Yield Loss in Ripped-One-Edge Lumber
From contributor J:
It depends on what defects you allow. The #1C yield by the grading rules is 67%-82% clear cuttings, so the average is 75% basic yield. Then add 10% for straight line ripping, and then 15% for ripping width and 10% for cutting lengths.
In a formula:
With the last number that came up, I might avoid this project altogether! Maybe round up to 700bf so you don't spook the supplier :)
From contributor W:
My rule of thumb is twice as much rough lumber as finished flooring. All depends on the losses due to cutting and machining. For 300sf of flooring I would start with 600bdf of lumber.
From the original questioner:
Contributor J, your reply was exactly what I was after. Thanks for the information, as it is a big help. As for avoiding the project altogether, based on its sign, it hasn't been too beastly. I've been at it about a year and a half or so (as time, money, and circumstances allow). Once completed, it will be a small, one person shop/studio.
From contributor O:
Your problem started when you requested the r1e. My orders have been short by 10%, at best, and 25% at worse. You have to understand that the supplier measures out the 450 bd ft then they straight line on edge. This is where a large discrepancy can occur. I now have a supplier that guarantees no more than a 7% loss for a straight line rip one edge. I have also had suppliers sell lumber based on pre-kiln and post kiln dimensions. 1000 bd ft put into a kiln is going to come out 950 at best. When I order from suppliers now, I tell them I want a certain amount of bd ft delivered after kiln and ripping one edge. That is the only way to get the amount of wood you want and to be able to compare prices among suppliers.
From the original questioner:
A couple of follow-up questions, if I may? Let's start with how the supplier, in this case my regional sawmill, determines board feet of their product. How is this calculation performed?
My guess is:
If correct, at this point, in theory, the 450bf order is now about 405bf (accounting for 10% loss due to ripping one edge). This should then yield an average of 75% usable stock based on the #1C grading yield. Then I should apply the average waste percentage calculation.
Which is the biggest area of "shrink," 1) the mill's initial estimation of total bf, or 2) the loss due to ripping one edge? I realize that my orders are small stuff/mice nuts compared with other orders. However, being informed and understanding what is customary helps me do a better job of planning and buying.
From Professor Gene Wengert, forum technical advisor:
Regarding the calculation of board footage at a sawmill, there is an article in the archives here. For hardwoods, it is the standard length (feet and no inches; always round down) times the width in inches and fractions and then divided by 12 and then rounded up or down to the closest whole number. Then, sum these values for every piece of lumber. Finally, multiply by the thickness (in quarters of an inch). Again, the answer is in whole numbers only.
Now when dried, there will be 6% shrinkage, but the industry uses 7%. The footage green is therefore reduced by 7% after drying. However, some folks add 7% back to the dry footage to obtain an estimate of the green footage. This is an illegal practice since 1977. The actual footage at the time of sale of KD lumber is called the net board footage. If you add 7%, then you have the gross footage. (In the terminology of contributor O above, it is illegal to sell lumber based on the pre-kiln-drying footage.)
The grade of No. 1 Common guarantees that there will be one or more (the number depending on lumber size) large clear rectangular areas (on the worst side of rough lumber) whose area totals 2/3 (67%) of the area of the lumber. For example, with a 4/4 piece of lumber that is 7-1/8" wide and 8 feet 10" long, you have 5 BF and are permitted to have one or two clear areas, each clear area being at least 4" x 2' or 3" x 3'. The area of these two clear cuttings must be 480 square inches. (As a variation, you can also have three clear areas that total 75% of the lumber's surface or 540 square inches.)
So, with this restriction on clear areas, and because this 67% is the minimum, and most lumber pieces will therefore have more clear area, it is easy to see why the actual yield when cutting small furniture parts, cabinet parts or flooring will potentially be quite a bit higher than 67%.
Note that when grading planed lumber, the clear area is judged from the best face. If I rip lumber on one edge and if the saw kerf is 1/8" and I remove 1/8" strip of wood, then this 1/4" loss on a 6" wide piece of lumber is 4%. A 10% ripping one edge loss is excessive on 6" wide lumber pieces.
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