'treated lumber' used to be copper chromium arsenate
now, it's mostly "ACQ" alkalyne copper quarternary
(not sure of any of the spelling as I'm not Capn Cut n Paste)
the latter supposedly is more health friendly or some bullchit, however it will create a galvanic corrosive situation with dissimilar metal fasteners unless extra heavy hot dipped galvanized are used I think (so many microns thickness)
but....if you're using your treated timbers for rafter applications, you can still special order the CCA treated, so you can put galvalume metal roofing and screw fasteners on
the other situation is with 'ground contact' they used readily sell a choice of .40 rated or .25 rated chemical treatment. I know the .40 was more rated for actual ground contact....but good luck most lumber is marketed to unknowing joe blow the rose bush planting weekend warrior. So now the latter .25 is almost all that's available at the big box lumber giants.
now if you really spec it out, you can order .60 or .80 or more for actual water contact like dock and pier construction
.25 is essentially garbage and that's what the consumer demands nowadays...cheap garbage for their weekend doghouse projects and birdhouses and doo-dads
hope this helps
