Deck Under a Hot Tub

100 Ib/fF is a typical code requirement (such as by ICC-ES) for uniformly distributed live loads, including those for decks. This corresponds to a rather low center-point load, which can be calculated using formula (7.42), given above  

For a 100 Ib/fF (0.694 psi), a corresponding center-point load, resulting in the same outer fiber stress, equals to only 31 lb. This load is 20-25 times lower than a break load at midspan for a typical composite deck board. Hence, deflection of 

The main (potential) problem with a hot tub on a composite deck is not a deck deflection or a flexural strength of deck boards but a (potential) creep of a plastic-based composite material, particularly in the South. This can lead to a partial sinking of a hot tub into the deck. Of course, joists would eventually stop the sinking even in a worst case scenario however, this might leave a permanent dent on the deck of a size of the hot tub base. Warranty would not cover the damage. 

As far as the author knows, there have been no cases of sinking of a hot tub into GeoDeck deck from the weight and heat, although it could and probably would happen. The best way to go would be to place the hot tub directly on top of the joists and build the deck around it. 

Another concern regarding placing a hot tub on a composite deck is a possibility of an accidentally breaking a board by impact, directed— by a not careful unloading of the hot tub—between joists. 

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Deflection of a deck under a hot tub, 279 Deflection of a deck under, 290 Deflection of a fence board, 287 Deflection of a hollow deck board filled with hot water, 290... 

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