Shrinkage of softwood demention lumber

[Hackermatack]

I am going to mill some of my own balsam fir and white spruce into 2 x 6 for a upcoming project. My mill has a scale for 6/4 to make the thickness the same as planed lumber. I want my lumber to match up with the store bought stuff so that if I run short a few sticks I can just go to town and buy them. Because I am milling fresh cut logs I will of course get some shrinkage. wondering if it will be enough to be concerned about. Thickness does not concern me as much as width, the lumber will be mostly studs and the walls will be finished both sides so a variation in width will make for a crooked wall. My first thought is to mill them oversize and either resaw or plane them to 5 1/2 after they have dried. Another thought is to mill them just slightly over width and hope they shrink to 5 1/2. Or maybe I'm just overthinking the issue. I'm needing about 400 2 x 6 x 10 so with the housing and lumber market red hot rite now my efforts will save me quite a chunk of change.

[scsmith42]

Lumber shrinkage is usually related to the orientation of the growth rings relative to the face of the board.  In oak, I typically allow 6% for thickness shrinkage in flat sawn oak and 12% in thickness shrinkage for quartersawn oak.

Said differently, oak will shrink 1/16" per inch of thickness if it's flat sawn, and 1/8" per inch of thickness if it's quartersawn.

Depending upon how exact you want your finished product to be, if you find out the shrinkage ratio for the species that you're milling and apply the percentage to the targeted thickness it will put you in the ball park.

[Hackermatack]

Thanks it seems that balsam fir is between 2.9 & 7 % depending on how it is sawed and I'm sure white spruce is different. So since I have mixed species and it will be sawed for quality and yield it looks like my best bet is to mill it all 6" and plane or resaw when dry. I will probably opt for resaw because I can do them 8 or 10 at a time on the mill. I guess this would also have the added benefit of removing any warp that happens while drying.

[GeneWengert-WoodDoc]

Continuing the thought above, in the 1950s, the industry decided on standard thicknesses, such as 1.5" x 5.5" for a 2x6.  This thickness applies at the time of grading, which is usually around 15% MC.  But due to player allowance, grain variation in shrinkage, and shrinkage variation from green to final MC, as well as species differences, there is no standard green lumber size at the sawmill.  Note that it is possible that the pieces if sold in a large, heated store might dry a little after grading, so might actually be 1/100" or so thin.

Working backwards, to make sure that you have enough thickness at the mill (1.50" final size), we often assume 4% shrinkage (0.06") from green to 15%, and a planer allowance of 0.10. so that we need a green size of 0.16" + 1.5" = 1.66".  Then this is the minimum thickness, so we want the average thickness to be 0.9" more for a good mill, giving an average or target size of 1.75".  To account for a little cupping in a species like southern pine, we might go up to 1.77".  We also might appreciate that maybe 1 or 2 pieces out of 100 might be 0.01 thin, but that is probably acceptable...there might be a small section of planer skip.  Large mills will refine this number to make sure they are very close to the perfect size, as even 1/100" extra at the saw costs money when you are making more than 25,000 pieces per shift.  Likewise, too many thin pieces are a big loss.

The example is for thickness.  The same approach is used for width,

Need specific help applying these concepts to your mill?  Note that many text book shrinkage values are green to 0% MC, so use half of that value for 12 to 15% MC.

[Hackermatack]

I don't think that I will worry too much about thickness because I am primarily making studs. I will purchase dressed lumber to make the shoes and plates to get uniform thickness. A variation of 1/4" or less in stud thickness will make little difference. If I keep an eye on things and keep a sharp blade on the mill I can saw very uniform lumber.

[Magicman]

I have never sawn balsam fir nor white spruce, but I would expect SYP 2X6's to shrink ~3/16".  In practical terms, if you oversize 1/8" you should be OK, at least that is what I would do. 

[GeneWengert-WoodDoc]

For the width, shrinkage and planer would be around 0.37" or around 3/8".

[LeeB]

With a decent blade you don't really need to plane band sawn studs.

[barbender]

Hackermatack, if you want them uniform, you have the right idea. Saw them 6" wide and resaw them10 at a time. I've tried sawing oversize to dry to a certain finished width, but it doesn't work well because no two pieces shrink the same. I've sawn softwood at 5 3/4" and that ends up real close on average, but be prepared to use some shims, etc.😊

[GeneWengert-WoodDoc]

The shrinkage number for a given species is the shrinkage from 30% MC (or wetter) to 0% MC.  Drying to 15% MC would mean we would use 1/2 of the published value.  Drying to 10% MC means we would use 2/3 of the published value.  However, the number is an average value and the numbers are for flatsawn width (tangential) and quartersawn width (radial).  In calculations of sawmill target thickness, we use the tangential value for both thickness and width as this is the maximum shrinkage (on the average).  So, this means that after drying, most pieces will be a bit thicker and wider than needed.

If we have green width of all pieces the same, then after drying we will have a variety of widths.  For a 2x6, we will see some pieces at 15% MC shrinking 0.22" and some only 0.11" with the others being in between.  This is due to grain variation within the wood.  Then, we would see some variation from tree to tree and also from the top of a tree to the bottom, and variations from heart to sap. 

So, if the green size is identical on each piece (unlikely due to the accuracy of the settings of a mill unless we use linear position era and blade variation or wobble), then after drying to the same MC (unlikely, so with varying moisture we will have varying final sizes), we will have grain variations causing variations in finished widths.  That is, three main causes of size variations in dry widths.  As this variation is likely unacceptable in the final product, even if the blade cuts a smooth surface initially when green, we will need to make the pieces have a uniform size after drying...saw or plane them. 

There is one exception...the size is measured at the time of grading, so if we grade the pieces green and they are exactly 5-1/2" wide, then we do not worry about dry sizes (although the customer or user might).  Such lumber would have a grading stamp indicating S-GRN.

Note that wood that is pressure treated is first dried and then sized and graded at this lower MC.  It is then treated using chemicals in a water solution, which increases the MC and swells the wood.  The wood is then sold (and used often) as a wet piece, larger in size than normal.  If this piece dries in use, it will shrink down to its dry size.  If low MCs are needed after treating, the piece is kiln dried a second time and it is called KDAT...kiln dried after treating.