ISO 9709:2005

INTERNATIONAL ISO
STANDARD 9709
First edition
2005-07-15

Structural timber — Visual strength
grading — Basic principles
Bois de structure — Classification visuelle selon la résistance —
Principes de base




Reference number
ISO 9709:2005(E)
©
ISO 2005

---------------------- Page: 1 ----------------------
ISO 9709:2005(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.


©  ISO 2005
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland

ii © ISO 2005 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 9709:2005(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms . 1
5 General. 1
5.1 Visual strength-graded timber. 1
5.2 Visual strength-grading operations . 1
5.3 Visual strength-grading principles of quality control. 2
6 Resource and sawn timber input requirements . 2
6.1 General. 2
6.2 Input requirements . 2
6.3 Control of inputs. 3
6.4 Reprocessing of previously graded material. 3
7 Visual strength-grading requirements. 3
7.1 Grader requirements . 3
7.2 Grading process . 3
7.3 Check on visual grading process . 4
8 Visual graded timber structural properties. 4
8.1 General. 4
8.2 Initial evaluation. 4
8.3 Periodic evaluation. 5
9 Product identification. 5
10 Documentation. 5
Annex A (informative) Example of a visual strength-grading timber standard — based on the need
for design values where a high degree of certainty of structural properties is required . 6
Annex B (informative) Example of a visual strength-grading timber standard — based on the need
for design values where a high degree of certainty of structural properties is not required. 19
Bibliography . 25

© ISO 2005 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 9709:2005(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
ISO 9709 was prepared by Technical Committee ISO/TC 165, Timber structures.
iv © ISO 2005 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 9709:2005(E)
Introduction
The general principle of this International Standard is that any type of visual strength-grading procedure is
acceptable, provided it is defined, controlled, and documented to the extent required to reflect the degree of
certainty of structural properties intended for the structural application of the product. The body of this
International Standard specifies the essential features common to all visual strength-grading operations. The
requirements are minimal so as to ensure maximum scope and flexibility in the application of a standard
applied to the visual strength-grading process as applied to timber. The annexes provide a detailed example
of a conformance standard resulting in strength properties having a high degree of engineering reliability and
a simple to apply standard resulting in strength properties where a high degree of engineering reliability is not
required.
This International Standard was based initially on the European Standard EN 518, Structural timber —
Grading — Requirements for visual strength-grading standards and modified to bring it into conformance with
ISO procedures and requirements.
The bibliography lists a number of additional standards referenced during the development of this International
Standard.

© ISO 2005 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 9709:2005(E)

Structural timber — Visual strength grading — Basic principles
1 Scope
This International Standard establishes the basic principles for rules and procedures governing the visual
sorting of timber for use in structural applications.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 13910, Structural timber — Characteristic values of strength-graded timber — Sampling, full-size testing
and evaluation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13910, and in Annexes A and B
apply. The terms and definitions given in Annexes A and B are representative of those in rules and
procedures governing the visual sorting of timber for use in structural applications.
4 Symbols and abbreviated terms
For the purposes of this document, the symbols and abbreviated terms given in ISO 13910 and in Annex A
apply. The symbols and abbreviated terms given in Annex A are representative of those in rules and
procedures governing the visual sorting of timber for use in structural applications.
5 General
5.1 Visual strength-graded timber
Visual strength-graded timber is sawn wood that has been sorted into structural or non-structural grades
according to visual criteria. The visual criteria identify physical features that may affect timber strength.
5.2 Visual strength-grading operations
A typical visual strength-grading operation shall be comprised of a visual grader who sorts an input resource
into one or more output grades (see Figure 1). Some of the timber may not meet the requirements of the
minimum specified grade.
In addition to the structural requirements, nonstructural or utility requirements may also be specified.
© ISO 2005 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 9709:2005(E)

