Bizarre warpage of PT 4x4s waiting in the shop.

[dbhost]

Okay this is a bit odd. This past spring, I picked up a bunch of lumber for a fence replacement project. Not enough to do the sides and the back, but enough for the sides and a few extra pieces..

One of the 2 remaining pressure treated 4x4 8 foot posts has gotten a warp in it that curves the end around about 22.5 degrees. A couple of the 2x4s are also getting this way.

I am assuming this is a humidity problem. How would I go about fixing this?

[LarryG]

That's not odd at all. That is standard operating procedure for PT material as all the excess moisture introduced in the treatment process dries. If you don't tie the stuff down as soon as you buy it, it's going to crawl all over the place -- and even when nailed or bolted down it's still going to move as much as it can.

You fix it by buying another piece.

[cgallery]

If you don't tie the stuff down as soon as you buy it, it's going to crawl all over the place -- and even when nailed or bolted down it's still going to move as much as it can.

You fix it by buying another piece.

So, here is a hypothetical question (sorry).

If the same board that twisted 22.5-degrees were to be clamped in such a way that it couldn't twist while sitting, and left for let's say a year or more before being unclamped, would it immediately twist? Would it eventually twist to 22.5-degrees, too? Or would the piece split or check badly while it was sitting clamped for the year?

I guess the question is, if the wood twists 22.5 degrees unclamped, is there really any way to prevent it from doing so over the long run?

[mschrank]

I guess the question is, if the wood twists 22.5 degrees unclamped, is there really any way to prevent it from doing so over the long run?

I don't think so. If wood wants to move, it's going to move. If it's clamped, nailed, screwed, or bolted in place, it will likely split from the stress of trying to twist.

PT is especially bad not only for the reason Larry mentioned, but also because mills use some of the worst wood for PT. Around these parts, it's usually some kind of true fir (not Douglas-fir) that is resinsous, knotty and doesn't have more than an inch or two of straight grain.

[LarryG]

If the same board that twisted 22.5-degrees were to be clamped in such a way that it couldn't twist while sitting, and left for let's say a year or more before being unclamped, would it immediately twist?

Good question. I don't think (heavy emphasis on "think") it would immediately twist; and if it's dry enough when unclamped, it might not twist at all, immediately or ever (normal wood movement aside). When air drying green lumber, you stack it flat in order to get it to dry flat -- the fibers in green lumber being considerably more pliable than those in dried lumber. If the PT board were restrained until the grossly excessive moisture from the treatment process had dried out of it, the board should come out of the clamps pretty close to the same shape as it was originally milled. If it's back to some reasonable level of moisture content, and in equilibrium with its environment, from then on it should be subject to the same amount of movement as any other piece of wood.

But that presumes that the one year in your hypothetical situation is long enough for the board to reach equilibrium with its environment. For a 1x6 deck board, a year should be enough. For a 4x4, it almost certainly wouldn't be. So I would guess that a piece that size would come out fairly straight but would again gradually twist over time, although probably not to the same 22.5 deg extreme.

Wood fares worst when its moisture content changes most rapidly, such as when it gets rained on and then dries under the hot sun. Since PT material is by definition used mainly outdoors, I have always assumed that it is the sun's baking out all the moisture that causes it to warp and check as badly as it does. That, and what Mike says ... the lumber used for PT is often pretty crappy stuff.

I need to break down and buy Bruce Hoadley's "Understanding Wood." It would be interesting to know what he has to say about all this.

[cgallery]

I need to break down and buy Bruce Hoadley's "Understanding Wood." It would be interesting to know what he has to say about all this.

Good idea, I can't wait to read your report.

[Uncle Cracker]

Do not tie the boards down! They will whip and throw your house across the street!

[cabinetman]

Do not tie the boards down! They will whip and throw your house across the street!

You're not far from the truth. Thinking that if you tie down PT lumber when you buy it that it may prevent twisting or warping is a pipe dream. That just won't work. PT lumber and any lumber for that matter will react to its conditions, such as one side or edge being damp and drying at a different rates.

The most you can do is try to provide an even drying of all the sides. Even with that there is no guarantee. IMO, analyzing the specific pieces and trying to determine how to tie it or clamp it and for how long is also false hope.
.

[LarryG]

Thinking that if you tie down PT lumber when you buy it that it may prevent twisting or warping is a pipe dream.

I keep forgetting that there are people in this world who just can't grasp the subtleties of a metaphor.

You might want to read my original post again, "cabinetman." Slowly. Carefully.

[Ed62]

I built a front porch about 7 or 8 years ago, using PT lumber. I used 2X8s for the sub fascia, and bolted them down to 4X4 PT posts because of likelyhood of twist. Maybe I got lucky, but there is no noticeable twist or other movement. If I had to do it again, I'd probably do it the same way.

Ed

[billfrommich]

About a month ago I bought some lumber from a big box store for a partition wall in my basement. From past experience I know how quickly it can start to deform once it's taken off the "bundle". As I was not sure I could use it right away I decided to clamp it to keep it straight until immediately before assembly in the partition. Notice in the picture below I added spacers between the boards so that they could be exposed to the atmosphere from all sides (you might also notice I'm trying out my bargain Stanley clamps from Big Lots). Well, luckily I clamped it since I still haven't gotten around to using it. The treated wood originally was a soft, soggy dark green - it has now dried (at least on the outside) to a moderately hard pastel green. If and when I use this wood, I'll post a followup, but I don't intend to "unclamp" it until immediately before use.

[cabinetman]

I keep forgetting that there are people in this world who just can't grasp the subtleties of a metaphor.

You might want to read my original post again, "cabinetman." Slowly. Carefully.

I keep forgetting that there are people in this world who just create poorly constructed metaphors. I read at a fairly fast rate. In fact last week I had a speed reading accident...I ran into a bookmark. Even with my regular reading rate, I pick up on metaphors and inferred sarcasm "LarryG"...believe it or not.
.

[billfrommich]

As a followup to my earlier posting I have some additional information of interest. When I first clamped the treated and untreated lumber shown in my attached photo, I tightened the clamps up to the limit of my hand/arm strength. I just checked up on the clamps - the clamps on the untreated lumber were still tight, whereas the clamps on the treated lumber had loosened (due to moisture loss) to the point where I could give them at least another complete turn of the clamp handle (movementof about 1/5" inches, judging the clamp thread pitch by eye). I guess I should have been checking up on the clamp tightness periodically all along.