Editing Lumber (section)

===Grades and standards===
[[File:The longest board in the world (2002).jpg|thumb|The longest plank in the world (2002) is in Poland (near Szymbark) and measures 36.83 metres (about 120 ft 10 in) long.]]
Individual pieces of lumber exhibit a wide range in quality and appearance with respect to knots, slope of grain, shakes and other natural characteristics. Therefore, they vary considerably in strength, utility, and value.

The move to set national standards for lumber in the United States began with the publication of the American Lumber Standard in 1924, which set specifications for lumber dimensions, grade, and moisture content; it also developed inspection and accreditation programs. These standards have changed over the years to meet the changing needs of manufacturers and distributors, with the goal of keeping lumber competitive with other construction products. Current standards are set by the [[American Lumber Standard Committee]], appointed by the [[United States Secretary of Commerce|U.S. Secretary of Commerce]].<ref>{{cite web|url=http://www.alsc.org/geninfo_history_mod.htm|title=American Lumber Standard Committee: History|website=www.alsc.org}}</ref>

Design values for most species and grades of visually graded structural products are determined in accordance with [[ASTM]] standards, which consider the effect of strength reducing characteristics, load duration, safety, and other influencing factors. The applicable standards are based on results of tests conducted in cooperation with the [[USDA]] Forest Products Laboratory. Design Values for Wood Construction, which is a supplement to the ANSI/AF&PA National Design Specification® for Wood Construction, provides these lumber design values, which are recognized by the model building codes.<ref name="WoodWorks Structural Properties and Performance">{{cite web|title=Structural Properties and Performance|url=http://www.woodworks.org/wp-content/uploads/Wood-design-structural-properties-performance-fact-sheet.pdf|website=woodworks.org|publisher=WoodWorks|access-date=7 May 2017|archive-date=26 March 2020|archive-url=https://web.archive.org/web/20200326010734/https://www.woodworks.org/wp-content/uploads/Wood-design-structural-properties-performance-fact-sheet.pdf|url-status=dead}}</ref>

Canada has grading rules that maintain a standard among mills manufacturing similar woods to assure customers of uniform quality. Grades standardize the quality of lumber at different levels and are based on moisture content, size, and manufacture at the time of grading, shipping, and unloading by the buyer. The National Lumber Grades Authority (NLGA)<ref>{{cite web|url=http://www.nlga.org/app/dynarea/view_article/1.html|title=National Lumber Grades Authority (Canada)|url-status=dead|archive-url=https://web.archive.org/web/20110811113224/http://www.nlga.org/app/dynarea/view_article/1.html|archive-date=2011-08-11}}</ref> is responsible for writing, interpreting and maintaining Canadian lumber grading rules and standards. The Canadian Lumber Standards Accreditation Board (CLSAB)<ref>{{cite web|url=http://www.clsab.ca/|title=CLSAB and Lumber Grading Quality |publisher= Canadian Lumber Standards Accreditation Board|website=www.clsab.ca}}</ref> monitors the quality of Canada's lumber grading and identification system. Their common grade abbrievation, CLS, [[Canadian Lumber Standard]] is well utilised in the construction industry.<ref name="Homebuilding">{{cite web | last=Jenkins | first=Steve | title=What is CLS timber and what DIY projects is it good for? | website=Homebuilding & Renovating | date=2023-09-03 | url=https://www.homebuilding.co.uk/advice/what-is-cls-timber | access-date=2024-08-22}}</ref>

Attempts to maintain lumber quality over time have been challenged by historical changes in the timber resources of the United States – from the slow-growing [[virgin forest]]s common over a century ago to the fast-growing [[Forest#Forest plantations|plantations]] now common in today's commercial forests. Resulting declines in lumber quality have been of concern to both the [[lumber industry]] and consumers and have caused increased use of alternative construction products.<ref>{{cite web|url=http://www.neo.ne.gov/home_const/factsheets/min_use_lumber.htm|title=Minimizing the use of lumber products in residential construction|publisher=Nebraska Energy Office|website=www.neo.ne.gov|access-date=2009-08-26|archive-url=https://web.archive.org/web/20170320140257/http://www.neo.ne.gov/home_const/factsheets/min_use_lumber.htm|archive-date=2017-03-20|url-status=dead}}</ref><ref>{{cite web|url=http://www.cfr.washington.edu/Research/factSheets/11-CINTRAmatsubstitute.pdf|title=Material substitution in the U.S. residential construction industry|publisher=[[University of Washington]], School of Forest Resources|url-status=dead|archive-url=https://web.archive.org/web/20100620142144/http://www.cfr.washington.edu/research/factSheets/11-CINTRAmatsubstitute.pdf|archive-date=2010-06-20}}</ref>

