Why are my joists smaller than standard size?

Question

I was preparing to replace a cantilevered rim joist on my house today. I bought a 2x10 treated board to replace it. I cut a piece to use as blocking and found that the board was much wider than the existing joists. I measured the board and it seemed to run wide, it was about 9-1/2". Then I measured my rim joist and another exposed joist in my basement and confirmed that the installed joists are only 9" wide.

I searched online and found that actual width of a 2x10 should be 9-1/4".

Why are the 2x10's on my house only 9"? Why are the green treat boards I bought 9-1/2"? My house was built in '88. Have the standards changed? Is there a tolerance to go along with the North American Standard lumber sizes? Did the contractor use factory seconds? I figure I'll have to rip half an inch off the new boards to match the old ones.

Here's the marking on the treated board

Answer 1

Yes, there are grading rules and tolerances for lumber, timbers, etc.

You are describing "dimensional lumber". Grading rules for that (especially where you live) is established by the American Softwood Lumber Association, PS-20. (Product Standard 20.)

If you look at Table 3, "Dimensional" you'll see for 2 x 10's, the minimum size is 9 1/2" for green and 9 1/4" for dry. (This can vary somewhat based on the species.) ...but the minimum size is 9 1/4".

Where I live, grading is primarily done by Western Wood Product Association. If you look on your joists, you'll see a black ink "Grade Stamp". I suspect you'll find several numbers and letters in a small grouping (about 1"x2" area) with one of the icons of WWPA. Next to that round icon, you'll see the "moisture" grade. If it reads GRN, then it was milled (cut at the mill) when it was green and had more than 19% moisture content. If it reads DRY, then it was milled when it was less than 15% moisture content. (There is a seldom used mark of S-DRY, which stands for Surface Dry and had a moisture content between 15%-19%.) Actually, if the stamp is smeared and "out of focus" to the point you can't read it, then it was milled "green".

I explained all that to let you know that your lumber could have been milled "green", but dried out, which caused the lumber to shrink.

If you read the text, you'll see that there is a tolerance of 1/4" in the width and 1/16" in the thickness...the 2x direction.

Also, the lumber could have been mis-cut and was rejected, so you won't find any grade stamp on your joists. The contractor could have bought the joists at a discount and was never caught by the Building Inspector.

By the way, the grain of wood in your picture indicates a better than a normal piece of lumber. If all the joists are similar, then I'd feel comfortable that it's structurally sound and would equal other lumber that is 1/4" larger.

Answer 2

Everything is correct on the answer by Lee Sam. I will add this.

Even when the wood is kiln dried, it will shrink more. When it is bought from the lumber yard or when the lumber yard gets it from the mill it is, or should be 9 1/4" wide. If it gets wet, it will swell wider. If it gets a chance to get really dry it will shrink even more narrow. This will also depend on how the wood is cut. Most logs at the mill are plain sliced or flat sawn, Same thing, depending where you are from. This type of sawing the lumber yields different types of lumber. Do a search for quarter sawn, rift sawn and flat sawn lumber. The quarter sawn wood will not grown in width as much as flat sawn wood, so there will be differences among lumber that is high in moisture content (MC).

This can be easily related to when the pressure treated lumber is considered. It is dunked in a tank with wood preservative which really saturates the wood, forcing the preservative into the grain. It in essence is really wet wood, therefore the swelling from the treatment with preservative makes it wider than 9 1/4". In time the wood will shrink back down. If it is a dry enough place, it will shrink to 9 1/4" too, and if it is in a really dry place, perhaps even smaller.

In many homes I have added more framing to, I have always had this issue. Crown your material, cut the little you need to get it in there snug, and if the floor system you are working on can handle it, raise up the other floor to the new level of the new material, and let it shrink back down in place. (This rarely works out that way, since bearing walls do not like getting raised, but on rare occasion, in other scenarios it can be done)