I'm replacing some rotted subfloor around a toilet, and I want to add blocking around the flange for extra support, and to give me a place to support the new pieces of subfloor (I'm not planning on replacing the flange). What complicates things is a pipe running parallel to the joists just an inch or so beneath the subfloor.
My thought was to first fill out the joist webbing (yellow) and then build a box (white) to surround the plumbing (red). The horizontal beams perpendicular to the joists could only be as thick as the joist flange due to the upper pipe.
Would this approach provide decent support for the replacement floor and the toilet? If so, what kind of materials would be ideal to use?