Height above grade for PT...?

Howdy,
I am building something (a covered bench) that will sit out in a meadow.
The lower material will be PT, the upper material is pine milled from our trees. How high up should I have the PT in order to be reasonably certain that I can avoid rot in the lower area for, say, 10 years?
Thanks for any tips,
--
Kenneth

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I would make the PT reach up to the point where the paint begins.
--

Larry Wasserman Baltimore, Maryland
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Isolation versus some absolute height would be preferable. Water will wick up whatever's in the ground, and there will be some splash. So if you isolate the untreated pine with some sort of impermeable barrier, say poly U glue on both surfaces and a tenon into the treated, you'll get the best result.
Note the principle in use with metal brackets to hold 4x4's above concrete.

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