Understanding the grading of American hardwood

This article will continue to answer questions we have received from buyers throughout the world. If you have a question regarding the rules for grading American hardwood lumber, or if there is any other topic you would like to see in future articles pertaining to hardwoods from the US, write to india@americanhardwood.org

Sometimes we receive hardwood lumber from the United States that is not entirely flat. The lumber is kiln-dried, usually around 8-10% moisture content, and the grading seems accurate. Is there some kind of rule for these boards that are not flat?

Yes, there is an allowable tolerance for these pieces. When lumber is going through the drying process, first on sticks in the open air or pre-drier stage, then in the dry-kiln, stresses occur in the boards.

The lumber will shrink in volume by an average of around 7%, depending on the species and thickness. These stresses will also cause the lumber to ‘move around’ as water is being removed. This movement is the deviation you mention from a flat board.

The NHLA rule for this is covered in several paragraphs in the Rule Book. Firstly, in the definition of a ‘cutting’ it states: “In the grades of Selects and Better, the entire board must be flat enough to surface two sides to Standard Surfaced Thickness. In the Common grades, just the individual cutting must be flat enough to surface two sides to Standard Surfaced Thickness, after it has been removed from the board.”

To calculate what a rough-sawn board must ‘clean-up to’ (surface to), the NHLA rule for Standard Surfaced Thickness calls for an inspector to subtract 3/16 inch (4.76 mm) for any board 1½ inch thick (38 mm) or less.

For all lumber thicker than this, an inspector would subtract ¼ inch (6.35 mm) to calculate the Standard Surfaced Thickness. In addition to the lumber cleaning-up, some allowable skips, chips, or torn grain are permitted on the board as well.

In looking at the lumber you are purchasing, first the grade must be established. If the grade is Selects, FAS1Face, or FAS, the entire board must meet the criteria for surfacing to standard thickness for surfaced lumber.

For example, a 1-inch or 4/4 (25.4-mm) board must clean-up to 13/16 inch (20.6 mm). If the grade you are purchasing is No. 1 Common, just the individual clear cuttings used in establishing the 66 2/3% yield must meet this standard surfaced thickness requirement.

Often we get complaints that rough-sawn lumber we are looking at has got some slight cupping in it. They are not taking into account that the lumber is 27- or 28-mm thick and when surfaced it will be slightly under 2-mm thick. A lot of imperfections will disappear in this surfacing process.

This rule for Standard Surfaced Thickness will also be applied to a board that has some slight season checking, slight fungal stain or any imperfection on the rough-sawn surface of the board.

The bottom line is that the NHLA rules recognise that when we subject green lumber to the rigors of the drying process, it is not the rough-sawn board we intend to utilise, but the surfaced, semi-finished board we are after.

In the AHEC publication, ‘A Guide to Sustainable American Hardwoods’, reference is made to No. 2A Common, but my supplier only quotes me No. 2 Common. He tells me they are the same thing. Are they?

Yes and no. Prior to 1990, the NHLA rules had one designation to this grade, which was No. 2 Common. There were 15 species that only required the No. 2 Common cuttings to be sound and not clear.

For example, if you were to purchase No. 2 Common basswood, you would receive lumber that was only required to have 50% sound cuttings. You could specify No. 2A Common and then you would receive lumber that was required to have 50% clear cuttings.

This rule was changed in 1990 to drop the No. 2 Common grade name and always use the No. 2A Common name for cuttings to be clear and No. 2B Common for cuttings to only be structurally sound. If the grade of No. 2 Common is used, then, technically, it could contain all the No. 2A and No. 2B that is produced.

The problem is that some suppliers have always used the No. 2 Common grade name and are slow to catch on! Just talk to your supplier and clarify that you are both talking about the same thing, and it shouldn’t be a problem.

I purchase Face and Better tulipwood and receive boards with knots and splits on them. My supplier says they meet the minimum required by the NHLA. Is there any grade better than FAS1 could specify?

The grade of FAS is the highest grade for hardwood lumber in the US domestic market, as far as the NHLA Standard Grades go. In any given parcel of FAS lumber, you will have boards that have some knots and splits, because the minimum requirement for FAS lumber is 83 1/3% clear (defect-free) wood.

This means that on a 12-foot board, there will be a minimum of 10 clear feet. In this same parcel there should also be boards that are 100% clear. The NHLA grading rules always talk about a range of quality, meaning that in any given parcel of a particular grade there will be a range of percentages from the absolute minimum to just under the next higher grade’s required minimum.

The price for each grade reflects this range of quality as well. You could ask your supplier to provide you with all 100% clear boards, but be prepared to pay for them because you would be upsetting the range of qualities for other shipments your supplier is putting together. Any exemption to the NHLA grades can be negotiated into a sales contract.

However, be sensitive to what you are asking for and what the additional costs may be! For more information on American hardwoods, and to learn more about different species and grades, visit: www.americanhardwood.org.



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