On Fri, 1 Dec 2017 21:03:15 -0800 (PST), Uncle Monster
wrote:

* He'd be better off to use pieces of the plywood under the joints .
Screw some ledger boards to the floor joists to hold the pieces up(flush
with the top edge of the joists) , effectively doubling the thickness
under the toilet . Construction adhesive is your friend . And don't
forget to screw the flange down to the new subfloor . (20 years as a
flooring installer , did my own subfloor repairs 'cuz the carpenters
just didn't understand what was needed .)
* --
* Snag


Perhaps an explanation of "ledger boards" is called for. I had to look it up. I saw something else about "sister boards". ^_^

https://www.thefreedictionary.com/ledger+board

http://www.dictionaryofconstruction....ger-board.html

http://www.concreteconstruction.net/...er-connector_o

http://homeguides.sfgate.com/repair-...ard-43156.html

[8~{} Uncle Floor Monster

Ledger boards are not always for flooring, don't ya know. I cut my
stucco back to the stud frame, hung a 20' 2X10" ledger using 4" lag
bolts - then was on the way for the patio cover. Made a custom closet
storage shelves for the bride with ledger boards.