We build decks in the Midwest, where code requires 32 inch deep footing holes. After calling JULIE (our state-wide dig-safe program) to check , digging our holes, cleaning them up, and tamping the bottoms, we fill the holes approximately half way up with concrete. The next day, after the concrete sets up, we come back and frame the outside perimeter of the deck, placing our 6x6 support posts loosely in the holes without backfilling. That way, we can square up the deck by taking diagonal measurements and adjusting the position of the corner posts - which are simply resting on the footings at this point - as necessary. Once we're satisfied that the frame is square, we wrap the bottoms of the posts with self-adhesive membrane and mix up enough concrete to finish filling up the holes. Then we can go ahead and start installing the deck joists.
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