Treated Lumber - Nominal vs. Actual


Old 07-25-04, 08:56 PM
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Treated Lumber - Nominal vs. Actual

While building a deck I noticed that two 2x10x8 AC2 boards had different measurements. One was 9 2/8" (standard actual size) and the second was 9 4/8". Is one board larger due to moisture in the wood and if so, will it shrink to 9 2/8"? I don't want to lay out my joists and have a wavy deck due to inconsistent dimensions for 2x10 joists. If the joists will shrink to 9 2/8", do I need to wait for them to shrink before adding the decking?

Thanks, DavesHammer
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Old 07-25-04, 09:42 PM
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Very good question and it's something thats affecting me right at this moment. I too am building a porch with the joists 12" on center. I'm laying down composite decking material so thats why i went with 12" on center. I quickly measured all my boards and placed the "smallest" ones at each end. I nailed all the joists in place at the ledger board on the house and layed them down on the outer beam. I then layed a line from one end of the deck to the other at the corner of the "smallest" joists. I then used an electric planer to zip the outer beam where the high points of the joists would rest making it level to the line. After that, i nailed everything in place. At this moment, all the joists are level at the top and i'm laying my decking down. It's all straight now but who knows what will happen later on.

Any "Pros" wanna add to this?
Old 07-27-04, 08:06 PM
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Actual Vs Nominal

Variations in the nominal widths of deck lumber occurs because of the moisture content in the lumber.

While one can find variations in the thickness of lumber of 1/4- 3/8ths inches between boards, variations of thicknesses within individual boards can also occur.

Since the lumber that is pressure treated has been milled to exact and rpecise nominal dimensions, there is never any reason at all to plane or mill treated lumber to achieve a 'flat' deck surface with joists of varying widths.

The proper way to deal with this situation is to install all joists at the same height by tacking them into place and only then installing and fastening the joist hangers that will support them. The joist will be aligned along to the top to assure a flat unwavy surface.

Even so, as the lumber dries there will be some variation in joist depths.

The only other way around the problem is to purchase what is called "kiln dried after treatment" lumber.

Kiln dried after treatment lumber is pressure treated lumber, either CCA ACQ or other...which has been milled to exact specifications, treated, then kiln dried to a precise 15% moisture content.

This kiln drying eliminates any variation in the lumber thickness and widths due to excess moisture content.

Kiln dried after treatment lumber is normally not a stock item at most lumber yards, is more expensive than conventional lumber, and must be special ordered. It is more expensive but affords greater control over the construction and finishing of projects used with it.

That said, all deck joists need to be aligned at the top then joist hangers individually fastened to each joist.
Old 07-27-04, 11:42 PM
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And Dregg,

Once your joists begin drying, they will 'crown' different amounts, so all of your planing will have been for naught in a couple of years. I DO hope you made sure all of the joists were placed 'crown up' -- otherwise, some will be crowning up, and others will be crowning down!!
Old 07-28-04, 03:34 PM
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Thanks for the info. I crowned all the pieces the same but i think you may have mis-understood the way i layed out the deck. You're right, no matter what, the wood will eventually dry and shrink a bit making most of this work worthless. However, i wanted to get the straightest deck possible at the start so when the wood does shrink, it won't be terribly noticeable. I know working with wood, you're not gonna be perfect....but you can get close.

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