RicodJour wrote:
On Jun 4, 10:36 am, wrote:
....
Does anyone know why the treated wood warps so much?

Because it's soaked with preservative chemicals, and all dimensional
lumber deforms as it dries. Treated lumber just has a lot more drying
to do and a lot bigger potential for deformation.

Not a disagreement, but some amplification (and not really so much for
you Rico as for reinforcement for OP)...

It's not nearly as much to do w/ the chemicals per se as it is simply
the water bearing the chemicals required to perform the treatment.

ALL lumber (dimensional or not) is subject to warp/bow as it dries.
The amount and general susceptibility is dependent on the particular
piece of wood (growth characteristics, sawing, etc.) within general
trends of the wood species itself.

It is correct that treated lumber as received is generally far wetter
than standard construction-grade lumber and so the drying process is
more of an issue. As noted in previous post, while not commonly
found, kiln-dried PT lumber is available (for a price, of course)...