to stain and fungal attack.
- For C18, D18, T11 and below: dote can be allowed
- Above C18, D18, T11: dote cannot be allowed
Clause A.4.1 of EN14081-1 requires that grading rules should take into account reaction wood (compression wood for softwoods and tension wood for hardwoods). The requirements for “taking into account” are not specified – so it would not be OK for a visual grading rule to be silent on the topic of reaction wood, but it might be OK to say it is permitted without limit if deemed not to be a problem for the resource.
Clause A.4.2 of EN14081-1 requires that other grade characteristics and strength affecting criteria (e.g. mechanical damage, included bark, damage to the living tree) shall be restricted in line with the already mentioned strength reducing characteristics. This is where visual grading rules can have an advantage as they are often tailored to certain well-used species and the defects that are common for them. However, now that grading is being applied to a wider range of species it should be confirmed that the requirements of a visual grading rule adequately cover those species too.
Which version of EN14081-1 applies – the “current” one or the one people have to follow?
Aside from reprocessing limit and twist (as noted above) the current EN14081-1:2016+A1:2019 and the actually harmonised/designated EN14081-1:2005+A1:2011 (but withdrawn) version have the same requirements for visual grading rules, but they are explained better in the EN14081-1:2016+A1:2019 version. The clause references are the same for both.
Which grading standards have assignments above C18, D18 and T11?
For reference, the visual grades and standards with have assignments “above C18, D18, T11” (according to EN1912 and assignments approved for EN1912) are listed below. These grades should therefore respect the stricter limits on fissures, distortion and dote.
There are assignments that exist in other places, such as the UK’s PD6693-1, and that should also be taken into account because EN1912 is not everything.
BS 4978:2007+A2:2017 SS
BS 5756:2007+A2:2017 TH1 (And adding more from PD6693-1 TH2, THA, THB)
ČSN 73 2824-1:2015 S10
ČSN 73 2824-1:2015 S10K
ČSN 73 2824-1:2015 S13
ČSN 73 2824-1:2015 S13K
DIN 4074-1:2012 S10
DIN 4074-1:2012 S10 & better
DIN 4074-1:2012 S10 & better & density
DIN 4074-1:2012 S10K
DIN 4074-1:2012 S13
DIN 4074-1:2012 S13K
DIN 4074-5:2008 LS10 & better
DIN 4074-5:2008 LS10K & better
DIN 4074-5:2008 LS13
DIN 4074-5:2008 LS13K
EN 16737:2016 STH
INSTA 142:2009 T2
INSTA 142:2009 T2 & better
INSTA 142:2009 T3
IS 127:2015 SS
NEN 5493:2010+C1:2011 C3 STH
NEN 5499:2007/A1:2011 T2
NEN 5499:2007/A1:2011 T3
NF B 52-001-1:2018 1
NF B 52-001-1:2018 2
NF B 52-001-1:2018 H1 (in H1/H3 grading)
NF B 52-001-1:2018 H2 (in H2/H4 grading)
NF B 52-001-1:2018 H3 (in H1/H3 grading)
NF B 52-001-1:2018 HS ST-I
NF B 52-001-1:2018 ST-I
NF B 52-001-1:2018 ST-II
NLGA:2017 J&P Sel
NLGA:2017 No.1 & better
NLGA:2017 SLF Sel
ÖNORM DIN 4074-1:2012 S10
ÖNORM DIN 4074-1:2012 S10 & better
ÖNORM DIN 4074-1:2012 S10K
ÖNORM DIN 4074-1:2012 S13
ÖNORM DIN 4074-1:2012 S13K
ÖNORM DIN 4074-5:2009 LS10 & better
ÖNORM DIN 4074-5:2009 LS7
SIA 265/2:2023 S10
SIA 265/2:2023 S10 & better
SIA 265/2:2023 S10K
SIA 265/2:2023 S13
SIA 265/2:2023 S13K
SIST DIN 4074-1:2009 S10
STN 49 1531:2001 S0
STN 49 1531:2001 SI
TS 1265:2012 Class 1
TS 1265:2012 Class 2
TS 1265:2012 Class 3
UNE 56544:2022 ME1
UNE 56544:2022 MEG
UNE 56546:2022 MEF
UNE 56546:2022 MEF-G
UNI 11035-1:2022 S1
UNI 11035-1:2022 S2
UNI 11035-1:2022 S2 & better
UNI 11035-2:2022 LS
UNI 11035-2:2022 LS1
UNI 11035-2:2022 LS2
UNI 11035-2:2022 LS2 and better