I'm doing a complete gut of a very tiny 3/4 bath (<35 sf). The original (1950s) subfloor consists of 1 x 8 planks placed diagonally to the joists. These were, in turn, covered by 1/2" plywood and, finally, a piece of 1/4" masonite. This combination nicely transitions height-wise to the hardwood floor on the other side of the door. Water has damaged all three layers, de-laminating and rotting the plywood and turning portions of about three of the boards into balsa wood. Only one floor joist shows evidence of (very minor) damage. One wall is an outside wall; it meets a 2 x 6 interior service wall.
My original thought was to remove and replace the damaged portions of the affected planks with new lumber, making sure that my cuts were on a joist and/or supported with new blocking as necessary between the floor joists below. But then I thought, "They don't build floors like this anymore, so maybe I should just cut out the old subfloor planks all the way to the wall plates, put down a new piece of 3/4" plywood and build the floor (backer board & tile) up from there." As an alternative, instead of going wall-to-wall I suppose I could cut the planks just enough to get rid of the damaged sections and put a smaller square of plywood in the space I created.
My concern here is making sure that I pick the option that will maintain structural integrity. In addition, this is a new kind of project for me and I'd like to find out how the pro's would do it.
Can anyone point me in the right direction?