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I recently bought some pressure treated yellow pine for building some window boxes. After building them, I sealed them with a product that would protect the wood for 3-4 years, according to the can. After building them, they sat outside under a covered porch for about a week until I installed and filled them. When I did, I noticed that the boards are pulling the screws out of the wood. The boards were straight when I bought them but have now begun to curve. Should I have purchased a different type of wood? What could be causing this? The damage is so bad, the boxes may not last the season.

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    Kiln dried after treatment(KDAT) is harder to find and more expensive but will be much less prone to warping and twisting. The wet treated wood sold in big box stores is crap. A 2x6 will shrink as much as 1/2 inch in width after it dries. Another option is to allow the wet stuff to dry out stacked with 1x2 strips separating the boards to allow full drying. May take a few weeks or months depending on humidity where you stack it.. also do not apply any stain or sealer until treated wood is completely dry. – Kris Apr 1 '20 at 15:34
  • I hate to ask, but are you growing food in these boxes? – Aloysius Defenestrate Apr 1 '20 at 17:07
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    The recommendation I received from my nephew who has been doing deck staining/maintenance/repair for a number of years now, is to let the PT dry for at least 3 months before applying any sort of additional coat. – FreeMan Apr 1 '20 at 18:19
  • Our rule is an inch a year (figuring penetration from both sides so 1 year for 2" stock), but that's for hardwood. – Harper - Reinstate Monica Apr 1 '20 at 23:50
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Treated wood doesn't always twist/warp/shrink, but in my experience more often than not it will to some degree. For something small like window planter boxes etc. I would use non-treated lumber and stain or paint with a quality exterior grade product. Obviously if the project is a deck or similar structure subjected to the inevitable wrath of Mother Nature then you should always use PT. Or as Kris suggested you could elevate and stack PT lumber to dry under controlled circumstances to mitigate warping.

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Is it normal for pressure-treated lumber to warp?

If it is from Home Depot and it doesn't warp then that is not normal.

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