Large Shrinkage of Treated Wood

[Rich Riddle]

I am remodeling the farm and installing a bench seat in the shower. To offer better security, I utilized solid wood to secure the seat. After cutting the 2 x 12, which were "dry", and waiting as suggested, here is the result of the real wood drying. The gap is huge but not important for it's purpose. Wow, does this stuff shrink.

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[Sean Tracey]

They make KDAT, Kiln dried after treatment. It is probably more expensive than regular pressure treated. I don't know how much it shrinks, but I would expect it would be less than wood that is treated but not Kiln dried afterwards.

That said, wood will move across the grain. I don't know that you will ever stop movement by mounting wide boards the way your picture shows. 2x4 Nailers could be mounted horizontally between the studs (1 1/2" side faces camera) where you want to put your fasteners. You can also double them up. That way, cross grain wood movement is not an issue. Not knowing what kind of bench or fastening method will be used, I am not sure if what I said will work, it is just a suggestion of possible wood arrangement to avoid or reduce shrinkage problems.

The way the boards are mounted now, any shrinkage or expansion in the wide boards will cause the studs to move.

If you need something wider than two 2x4's, You could mount wider doubled up 2x material so the face faces the camera and the cut ends (like a header) are butted between the studs. The expansion across the width will not move the studs but will go up and down.

[Mac McQuinn]

Using treated wood for an interior project is different than outside. Humidity levels inside are much lower which really drys and shrinks the wood. Even when using boards this wide outside, if you space them with a brad (deck), the gaps will enlarge due to the wood shrinking, perhaps to a 1/4" over time. My guess is the evaporation of the chemicals in the wood also add to this shrinkage.
Mac

[Rich Riddle]

The reason it's designed that way is because the pressure treated wood is screwed directly into the foundation. I didn't design it that way, and the size constraints didn't allow for any variance. Consequently, I feared a horizontal piece might not hold up, literally. Someone would be in a bit of trouble if the wife was seated on a shower seat and it broke free from the wall.

[John K Jordan]

Yes! I learned 40 years ago not to use careful joinery techniques on pressure treated wood. Most of it is so wet and as you say, shrinks like crazy. Twists and warps too if insufficiently unrestrained. I've used a lot of 2x6 PT for deck boards and often ended up with 1/2" gaps. No problem in that case, either - large gaps between decking boards are actually better since they tend not to get packed with debris.

Also, fasteners tend to loosen as the wood dries. For structural support I like to use bolts where possible, otherwise lag screws into properly sized holes.

When buying just a few boards from a place that lets you pick through them, I might look for the lighter weight pieces. Sometimes the pieces that were on the top of the bundle are dryer.

JKJ

[Sean Tracey]

The reason it's designed that way is because the pressure treated wood is screwed directly into the foundation. I didn't design it that way, and the size constraints didn't allow for any variance. Consequently, I feared a horizontal piece might not hold up, literally. Someone would be in a bit of trouble if the wife was seated on a shower seat and it broke free from the wall.

A door header would be supported by jack studs that go all the way to the foundation and are also attached to the studs next to them which I think they call king studs. The jack studs would also give additional fastening surface on their own.

I inititially thought the wide boards were attached to the studs along the edge of the wide board which is why I thought the board would continue to move the studs due to expansion and contraction of the wide boards. This would be bad as any tile backer attached to the studs would be affected and could crack.

Perhaps the wide boards aren't fastened that way and the effect of expansion and contraction won't affect the studs.

[Danny Hamsley]

Pressure treated pine comes out of the pressure treatment cylinders sopping wet. Just like green lumber sawn from a fresh log.