I'm installing a fiberglass whirlpool tub. The directions specifically say the tub is designed to be held up by the outside of the tub not the floor. I've installed 2x6 ledger board on the long wall, a 2x4 on the rear wall and a 2x2 on the front wall. It just doesn't seem to be enough to support a tub that'll hold 75 gallons of water. I only used smaller boards because the motor or overflow were in the way so I couldn't use wider boards.