Closet Flange

G'day. In installing a new toilet I accidentally popped the closet flange off of the 3" copper riser. I replaced the flange with a multi-purpose PVC replacement flange that slipped into the riser. (the replacement riser was the type that had a 6" tail-piece with a gasket on the outside of it).

I have an issue in that the sub-floor doesn't go underneith the flange tight to the riser. Rather it was cut around the peremeter of the old flange, therefore, i do not have any thing to fasten the replacement flange to.

So now I have a toilet that is essentially held in place by gravity. It will wobble and can twist around on its axis.

Can someone please guide me on how to remedy this? My thought was to use thin-set mortar to set the toilet in place. will this work? Should I use a different type of mortar or a different solution?

Thanks.

an idea from someone who knows no better

The flange should be properly attached. That's the only way, in my non-professional opinion. Particularly with your repair flange in there as you don't even have a solid connection to the riser, let alone to the floor.

I once had a similar situation. I cut blocks of 2x3 lumber glued and screwed them underneath to bridge the hole beside the riser. Then cut a piece of plywood to fit around the riser to fill the gap and glued and screwed that in place. Then when the flange went on I screwed it down into the 2x3 blocks.

Can you access the area from below? That would make it easier. In my case I had to do everything from above and it was a challenge.

We sometimes use these to hold the closet bolts.
They have a pattern for the hold down screws a little farther out.

Wow, this really ruined my Sunday. I just finished! I cut 2 2x3's into 14" lengths, fed them down into the joist area, and glued them into place using PL. After I gave it a few hours to dry I used 3" deck screws to secure the flange in place. I reinstalled the toilet in place and it is now firmly secure. To make sure i had my wife stand on the edge of the bowl and do a 2-1/2 back flip with a 1/2 twist. A perfect 10.

thanks for your help, Gang.