In preparation for ceramic flooring in the bathroom, I am replacing the subfloor. I pulled out all old material down to the floor joists and have purchased 3/4" treated plywood, thinset and Durock to go over that. Joists are 2 x 8, 16" O.C. The problem is that the bathroom, which is only 5' wide, happens to be over an area where two 13' spans meet (supported by cinder block wall in basement & attached to each other with a foot long 2 x 8). As you can imagine in a 58 year old house, the joists slope away from the support a little in each direction. I know that the floor doesn't have to be level, but it needs to be flat. I was thinking of sistering 2 x 8's alongside the joists on the short section (2') to put them on the same plane as the 3' section, which slopes less. If I do this, the sistered boards would end up about 5/16" above the existing joists at the wall. Or, I could sister along the entire 5' span and make it level, which would have the sistered joists above the existing ones on both sides. Suggestions?

Also, 12d, 16d, or wood screws?

 

While I'm sure you'll get suggestions to re-level the entire section... I kinda like your DIY plan.

Use wood screws.... construction adhesive between the layers.... a layer of 1/2 plywood glued between the layers would be even better as a stiffner.....