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I am finishing a basement bathroom and preparing to lay down vinyl flooring. Before I do that, I wanted to check how my toilet flange would fit sitting on top of the finished floor, and I noticed that the concrete hole is so large that I will not be able to screw into anything under the finished floor when I put it in. What should I do? Should I try to find some material to fill in the space? look at something to put under the flange to keep it stable? I'm guessing that I shouldn't just screw it into the vinyl click-lock flooring I'm going to use for the finished floor...

Rough in hole

Flange sitting in hole

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Pour concrete (or mortar mix) to fill the space and embed your bolts into.

Wrap or otherwise protect the pipe. The UPC says:

312.1 ...Voids around piping passing through concrete floors on the ground shall be sealed.

312.2 Piping in connection with a plumbing system shall be so installed that piping or connections will not be subject to undue strains or stresses, and provisions shall be made for expansion, contraction, and structural settlement. No plumbing piping shall be directly embedded in concrete or masonry....

312.10 Sleeves. Sleeves shall be provided to protect piping through concrete and masonry walls and concrete floors....

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  • Can that concrete butt up against the side of the pipe? Or should I put something in as a spacer? – djohnson10 Feb 27 at 18:36
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    I would wrap the pipe - I used sill seal, but similar packing foam (thin, closed cell) would do as well. I think code calls for some sort of separation, but I don't know that it gets specific about what. – Ecnerwal Feb 27 at 19:37
  • @Ecnerwal why wrap? We don't in our jurisdiction, and I'm curious as to the purpose. – P2000 Feb 28 at 6:26
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    @P2000 see edit. Direct embedment can lead to cracking the pipe if/when the slab cracks or moves, among other things. – Ecnerwal Feb 28 at 13:47
  • Good point. I wonder if this depends on the material of the pipe, and it's less of a problem if ABS. – P2000 Feb 28 at 17:12
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You can fill with concrete or sanded cement, and in our jurisdiction it can be in contact with the pipe (see comment by @ecnerwal)

It's common to leave space around a shower drain pipe in a concrete floor, so that there is some movement in the pipe to adjust for the precise alignment with the shower base drain hole.

Also, a flange that's glued to the pipe, which in turn is cemented in, does not have to be bolted. My plumber did not bolt mine, and it's 100% firm after years.

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It looks like your flange is a 3' internal model (goes into the pipe). This means you can fill the hole around the pipe almost to the top without having an issue with the glue joint. Tape around the pipe with some heavy duty duck tape and fill the hole with Sand mix(no stone). The tape is in case the flange breaks and has to be changed at a later date. The outer pipe surface will remain fairly clean. After that install your flooring and glue your flange down to the floor. Flange should be resting on the new flooring. With a slab floor, I always like to seal around that connection with a little silicone. No other bolts but the toilet flange bolts -aka-Johni bolts required.

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  • "The tape is in case the flange breaks", what do you mean with this? – P2000 Mar 1 at 15:55
  • @P2000 I think Rich is thinking of wedging the flange on instead of gluing it and once you bolt it down it would fine. Maybe they'll pipe in and correct me if I'm wrong. – Micah Montoya Mar 1 at 19:59
  • I've had instances where pvc flanges have cracked by the channel where the bolts attach. I replace the internal flange with one that goes over the 3" pipe I never liked using 3" internals but I do use 4". Tape will preserve the integrity of the outside of the pipe to make it easier to replace it with a new external flange if necessary. I buy pvc flanges with the stainless rims (they bend) and sand mix because it's easier to scrape out for the replacement. Might be making it easier for the next guy but hopefully it was going to be me. Not the only way but the method I like after many jobs – Rich Mar 1 at 22:18
  • @Micah Montoya No I glue them down. I've never screwed a flange down to a concrete floor. I've seen them fail (above) but not due to the glue joint failing. Not talking Johni bolts right? And what is the method of wedging the flange? – Rich Mar 1 at 22:24

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