Underlayment over diagonal subfloor
I just did exactly the same thing as you about a month ago, only I installed 5/16 inch fir underlayment over the 1X6 fir subfloor.
The original direction of the underlayment was with the top grain of the underlayment parallel to the bathtub, and that was also parallel to the direction of the floor joists. So, the original builders cheated on grain direction, and I did as well when installing the new underlayment. The reason was because the bathtub had a contoured side, and it was simply much easier to put down a 5 foot long and 4 foot wide piece of underlayment to cover most of the floor, and then scribe the contour of the tub into two 12 inch wide pieces to butt up against the tub.
I figured that if that building lasted nearly 50 years with the underlayment going that way, it would last another 50 if I put it the same way.
If you also have a toilet in that bathroom, and it hasn't already been done, now would be a good time to replace the toilet drain pipe with plastic (PVC or ABS). There's a good way of using plywood to replace the 1X6 boards you cut out, and I don't just mean replacing 3/4 inch thick boards with 11/16 inch thick plywood. Also, you need to be careful when cutting these boards because they go under your bathroom walls.
Last edited by Nestor_Kelebay; 08-14-2008 at 11:10 PM.