Our 1968 townhouse has lightweight concrete poured on ~1/2" plywood for the 2nd story subfloor. The old cast iron sanitary lines were removed and replaced with PVC. As as result of the removal (as well as just poor 60s construction), some of the lightweight concrete around the old flange was broken out. I'm now ready to install the new toilet flange and I'm not quite sure what I should anchor it to.

View from above. This would be standing in front of the toilet looking at it.


View from above at a little more shallow angle. A chunk of lightweight concrete is missing from the left side where the thinset is globbed from the tile installation.


View with flange set upside down on pipe. This way you can see the relation of the holes to the missing concrete.


View from below. In this orientation, the big missing chunk of concrete is on the right (where the 2x4 is).



My plan at this point is to just get some Tapcon screws and try to install them in all 6 holes. Is that enough or do I need to drill all the way through the plywood and install some additional blocking underneath to anchor the ring to the floor?

You would be better off, in my opinion, anchoring to the floor. Lightweight concrete is not going to be a reliable anchor evidenced by the piece that broke off.

***edit*** I would also fill the void created by the broken concrete with concrete patch of some sort so the flange sits level.

Could you screw & glue a larger plywood patch under the subfloor that would fit tighter around the drain? Set in that T-nuts that you could then use to bolt the flange to? Also use that as a new base layer to pour in some new cement or grout for the flange to rest against.

When you say anchor to the floor, I'm assuming you mean all the way through to the plywood?

Also, I could probably do what you both suggested without too much trouble. My only question would be, what should I used to fill the void? Is thinset mixed dry enough? Or am I opening up a 40 lb. sack of Quikrete for 2 cups worth?

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When you say anchor to the floor, I'm assuming you mean all the way through to the plywood?

Also, I could probably do what you both suggested without too much trouble. My only question would be, what should I used to fill the void? Is thinset mixed dry enough? Or am I opening up a 40 lb. sack of Quikrete for 2 cups worth?

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Yes, anchor to the subfloor consisting of the existing plywood below the thinset as well as into the additional piece of plywood they recommended that you attach from underneath the current plywood subfloor.

Either thinset or grout will do good as all you are needing to do is get the surface filled in and flat so the flange has a flat surface to sit on when it's secured in place.

Id go with what KENNETH said

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Id go with what KENNETH said

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Add one more for me. That's exactly what I would do.

Thanks for the feedback. I'm planning to do most of what was suggested. There isn't any room (thickness) around most of the drain pipe to fit in T-nuts without having the flange sit too high.

I've got the plywood cut and screwed underneath. The plan is to fill the void with grout up to the bottom of the tile so the flange will sit evenly. Once that is all set-up, I'll drill through the grout and concrete and run screws all the way to the old and new plywood layers to pull it all together.

I would go with kenneth's idea too. Maybe use some Sika Flex or similar around the pipe below the flange. Then go with foam ring seal versus wax seal

It's a good idea, but I hope you aren't expecting to get any mechanical strength out of the grout. It's a good filler material though.

No, I realize that the grout isn't serving any purpose but to be a level-up and keep the metal ring from being completely unsupported in some areas. That's why I'm still planning on running some screws all the way through both the new grout, existing concrete, and into the plywood decking below.

Forgot to update this. I followed the suggestions on here and cut a piece of OSB and glued/screwed it around the drain pipe to block off most of the hole. I had to improvise with a bit of cardboard for the portion on the back side of the pipe opposite the new piece of OSB.

This pic is after the thinset was already poured/placed, but you get the idea.


I wrapped a bit of cardboard around the drain line to try to keep the thinset from bonding to the PVC.


I mixed a bit of thinset to what I thought was a dry enough consistency that it wouldn't flow, but that was not the case.


After the thinset got it's initial set, I pulled the cardboard out. It left a small gap, but not big enough to get the flange around easily.


After letting it set up for about 6-8 hours, I took a screwdriver and scratched away enough thinset around the pipe to give me room to set the flange. I left the thinset to set overnight.

The next day I cut down the drain pipe and glued the flange in place. I used the Tapcon drill bit to drill through the existing concrete sub-floor as well as the new thinset and OSB, followed up with the 1/4" x 3-3/4" Tapcon screws.

Here's the finished product. The flange is solid as a rock, so I'm not worried about anything moving anymore.


Thanks to everyone who offered ideas and opinions!

Got a question...is the toilet not going to be parallel with the floor tile in one direction or the other?

Reason I ask, is the slotted holes don't appear to line up properly to install the toilet parallel to the floor tile. Am I missing something?

Parallel Schmarallel

It's an optical illusion caused by me not taking the picture dead on parallel with the floor tile. Both bolt slots are the same distance from the wall as well as from the grout joint in the tile. Believe me, if the toilet wasn't going to be perpendicular by even 1/4" I would be doing it again. My OCD wouldn't be able to take it.

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It's an optical illusion caused by me not taking the picture dead on parallel with the floor tile. Both bolt slots are the same distance from the wall as well as from the grout joint in the tile. Believe me, if the toilet wasn't going to be perpendicular by even 1/4" I would be doing it again. My OCD wouldn't be able to take it.

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Yet your OCD allowed THAT???

Glad the fix worked out! Looks good!

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Yet your OCD allowed THAT???

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Touche