Even though the floor has a definite slope, I figured I had to replace some bad boards in the subfloor. Here are the boards I’ve replaced so far. I probably need to do one more, but I think I’ll use a 1″x8″ instead of a 1″x6″, since there’s a bigger hole to fill.

The edge of Durock toward the bottom of the picture is at the point where the floor starts to slope down. It’s about a 1″ drop total.