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I'm redoing my bathroom and had to replace a toilet flange. The old one was rusted cast iron whose flanges had all but disintegrated, and I ground down what was left of it with an angle grinder. It was fastened directly to the pipe, not the subfloor--which is concrete (house is on a slab). Here's what the result looks like:

enter image description here

Then I bought a new flange that fits inside the existing pipe. Unfortunately I haven't found a good way to secure it to the subfloor, which is old (very hard) concrete. I tried Tapcons with little success. The problem is the angle of the concrete I need to drill into: enter image description here

The walls of the existing concrete are so steep that it's almost impossible to drill pilot holes.

So what should I do? I've contemplated pouring some new concrete to fill the void and embed threaded rods in it to which I can fasten nuts though the flange, but I worry that it won't adhere well to the old concrete and tightening the nuts will just pull it loose.

What do you guys think?

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  • In the days when we used cast pipe we would fill the void with lead and use brass wood screws for a case like this. Filling with concrete may work but that's not much material it may break up. I would consider epoxy.
    – Ed Beal
    Feb 7, 2017 at 0:09
  • The go-to solution for drilling a cone like that would be to drill through a block of wood laid across the top. I'm partial to the epoxy idea.
    – Matthew Gauthier
    Feb 7, 2017 at 4:49
  • No room for wood, I'm afraid. Also there's nothing to anchor the wood to without drilling into concrete, which is the original problem.
    – iLikeDirt
    Feb 7, 2017 at 4:54
  • I would also vote for the epoxy idea. Clean out the area in the gap as really well as you can. Include getting all the scale and corrosion off the outside of the pipe as well. If you do this cleverly you should be able to mount bolt studs from the new flange that extend down into the epoxy. It may be necessary to chisel out some of the existing concrete where the embedded studs will hang into the epoxy. Make sure to use studs that are not likely to rust any time soon. One choice would be to select a top grade stainless steel product.
    – Michael Karas ♦
    Feb 7, 2017 at 5:00
  • You misunderstand. I mean you put a 2x4 on the floor, put a knee on it, then drill straight down with the hole in the wood keeping the bit straight. Cheapo portable drill-press. (Alternately you drill the wood first, then put it down so you can see where the bit is.) Also, since you have an angle grinder a masonry cutoff wheel might be preferable to a chisel if you can fit it. Less chance of shattering the floor you want to keep.
    – Matthew Gauthier
    Feb 7, 2017 at 19:09

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I would drill out and break out more concrete to develop a hole that would hold a new concrete plug/doughnut around the pipe, rather than the difficult pointy cone you have now.

So, drill from the flat surface you have, lots of holes, and chisel carefully between holes.

My personal experience with Tapcons and "hard concrete" is abysmal. In green (new, fresh) concrete they seem to work OK most of the time.

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  • That worked perfectly! I embedded nuts deep in the new concrete and fastened the flange down with machine bolts.
    – iLikeDirt
    Feb 27, 2017 at 22:05

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