Correct thickness for 4/4 and 8/4 lumber

Q. What is the correct thickness for 4/4 and 8/4 lumber? Our lumber seems to thin at times, especially quartersawn.

A: Based on your company’s products, I think you are concernedabout hardwoods. The answer is different for softwood lumber.

The first key is that the thickness of hardwood lumber ismeasured at the time of grading (and the same for softwoods). This isreasonable, as a change in moisture can result in a change in thickness, butmoisture changes are basically out of control of the supplier.

A second key with hardwoods is that the thickness is measuredonly in the sections or areas of the lumber that are used to establish thegrade of the lumber. So, for example, with No. 1 Common lumber that requires inmany cases a clear area of 67 percent, it is only the thickness in this 67percent area that is critical. Also, in case you wondering, the thicknessmeasurement does not worry about warpin the lumber; thickness measurement is an independent event or measurement.

Third, the allowable thickness (allowable based on the NHLA’sRule Book), varies by moisture content, and if the lumber has been planed or isrough. So here is a summary of the thickness rules for 4/4, 5/4 and 8/4; seethe Rule Book for the complete set of rules for all cases.


Green or Air-Dried


For green, freshly sawn lumber, to be called 4/4, the thicknessin the grading area must be over 1.00 inch; for 5/4, 1.25 inch; for 8/4, 2.00inch. However, lumber does shrink in thickness (and width) when it goes fromgreen to air-dried. The shrinkage varies with species. Also the shrinkage isroughly twice as much in thickness with quartersawn as with flatsawn. A goodestimate is that the lumber will shrink around 3 percent in thickness or lessgoing from green to air-dried. For 4/4, 3 percent is 1/32 inch. So, a smartsawmill will saw 4/4 thick enough so that the thinnest spot is 1-1/32 inchto avoid getting thin lumber after air-drying. Of course, if the lumberis not graded after air drying, then the air-dried thickness is not an issue. (Dueto warping, the lumber might have to be even thicker in order to be able to beplaned successfully, but that is another issue.)


Kiln-Dried Rough


For kiln dried rough lumber, the NHLA has a “kiln-driedrule.” This rule, applied when roughkiln-dried lumber is graded, allows the thinnest spot in the grading area to be1/16 inch thinner (and a very few pieces can be 3/32 inchthinner) for 4/4, 5/4, 6/4 and 7/4 lumber; that is 15/16 inchfor 4/4, 1-3/16 inch for 5/4, etc. For 8/4 and thicker, theallowance is 1/8 inch (with very few allowed 3/16 inch), sothe minimum thickness in the grading area is 1-7/8 inch. In most cases for 4/4and 5/4, and for 8/4, this allowance is adequate to account of rate shrinkagethat occurs in the drying operation. It might be a bit troublesome for 6/4, 7/4and over 8/4, especially with high shrinkage species and for quartersawn. So,again the sawmill might cut just a bit thicker than the minimum, just in case.


Kiln-Dried Planed


Finally, sometimes kiln-dried lumber is planed or surfaced on twosides (S2S) prior to grading. In the case of planed lumber, an extra allowanceis given. For 4/4, the surfaced thickness can be as thin as 13/16 inch; for5/4, 1-1/16 inch; for 8/4, 1-3/4 inch.

A special note about planning. For hardwoods almost all gradingis done from the poorer side of the lumber. However, for planed lumber, theinspection for grade is based on the better side or face.

Special note about varying thickness. Normally, the thicknesswill be fairly uniform within an individual piece of lumber. However, if thedifference between the thinnest and thickest spot for rough lumber varies toomuch, then the title “miscut”is added to the description. There is not grade change in this case. Of course,the minimum is still the same as mentioned above. The variation for miscut 4/4and 5/4 is over ¼ inch; and for 8/4, 3/8 inch over.


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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.