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When I replaced a toilet, I discovered that my toilet flange was not actually secured to my concrete subfloor at all and had come out of level, causing a leak between the bowl and the floor on the side that sunk beneath the finished floor. I removed the old flange and discovered that the original builder likely used too much padding around my 4" PVC waste pipe when they poured the slab, leaving approximately 3/4" distance between the waste pipe and the slab. The gap is much wider than that at the top of the pipe where the flange would be secured, but then narrows considerably. Any flange won't be able to rest on surrounding subfloor without pouring new concrete down there.

I plan to wrap the pipe to prevent anything from entering, then pour some sand as backfill down there and ultimately finish it with at least 4-6" of concrete at the top to bring it level with the finished floor (like these videos: https://www.youtube.com/watch?v=Szlhjn1Hfe0, https://www.youtube.com/watch?v=XKxYSOoiGe8, https://www.youtube.com/watch?v=LzPmmN6DAZY)

The problem I see is that the crevice is deeper than in those vids and it becomes so narrow, that if my concrete mix is at its ideal consistency, it's doubtful that I'll be able to work it into the crevice without significant air pockets. Ideally, I'd like something relatively soupy that can just be poured down there, but I realize that a soupy/pourable consistency may compromise the integrity of the mounting surface.

Quikrete makes a product (see vid here) that can be dry poured, which I think may be a good option, but I'm concerned that it could expand while curing and crack my waste pipe.

Whatever I use, I plan to then secure the new flange to the surface using Tapcons.

What's the best way to fill in a gap like this?

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Anchoring cement is designed to be pourable and, while not a great finished surface with respect to wear durability, should secure your plumbing with excellence and aplomb.

  • Awesome, this is what I was looking for! My only concern with using something like anchoring cement is that it could damage my PVC waste pipe while it expands during curing. From what I've read, most anchoring cement expands when it cures. Thoughts on that? – cfx Jan 10 at 21:48
  • Expansion is minimal--just enough to snug up any slop in the system. I don't consider it a concern. You could always install it in several "lifts" of 2 inches or so and watch how it does. – isherwood Jan 10 at 22:01
  • Thank you very much, will definitely give that a shot. Final quick question: I assume that since it hardens tougher than traditional concrete, Tapcons will work well to fasten the flange into the newly poured anchor cement? – cfx Jan 11 at 0:35
  • I don't see why not. You could also plunk any corrosion-resistant fastener into the uncured cement. – isherwood Jan 11 at 0:36
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Some builders always leave a space aground the waste pipe, I know I used to use Styrofoam plugs so I had a perfect finish around the pipe after the concrete was set I used a hammer to pull the Styrofoam and then set the flange, at this time I usually filled the void with hydraulic cement it won't crack the pipe and helps secure and seal the floor.

  • Right, but this builder left way too much space, or the plumber who did the initial install chipped away too much concrete so that the flange's holes don't line up with any substrate whatsoever, leaving the flange supported entirely by the waste pipe. Either way, during my pour I'll be lining the pipe with styrofoam to leave a gap. Also, whatever I use will need to harden enough for me to use Tapcons to secure the flange once it hardens. – cfx Jan 10 at 15:50

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