Large hole around toilet drain pipe - Help!

[Mike1987]

So we had a toilet that started leaking. My house was built in the early 60s, so our waste line was not surprisingly 4" cast iron. I took the toilet off, and the cast iron flange had deteriorated. I cut it off and got rid of the lead and such. However, when I went to install the new oatey "cast iron closet flange replacement" my screws just would not grab. Apparently around the waste pipe, whoever did the bathroom remodel for the previous homeowner cut a huge hole in the upper layer of the subfloor. They just filled it in with what I think was grout from the tile. I removed that grout as it too was deteriorating. I included a picture of what the pipe currently looks like. The lower layer of the subfloor immediately around the pipe isn't in great shape. It's not crumbling, but it's not strong.

So how do I handle this? My thought was to cut some 3/4" plywood and fit that around the pipe from beneath (in my basement) and screw the two plywood pieces up into the subfloor. Then I was going to cut some shims from a treated 2x4 so that that the tops of the shims would be flush with the tile. I'd screw those shims to the subfloor. Finally I was going to install a Big Sioux closet flang floor support to rest on the shims and tile before installing the oatey cast iron closet flange replacement. Will that work?

Or would be easier to simply fill in the area around the waste pipe with something (like concrete)? Please let me know you guys' thoughts! Thanks.

 

[Breplum]

I might use a 4 x 2 insta set closet flange and use plywood underneath for attachment, either long 1/4x20 stainless steel all-thread or whatever length GRK non corroding screws (like for P.T. wood)