Dimensional shrinkage of pressure treated lumber.
jayh25
| Posted in General Discussion on
I’m building a deck to an existing elevation using 2 x 8 pressure treated lumber. It measures 7 1/2″ and the wood has a moisture content that ranges anywhere from 30% to 50% on these new boards with an average rating of near 35%. The small town home center said that the lumber was kiln dried before treatment. What can I expect the lumber to measure after it has dried out?
I’m expecting the lumber to dry to somewhere near 9%.
This matters twice here because the 2×8′ joists will rest on a 2×8 beam.
Thank you all.
Replies
I would figure 7.25" just because that's what it probably was before the treating process.
Different species will 'shrink' at different rates because they absorb at different rates. Some of the wood associations will have that information.
Ask the small town home center what the mill dimensions were after kiln drying, they won't get smaller than that.
Most mills undercut 'to be treated' lumber because of the swelling. This is why engineers and load calc programs alter the loading values of pressure treated beams. It's only as good as it's original dimensions.
I thought KDAT wood was preferred?
Thank you all for your thoughtful answers.
I did call the lumber center to ask what the dimensions were after kiln drying and I was told to call back tomorrow to ask for the one person who may know. If he doesn't know I'll see if he can make a call.
I will let everyone know how I make out.
Jay,
Here in Nw Oh. I would expect MC equalizing somewhere close to 12 -14.
How many bearing points under the beams?
Lowering it could be a btch. Raising it with solid shim stock like plastic (iron workers use) might be easier if necessary.....
Maybe.
I ended up calling Madison Wood instead of the home center this morning. Very quickly I spoke with a person who told me that when the 2x8 lumber arrives it measures 7 1/4".
I was expecting something closer to the 7" range.
On a side note, but hopefully helpful to someone else here, M.W. has a video on youtube where they suggest separating the deck lumber into three piles. Those that measure 7 1/2", those that measure 7 3/8", and those that measure 7 1/4". After making piles of each, install them together working from 1/2" to 1/4".
Which makes me question the answer that M.W. gave me. If there are some at 7 1/4" in the pile what are those going to dry to?
I might just have to chill out here. I'm going to anticipate 5/16" shrinkage on the joists and beam for a total of 5/8" from the width I have of 7 1/2" and go with that. Wood is wood and I can only ask for an average.
I will say that I got the impression that I was the first person to ask the people I spoke with this question! For better or worse.
Thanks all and have a good day!
I have never ( 47yrs ) worked with a 2x8 get narrower than 7-1/8”, and see most commonly ....7-1/4. Wet treated shows up 7-1/2 - 3/4.
NW Oh, not the desert.
All wood changes (w/ respect to tangential, radial, and longitudinal axis) dimensionally with changes in moisture content. Your best bet in order to avoid errors due to critical assumptions is to buy a moisture meter and see where the moisture content (M.C.) of your lumber equilibrates with respect to where you are putting it into service. If its real critical that each width be approximately the same (within a 1/16"), then plane each piece down to same height after they reach equilibrium. If the service condition is such that M.C. will change substantially in service, then you're most likely pissing in the wind with your design details.
IMO a final 7.25" equilibrium width is as good of a guess as any. If you want to avoid the time and cost of measuring to equilibrium, then spend that extra time and money on pressure treated lumber that is kiln dried after treatment (as edwardh1 intimates).