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I used 6 - 6x6 posts with dounble 2x10 beams.You really need the deck to hold 100+ PSF, maybe closer to 125 to 150 PSF depending on the gallons that the tub holds, the weight of the empty tub and the # of people the tub can hold. Add possibly a snow load and you got a pretty brrfed up support system. My tub's deck probably can hold over 150 PSF but I have no worries about it. My deck is about 19 to 23" high and sits lower than the main deck.MAke sure you don't skimp, it's too much weight to be fooling around with.Here's some photoa:
Our inspector would of required you to notch thoses 6 bys and set those beams in the notch and then bolt.