"Art" wrote:

We are pulling out a Jacuzzi brand whirlpool and replacing it with a
Sanijet. According to Jacuzzi, they require floor strength of about 55 lbs
per sq feet. Surprisingly, Sanijet wants 100 lbs per sq feet. I am looking
for ideas on strengthening the floor. I have access from underneath. I
plan on adding an additional piece of sheet rock on top and screwing it to
the joists.

On a floor, I'd strongly suggest an additional layer of PLYWOOD, not
sheet rock (aka wall board), at least 3/4" thick.