Figure 1 — Schematic of visual strength-grading operation
5.3 Visual strength-grading principles of quality control
Visual grading is one element of quality control operations. This International Standard requires that the
quality control related to the visual grading operation is undertaken by placing checks on the three
components of the grading operation: 1) the resource and sawn timber inputs; 2) the visual sorting process;
and 3) the graded timber output (see Figure 1).
NOTE In theory it is possible to control quality either
a) by control on the resource input and the visual sorting operation, or
b) by checks of the visual sorting operation and of the quality of the output grades.
However, in practice it is not feasible to rely solely on the checks on the output grades because of the high variability and
complexity of timber, and because of the large sample sizes that may be required to reliably measure the 5-percentile
strength values.
6 Resource and sawn timber input requirements
6.1 General
The input resources shall be identified in terms of all parameters that may affect the output of the visual grade
sorting operation.
6.2 Input requirements
6.2.1 Resource
The parameter that shall be identified is the timber species or mixture of species.
Other parameters that may be identified are
a) silvicultural practices used,
b) log source,
c) log size,
d) cutting pattern used to manufacture sawn timber from logs, and
e) any other parameters deemed to be important.
6.2.2 Sawn timber
Parameters that shall be specified are
a) condition (such as seasoned, unseasoned, etc.),
2 © ISO 2005 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 9709:2005(E)
b) moisture content and moisture content range, and
c) any other parameters deemed to be important.
6.3 Control of inputs
A periodic check on the resource and sawn timber inputs should be defined and specified.
6.4 Reprocessing of previously graded material
If major reprocessing of previously graded material is permitted, then any requirements for re-grading of the
material should be specified.
7 Visual strength-grading requirements
7.1 Grader requirements
The grader shall be qualified to grade timber accurately at the necessary operational speeds and to evaluate
the visual quality of all grades and sizes that the grader will encounter in commercial visual grading operations.
7.2 Grading process
7.2.1 General
The grading process shall be specified. During grading, methods shall be in place to ensure that the timber
species and the timber moisture content comply with the requirements specified.
The detail required in the standard is directly related to the reliability of the stated/claimed structural properties.
 Annex A, an informative annex, provides a detailed example of a conformance standard resulting in
strength properties having a high degree of engineering reliability.
 Annex B, an informative annex, provides an example of a conformance standard resulting in strength
properties where a high degree of engineering reliability is not required.
7.2.2 Rules to satisfy the structural requirements
To ensure adequate structural properties, limitations shall be specified on one or more of the following
features:
a) knots (type, size and location);
b) slope of grain;
c) rate of growth;
d) fissures (shake, checks, and/or splits);
e) moisture condition; and
f) any other features that are deemed to be important.
© ISO 2005 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 9709:2005(E)
7.2.3 Rules to satisfy the utility requirements
To ensure adequate visual quality, limitations may be specified on one or more of the following features:
a) crookedness;
b) dimensions and tolerances;
c) fungal decay;
d) insect damage;
e) sapstain;
f) squareness;
g) white speck; and
h) any other features that are deemed to be important.
7.3 Check on visual grading process
A periodic check shall be required to assess the accuracy of the grading process. If a check indicates that the
process is inadequate, then appropriate measures may be specified to modify the process so that the process
is adequate.
8 Visual graded timber structural properties
8.1 General
The critical properties of strength-graded timber are structural properties. These structural properties may be
incorporated in the visual strength-grading standard or may be in other appropriate standards referencing the
grades determined using the criteria of the visual strength-grading standard. These properties shall be as
defined and measured as specified in the test methods in ISO 13910.
The structural design properties shall be determined from tests on timber having a defined moisture content, if
the tests are conducted on timber having a moisture content that differs from that specified by the procedure
conforming to this International Standard, the properties resulting from the tests shall be adjusted (using
sound engineering principles) so that the structural design properties reflect the intent of these basic
requirements and/or the applicable associated design codes.
8.2 Initial evaluation
Once the grading operation has been selected, evidence shall be provided that the resultant output grades
have the structural properties stated for the material. This evidence may be linked to other mills carrying out
equivalent sorting procedures.
For cases where such evidence is not available or it is not appropriate to link the evidence to other mills, an
initial test program should be specified. The requirements for this test program should be based on sound
sampling principles and the tests for the structural properties shall be based on the test procedures specified
in 8.1.
For cases where a high degree of certainty of the structural properties is not required, then the structural
properties of the material may be based on other mechanical or physical properties representative of the
grade being evaluated providing these properties have been defined and related to the test procedures as
specified in 8.1 and continue to be used as the basis of the sorting process.
4 © ISO 2005 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 9709:2005(E)
8.3 Periodic evaluation
Direct measurement of the structural properties of full-size timber shall be undertaken if there is a reason to
expect that the structural properties of visually graded lumber have changed and may also be specified to be
undertaken at periodic specified intervals.
9 Product identification
A product identification mark on the timber shall be specified to indicate the document conforming to this
International Standard on which the sorting is based, the grade and/or strength class, and the producer
responsible. The product identification mark may also include other information deemed important.
Each piece of timber shall be marked except for high quality strength-graded timber intended for structural as
well as appearance purposes. For this high quality appearance timber, each shipment shall be accompanied
by documentation containing the product identification requirements specified in the standard.
10 Documentation
Documentation requirements shall include:
a) the standard on which the visual strength-grading process is based;
b) specifications of the resource input;
c) specifications of the visual grade sorting process;
d) specifications for the timber grade criteria;
e) methods for assigning and confirming a grade; and
f) specifications for the identification of the product.
Documentation requirements may include:
g) specifications and control checks of the resource input;
h) specifications and control checks of the visual grade sorting process;
i) specifications and control checks of the structural properties;
j) materials identifying the allocation of responsibilities for quality control operations; and
k) other specifications or materials deemed to be important.
© ISO 2005 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO 9709:2005(E)
Annex A
(informative)

