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Old 10-20-2009, 11:23 AM   #1
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Standardization for framing lumber

I a doing some work on a house that was built fairly recently. Permit was pulled in 2001. The floor joists are 2x10. Looking at the joists, it is evident that some joists are deeper than others. The existing joists differ by as much as 3/8 inch in the 10" dimension. 2x10's that I purchased at HD show even larger mismatch. Some of the boards I purchased are about 1/2 inch deeper than the smallest existing joists.

The depth of the joists is an issue because I want a sheet of plywood to sit flush on the underside of the floor.

Most of the existing boards have marks indicating that they are graded (#2) framing lumber.

Anybody know if there is a standard that stipulates min and max dimensions for framing lumber. Any idea why boards would run small ?
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Old 10-20-2009, 01:07 PM   #2
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That certainly sounds wrong to me. There is a standard size for lumber ( and framing lumber or not, a 2x10 should be 2x... er, nope, it should be 1.5 x 9.25" :-) It's possible the builder had some 2x12s and ripped them down; but that wouldn't account for seeing the problem in HD.
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Old 10-20-2009, 03:46 PM   #3
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Depends on the mill. I've had some in the same sling, from the same mill be off by 1/4". Box stores sell 1/4" lath and 1/16+ cardbord drywall shims.....
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Old 10-20-2009, 06:24 PM   #4
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It's common for lumber to shrink after a house is closed in. I often find 2x4s to measure 3 3/8 to 3 1/4 instead of 3 1/2. With 2x10s normally being around 9 1/4, the shrinkage can be considerable.
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Old 10-20-2009, 06:36 PM   #5
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I don't know what area you are from, but around here, Philadelphia area of PA, framing lumber dimensions vary considerably once you go above a 2x6.

2x8s can be anywhere between 7 1/8 and 7 3/4
2x10s can be anywhere from 9 1/8 to 9 7/8
2x12s can be anywhere from 11 to 11 7/8
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Old 10-20-2009, 08:13 PM   #6
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I've also seen a lot of range in the sizes of wood

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Old 10-20-2009, 08:20 PM   #7
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This is good to know. I was planing to build a server rack out of wood. Bad idea maybe? needs to be rather precise...
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Old 10-21-2009, 11:34 AM   #8
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When you get new 2x10s and bigger not only is there variation between pieces but if you measure each end there is often a difference on the same piece. Then add the shrinking and the joists in an established floor can vary quite a bit. As mentioned shims are an option. Some folks run strapping (1x3 or 1x4) perpendicular to the joists and shim those where they cross the joists to get a flat ceiling.

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Think about building your server rack with carpentry grade boards rather than framing lumber. "Lumber" is 2x (which is actually 1.5") or bigger stuff and has a lot more dimensional and trueness variation (and edge quality and knot issues) than what you want. Construction quality "Boards" are typically 1xs (which are actually 3/4" thick) and seem to have less dimensional variation than lumber but still trueness, edge quality and knot issues - you might get by with them but if you go with a carpentry quality board the quality will be better and dimensions thicker than 3/4" are available.
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