Hi Bob:
If you have the bath’s floor joists exposed, I would definitely sister on another 2X8, as you are adding about 1,200 pounds to this area with bathtub, water and people. Also, I would preload the joist. Ok, what does that mean?…….
Cut the joist to length (13′?) and put up against the existing joist, crown up (with the natural bow of the board facing upward) to make sure it fits. Measure to the center of the top of the old joist and put in a #8 or #10 wood screw (not drywall screw) so that only 3/4″ of the head is sticking up. Put construction adhesive on the new sister joist and put up against the old joist. Then with a 4′ (or so) bar, put the nail pulling end under the head of the screw and lever down on the new joist until the tops of the two joists are even. Hold this while someone nails, or better, screws the two joists together. This will lift the old joist while pre loading the new joist, and it ensures the tops are flush before putting down the sub floor. Be sure and take the screw out when you are done.
Hope this helps
Bruce M