Example of a visual strength-grading timber standard — based on the
need for design values where a high degree of certainty of structural
properties is required
This is one of a number of possible systems provided as an example only - not a system proposed for
universal usage.This example is a practical implementation of the visual strength-grading principles defined in
the main body of ISO 9709, as applied to the following case:
 rectangular timber;
 for structural applications;
 requiring strength characteristic values within 5 % of the expected values.
The layout corresponds to that of an International Standard (not a normal Annex) to clearly show how an
International Standard in this domain should look. It includes both
 normative elements (Scope, Normative references, Terms and definitions, Symbols and abbreviated
terms, Requirements, Sampling, Test methods), and
 supplementary informative elements (Bibliography).
Informative annexes (like this Annex A) and normative annexes may also form part of an International
Standard.
NOTE The Scope does not usually form part of an annex, but is included in this example for completeness purposes.
A.1 Scope
This standard provides an example of the grading procedures for producing visually sorted strength and
stiffness graded rectangular timber for structural applications requiring strength characteristic values within
5 % of the expected values.
It is applicable for timber that is graded in the seasoned state.
It may be applicable for timber that is graded in the unseasoned state providing the structural design
properties for the timber are modified to reflect the intent of this standard and/or to the associated design
codes.
A.2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 9709, Structural timber — Visual strength grading — Basic principles
ISO 13910, Structural timber — Characteristic values of strength-graded timber — Sampling, full-size testing
and evaluation
6 © ISO 2005 – All rights reserved

---------------------- Page: 11 ----------------------
ISO 9709:2005(E)
A.3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
A.3.1
compression wood
abnormal wood that forms on the underside of leaning and crooked coniferous trees
A.3.2
fungal decay
not sapstain but wood that has been damaged by fungal attack
A.3.3
grade
population of timber derived from a specified resource and by applying a specified sorting procedure
A.3.4
insect damage
damage that has been caused by insects
A.3.5
knot area ratio
KAR
ratio of the sum of the cross-sectional knots to the cross-sectional area of a piece of timber
A.3.6
knot
portion of a branch or limb that has become incorporated into a piece of timber. In timber, knots are classified
as to form, size, quality, and occurrence
A.3.7
margin knot area ratio
MKAR
ratio of the projected cross-sectional area of all knots intersected by the margin areas of the piece
A.3.8
pith
small soft core in the structural centre of a log
A.3.9
pocket
well defined opening between the rings of annual growth which develops during the growth of the tree and
typically contains resin or bark
A.3.10
sapstain
natural variation from the colour of the sapwood
A.3.11
seasoned timber or dry timber
timber with an average moisture content of 15 % or less at the time of grading
A.3.12
structural requirements
grade requirements that affect the structural properties of the timber. Structural features are: knots, slope of
grain, fissures, and any other features that may cause a decrease in strength properties to an amount which
threatens the serviceability of the piece
© ISO 2005 – All rights reserved 7

---------------------- Page: 12 ----------------------
ISO 9709:2005(E)
A.3.13
thickness
lesser dimension perpendicular to the longitudinal axis of a piece of timber
A.3.14
total knot area ratio
TKAR
ratio of the projected cross-sectional areas of all knots intersected by the total cross-sectional area of the
piece
A.3.15
unseasoned timber
timber with moisture content greater than 20 % at the time of grading
A.3.16
utility requirements
grade requirements that do not affect the structural properties of the timber and/or to an amount which
threatens the serviceability of the piece
A.3.17
visual graded timber
sawn wood that has been sorted into structural or non-structural grades according to visual criteria. The visual
criteria identify visible physical features that affect timber strength, utility of the product and/or the visual
quality of the product
A.3.18
white speck
white or brown pit or spot in wood caused by the “Fomes Pini” fungus that only develops in the living tree and
does not develop further in service
A.3.19
width
greater dimension perpendicular to the longitudinal axis of a piece of timber
Other features such as bow, cup, fissures, knots, rate of growth, slope of grain, spring, twist and wane should
be defined in the appropriate subclauses and by referencing the features to an illustration.
A.4 Symbols and abbreviated terms
A.4.1 General notation
E modulus of elasticity
f strength
N sample size
CV coefficient of variation
A.4.2 Subscripts
0,05 5-percentile
data value measured on data sample
mean mean value
8 © ISO 2005 – All rights reserved

