Compensate for wet PT lumber shrinkage?
canoehead2
| Posted in Construction Techniques on
The PT 6×6 posts I cut today for my porch project were still very wet inside. Should I plan for some shrinkage in the direction of the grain? Any rule of thumb (e.g. shrinkage per foot of wet PT lumber)?
Replies
I haven't seen p.t. shrink in length and don't know of anyway you could compensate for it dimensionally other than to go back after it has dried and tighten up lags, nails etc.
Won't shrink in length, but I usually allowed 1/8" when laying 2X6 for decking.
Posts WILL twist when drying unless well fastened at top and bottom. Expect some splitting too. If that bothers you, just veneer it with 1X rough cedar after it's dried for about 6 months.
My posts hold up a header so they're secured at the base (with a standoff atop a concrete pier so no wicking) and a post cap secured to the header.
canoehead2,
the complete answer would scare the pants off you. The short answer is don't worry about it,, thousands of posts are put up every year without a care in the world, you're actually overthinking this.
Now the complete answer is wood air dries at the rate of about 1 inch per year* however since you have them installed in soil which will permantly wick water up thru the ground it will technically never dry.
* actaully 1 inch per year is for wood that is about one inch thick, the thicker the wood the slower the loss of bound water and a 4x4 post might take six years or more to dry out completely.. please do not be confused by the KD stamping on some wood used in construction.. the standard for that is 19% moisture plus or minus 2% which means that untill the wood gets below 12% it's liable to mold or rot..
Do you make this stuff up all by yourself or do you have help coming up with this?" Now the complete answer is wood air dries at the rate of about 1 inch per year"Hoadley: "When materail comes into equilibriuym wieght with the designed evniroment, it's ready. DON'T PAY ANY ATTENTION TO TO GENERALIZATIONS LIKE 'ONE YEAR OF DRYING FOR EVERY INCH OF THICKNESS'. Such rules have no way of accounting for the TREMENDOUS variations in speices characteristics or atmospheric conditions."He says that 4" basswood or pine blanks for carving can dry in less than a year." however since you have them installed in soil which will permantly wick water up thru the ground it will technically never dry."You are making that up also. He never said that it was in ground.But since I know of no place on the earth where people live that atmospheric moisture is zero there is no wood anyplace that is trying dry."please do not be confused by the KD stamping on some wood used in construction.. the standard for that is 19% moisture plus or minus 2% which means that untill the wood gets below 12% it's liable to mold or rot.."Really you must use out of work commedy writers to come up with some of this stuff."Wood Moisture Content (MC) and Decay Conditions: * Optimum decay condition, MC 25%+
* Marginal decay condition, MC 20 - 25%
* No decay, MC 20% or less Fast Facts on Lumber and Mold * Discoloration aside, molds generally have little effect on the structural integrity of framing lumber.
* Mold needs a moisture content (MC) of at least 20% to survive on wood.
* Southern Pine dimensional lumber is typically kiln-dried to maximum 19% MC or less. The moisture content will be identified on the grade stamp. "The Southern Pine Council"Surface moisture on unseasoned framing lumber, appearing as the wood dries, may create conditions for mold growth. However, once the moisture content of the wood falls below 20 percent, mold growth can no longer be supported. Depending on the climate, framing lumber will dry to below 20 percent moisture content during the construction and before the building is enclosed."Western Wood Products Association.PT lumber is KD to 19%. Then when it is treated it retains alot of FREE water. That is not bound into the cells and will dry withing in a few weeks in most weather back to 19%. Then it will slow aclimate.But this is PT lumber. So no problem with mold..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
BillHartmann,
too bad Jon Arno passed away recently, he could provide you with a proper education about wood..
I use my moisture meter to determine wood dryness not some rule of thumb, but for the millions who don't own a moisture meter the one inch per year is as valid a guildline as anything..
You've spent a great deal of time worried about commas and periods..
Can you argue that KD 19 means a moisture content of 19%? plus or minus 2%?
I like little comments that you included which really says what I was attempting to say.. things like depending on climate construction lumber will dry to below 20% moisture content during construction....
"too bad Jon Arno passed away recently, he could provide you with a proper education about wood.."Yes, John Arno was a good resource.But so is Bruce Hoadley.FWW Jan 1976 # 1
...
p 1: Wood; a look at this fundamental material. author: Hoadley
p 12: Water and wood; the problems of a difficult pair. author: Hoadley
p 16: Drying wood; the fundamental considerations. author: Hoadley
...FWW Jan 1977 # 2
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p 8: Measuring moisture; portable meters prevent guesswork and grief. author: Hoadley
...FWW Jan 1979 # 3
...
p 14: Wood has to breath; doesn't it. author: Hoadley
p 16: PEG for the woodworker; what you always wanted to know about polyethylene glycol-1000. author: Hoadley
...FWW Mar 1983 # 39
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p 92: As dries the air, so shrinks the wood; why woodworkers keep a weather eye on relative humidity. author: Hoadley
p 92: Wether, temperature and humidity. author: Hoadley
..."Can you argue that KD 19 means a moisture content of 19%? plus or minus 2%? "KD 19 means that the moisture conetent was 19% when it comes out of the kiln. I have not seen any reference from a know source that it is plus/minus 2%.And in most places by the time that it is installed it will be less. The exception would be PT lumber and lumber stored uncovered during a constant raning season.HOW COME YOU ARE ARN'T DEFINING YOUR COMMENT THAT THE WOOD HAS TO BE LESS THAN 12% TO PREVENT MOLD.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Frenchy,The comment about construction lumber drying to below 20% was regarding green lumber (S-GRN) stuff that is surfaced at the mill above 19% moisture and that can grow superficial mold intially, but then air dries in the open house frame before being closed up.This does not mean the KD lumber may leave the mill above 19%.
Installed in soil!? I'm insulted that you would think that I would do that! ;)
Posts are on a standoff type post base (Simpson) tied to top of a pier with a J bolt. The posts hold up a header and I was wondering if I should have set the header an 1/8" higher than level to account for shrinkage.
In the end I just made it level (did it today). Used my new Hot Shot laser level that I bought yesterday :) Beats a tight mason's line and line level!
canoehead 2
short answer, no! wood doesn't shrink lengthwise enough to make any differance..
--"please do not be confused by the KD stamping on some wood used in construction.. the standard for that is 19% moisture plus or minus 2%"Actually the KD stamp means the lumber was dried to 19% or less...so minus 2% is acceptable, but plus 2% and the mill can get busted.
How tall is your post?
Wood does shrink in length and width, but length is usually ignored (the shrinkage in length is typically 0.05% as the wood dries from saturation--25-30% moisture to your outside EMC of say 15%) In the case of a 8' post it would shrink about 1/16" out of 96".
In width, your post will loose about 3-5% of 5-1/2 or 6" which is something around 1/8" to 1/4."
In short, though your post is about 20 times longer than it is wide, it will still loose more than twice as much width as length.
BTW, you can pay a premium for KDAT (kiln dried after treatment) stamped lumber that is brought back down to 19% or less and will not shrink in any measurable way.
About 6' so given your figure, I'm just fine.
Like someone posted earlier - I'm over thinking this thing.
No sweat...there is plenty of over-thinking going on all the time here at Breaktime...hey I'm the nut that calculated long axis PT lumber shrinkage--so I'm over-thinking right along with you.
Yes, those figures were impressive! :)
Your question and part of my answer (highly edited) made the Breaktime column in the current issue of FHB.
I guess that makes me an expert on...shrinkage.
Cool. I haven't bought an issue in a while.
Your answer was very informative.