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I am in the planning stages of a bath remodel. We are doing away with a walk in closet and adding a whirlpool type tub (Kohler K 1244-F). The installation notes of a related tub (K-1144) specifically states “Unless otherwise specified, floor support under whirlpool must provide for a minimum of 80 lbs./square foot loading.” I am looking to install it with an “overkill” for the load rating on the floor.

The floor I am building this bath on is 2×8 16” OC with approx 14 span (some type of fur, maybe SYP, but I don’t know softwood identies very well especially in a dark ceiling). I am planning on mounting the tub on a 6” platform where the base of the platform will run 90 degrees to the joists (if that matters).

From what I can tell from the web and doing some math (maybe not correctly), it appears that I will have to beef these things up to handle the load. Is that the case? If so, how much more beefy do they have to be?

Beefing up the joist would require the floor in the master bedroom to be opened and…, and …

Well, it would not be a DIY job in any case.

So can someone either explain the math or point me to a table to figure out the correct loading? Would it be as simple (maybe not so simple) as opening the floor and installing a sister or a second joist at 8” OC?

Thanks
Larry