Shrinkage Allowances for Sawing Southern Yellow Pine

      What dimension should SYP be sawn to, in order to arrive at the desired thickness after drying and planing? November 14, 2009

Question
I am interested in cutting SYP for construction purposes and would like to know the exact dimensions to cut for air drying to get as close to kiln dried and planed lumber. At this point there is neither kiln nor planer available. Also, there seems to be a lot of different opinions about sticker sizes - do the stickers change with different lumber sizes?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Lumber size has various components, so what one person does may not be correct for you. What you do is decide what your planer allowance is, add for shrinkage, add for sawing variation and this, added to your final dry planed size, gives you the average target green size.

Sticker size is 3/4" for slowly dried SYP, but HT kilns often use 1.0" with their high air flow and narrow loads. Again, it depends on your specific situation, kiln design, and so on. There is no one correct answer; what one person does may not be right for you.



From the original questioner:
Thanks for the reply, Wood Doc. Received a copy of your Woods to Workshop yesterday from Wood-Mizer. Learned about it from the DVDs they have about sawing. For this novice it is the most helpful info to date.

The sticker question was actually about width and thickness, and do sticker sizes change with lumber size? Since I have neither kiln nor planer, I am trying to determine the sizes to cut framing material, etc. to a dimension that air dried SYP will shrink to. In other words, if you were cutting 2x4's, 2x6's, etc., what dimension would you cut the lumber to air dry to 1 1/2 x 3 1/2 and 1 1/2 x 5 1/2?



From contributor A:
I saw over 500,000 bdft a year of SYP and sell most of it air dried. The largest percent is sawn 1 5/8 x 4 or 6 or 8, etc. and is used as it comes off the Wood-Mizer mills. If it is to be dried in the kiln, I saw it 1 3/4 thick by width. This will plane down to store size with no problem. Lumber to be finished at 3/4 needs to be sawn 1 inch thick. A 6 inch board will shrink about 1/4 of an inch when at 12 to 16% MC (my wood is tight grained).

I use mostly 1 x 1 stickers for everything but timbers, and there I use "H" stickers or 2 x 2's. I dry mine in a Nyle DH200 and have had no problems with it.


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From Professor Gene Wengert, forum technical advisor:
A really good planer will have 4/64" (= 0.06") planer allowance, about 0.1" sawing variation (3 x std dev), and 4% shrinkage which for 2" nominal size is 0.07. So that gives for a final thickness of 1.50" a green size of (0.06 + 0.10 + 0.07 + 1.50) 1.73" average.

Note that this assures that almost no pieces will be too thin. In other words, we are using the maximum sawing variation and the average shrinkage in the tangential direction (thickness of quartersawn lumber). We seldom have quartersawn and all but a few pieces will have much less sawing variation. This high level of assurance (100% being thick enough) may be too costly, so a bit smaller target size may be appropriate. For each 1/32" reduced target, there will be an increase of yield from the log of about 2%.



From contributor P:
I found a couple charts online called "Standard Dimensions of Hardwood Lumber" and "Standard Dimensions of Softwood Construction Lumber" [link removed]


From Professor Gene Wengert, forum technical advisor:
In the chart that is linked to above, there are several errors. First, they list hardwood lumber grades for "First" and "Second." As separate grades, these were abolished over 30 years ago. They were put into one grade called "Firsts and Seconds" but this grade was abolished in 1992 and the new grade is "FAS." They also omitted FAS-One-Face (F1F).

Also, they should have indicated grades 2A and 2B Common and not just 2 Common.

I might also question their use of "board" for 8/4 lumber. Further, there are standard lengths for hardwood lumber; I do not recall ever seeing hardwood lumber being marketed or referred to as random length.

They should have mentioned that the surfaced thicknesses for S1S are not standard and for S2S are grading standard thickness. In both cases, such thicknesses are for dried lumber and are what the grader uses for judging the grade of the lumber. In truth, the actual thickness of planed lumber is generally not 3/16" under the nominal for 6/4 and less and under 1/4" for thicker, but will actually be a bit thicker. For example, for 4/4 dried and surfaced, there is a lot of 15/16" hardwood (and also a lot of 3/4" which is under). There is no industry wide standard for actual surfaced thickness. Similarly, for green or air-dried 4/4, the grading standard is 1.00", but most hardwoods are 1/16" to 1/8" thicker on the average. The 1.00" is a minimum and not the actual.

Also, the thickness of softwood dimension lumber that is sold with the designation of green or dry (or in between) is the thickness at the time of surfacing and does not change depending on the MC. That is, S-GRN, KD19, KD15, etc. all have the same thickness at the time of surfacing. Here is the standard: "When rough (not planed or surfaced) green lumber is specified, the minimum rough green thickness of finish and common boards, dimension and timbers of sizes 1 or more inches nominal thickness shall be not less than 1/8 inch thicker than the corresponding minimum dressed green thickness, except that 20 percent of a shipment may be not less than 3/32 inch thicker than the corresponding minimum dressed green thickness."

Note that the softwood thickness table referenced says it is for green dressed sizes, so this extra thickness cannot be used as it is for rough lumber. Dressed lumber must be the correct thickness at any MC.



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