Machine stress-rated and machine-evaluated lumber are readily available for end-uses where high strength is critical, such as [[truss]]es, [[rafter]]s, laminating stock, [[I-joist|I-beams]] and web joints. Machine grading measures a characteristic such as stiffness or density that correlates with the structural properties of interest, such as [[bending strength]]. The result is a more precise understanding of the strength of each piece of lumber than is possible with visually graded lumber, which allows designers to use full-design strength and avoid overbuilding.<ref>{{cite web|url=http://naturallywood.com/Wood-Products/Wood-Performance/Grades-and-Quality.aspx|title=Naturally:wood|url-status=dead|archive-url=http://arquivo.pt/wayback/20160522205856/http://naturallywood.com/Wood-Products/Wood-Performance/Grades-and-Quality.aspx|archive-date=2016-05-22}}</ref>

In Europe, strength grading of rectangular sawn lumber/timber (both softwood and hardwood) is done according to EN-14081<ref>{{cite journal|title=Strength grading of sawn lumber/timber in Europe: an explanation for engineers and researchers|first1=Dan|last1=Ridley-Ellis|first2=Peter|last2=Stapel|first3=Vanesa|last3=Baño|s2cid=18860384|date=1 May 2016|journal=European Journal of Wood and Wood Products|volume=74|issue=3|pages=291–306|doi=10.1007/s00107-016-1034-1|url=https://napier-surface.worktribe.com/169717/1/EJWP%20ridleyellis%20et%20al%20submission%20Mar%202016%20revised%20after%20proofing%20check.pdf}}</ref> and commonly sorted into classes defined by EN-338. For softwoods, the common classes are (in increasing strength) C16, C18, C24, and C30. There are also classes specifically for hardwoods and those in most common use (in increasing strength) are D24, D30, D40, D50, D60, and D70. For these classes, the number refers to the required 5th percentile bending strength in newtons per square millimetre. There are other strength classes, including T-classes based on tension intended for use in [[glulam]].
* C14, used for [[scaffolding]] and [[formwork]]
* C16 and C24, general construction
* C30, prefab roof trusses and where design requires somewhat stronger [[joist]]s than C24 can offer. TR26 is also a common trussed rafter strength class in long standing use in the UK.<ref>{{cite web|url=http://blogs.napier.ac.uk/cwst/what-is-tr26/|title=What is TR26? |publisher= Centre for Wood Science & Technology|date=1 December 2015}}</ref><ref>{{cite journal |last1=Ridley-Ellis |first1=Dan |last2=Gil-Moreno |first2=David |last3=Harte |first3=Annette M. |title=Strength grading of timber in the UK and Ireland in 2021 |journal=International Wood Products Journal |date=19 March 2022 |volume=13 |issue=2 |pages=127–136 |doi=10.1080/20426445.2022.2050549 |s2cid=247578984 |issn=2042-6445|doi-access=free }}</ref>
* C40, usually seen in [[glulam]]

Grading rules for African and South American sawn lumber have been developed by ATIBT<ref>{{Cite web |url=http://www.atibt.com/ |title=ATIBT |access-date=23 July 2014 |archive-date=16 May 2014 |archive-url=https://web.archive.org/web/20140516233446/http://atibt.com/ |url-status=dead }}</ref> according to the rules of the Sciages Avivés Tropicaux Africains (SATA) and is based on clear cuttings – established by the percentage of the clear surface.<ref name="fordaq">{{cite web|title=African and South American sawn timber|url=http://www.fordaq.com/fordaq/html/quality_africa_sata_En.htm|website=www.fordaq.com|publisher=Fordaq S.A., The Timber Network|access-date=7 May 2017}}</ref>