---------------------- Page: 13 ----------------------
ISO 9709:2005(E)
m bending
target target or specified value for the grade
A.5 General
A.5.1 Visual strength-grading operations
The visual strength-grading operation shall be comprised of one or more visual graders who sort an input
resource into two output grades. Some of the timber may not meet the requirements of the minimum grade.
Structural and utility requirements are specified for the visual strength-graded timber.
A.5.2 Principles of quality control
The visual strength grading is one element of the quality control operations. This standard requires that the
quality control be undertaken by placing checks on the three components of the strength-grading operation:
the resource input, the visual grade sorting, and the graded timber output.
A.6 Resource input requirements
The input resource shall be defined in terms of all parameters that may affect the output of the visual grade
sorting operation.
Parameters that shall be defined are
a) timber species or mixture of species,
b) log source,
c) log size,
d) silvicultural practices used,
e) cutting pattern used to manufacture sawn timber from logs,
f) seasoning condition at the time of visual grade sorting, and
g) moisture content.
Limits on resource parameters shall be stated in a quality manual.
If major reprocessing of previously graded material is undertaken that may reduce the structural design
properties or lower the visual grade, then the material shall be re-graded.
A.7 Visual properties
A.7.1 Grader requirements
The grader shall be qualified to grade timber to the requirements of this standard at the necessary operational
speeds and to evaluate the visual quality and sizes being manufactured.
© ISO 2005 – All rights reserved 9

---------------------- Page: 14 ----------------------
ISO 9709:2005(E)
A.7.2 Visual grading requirements
A.7.2.1 Definitions of features
In addition to the features defined in A.3, the size of a knot is assessed by the knot area ratio (KAR)
(see A.3.5 and Figure A.1). In addition, the location of a knot is defined in terms of edge, face and margin
areas (see Figure A.2).
Two types of knot area ratios are defined:
 the margin knot area ratio (MKAR) (see A.3.7); and
 the total knot area ratio (TKAR) (see A.3.14).
Fissures are measured as illustrated in Figure A.3.
The slope of grain (see Figure A.4) is assessed over a distance sufficiently great so as to avoid the influence
of local deviations.
To assess rate of growth, measurements are made on one end of the piece, and expressed as the average
ring width in millimetres along a straight line 75 mm long normal to the growth rings, passing through the
centre of the end of the pieces [see Figure A.5 a)]; or commencing 25 mm from the pith when it is present [see
Figure A.5 b)]. When a line 75 mm in length is unobtainable, the measurements are to be made on the longest
possible line normal to the growth rings and passing through the centre of the piece.
A.7.2.2 Limits for structural features
The following limitations shall be applied so as to meet the visual grade structural requirements.
Table A.1 provides the limits for ensuring adequate structural properties. Two visual strength grades, denoted
by G1 and G2, are specified. To qualify for a grade, a piece shall not contain characteristics which exceed the
limits given in Table A.1.
Sapstain is not a structural defect and shall be acceptable without limitation.
Any piece which contains defects such as compression wood, insect damage, fungal decay (not sapstain),
physical damage, combinations of knots and/or other characteristics, which can cause a decrease in strength
properties to an amount which threatens the serviceability of the piece, shall be excluded from the grades.
It shall be permissible for pieces to be accepted, where the reduction in strength caused by the abnormal
defect or damage is obviously less than that caused by the defects admitted by the grade, subject to the
provision that these abnormal defects are of a type which will not progress after conversion and drying, e.g.
white speck derived from the standing tree.
10 © ISO 2005 – All rights reserved

---------------------- Page: 15 ----------------------
ISO 9709:2005(E)
Table A.1 — Grade limits for ensuring structural properties for G1 and G2 grades
Feature Feature limit
Grade G1 Grade G2
Limitation on knot sizes Either Either
MKAR u 1/2
MKAR > 1/2
TKAR u 1/2
TKAR u 1/3
or or
MKAR u 1/2
MKAR > 1/2
TKAR u 1/4
TKAR u 1/2
Slope of grain 1 in 6 1 in 10
Average width of annual rings
u 10 mm u 6 mm
(rate of growth)
Fissures  not through thickness unlimited
L/2
 through thickness u600 mm
2b
Pockets Unlimited if shorter than half width of piece, otherwise
(resin pockets and bark pockets) same limits as for size of fissures
NOTE
L is the length of piece
b is the width of piece
TKAR is the total knot area ratio
MKAR is the margin knot area ratio


a)  Axonometric view, showing a group of knots b)  Front view of projection plane showing
and their p
...