The plumber broke up some of the slab, removed an old cast iron Y and put in a new elbow and closet flange. He left back filling and patching the slab to me (my choice). The edge of the hole is irregular and chiseled at an angle.

My question is what material do I back fill with? Sand, pea stones, cracked gravel? My first thought is tamped sand to support the new piping. How deep do I then want to pour concrete? 4 inches? 6 inches?

The hole is about 2' x 3'. The slab is below grade - there is just dirt below there. The house is 100 years old.

  • Is the slab on the ground? Or is there open space below it (like another floor or crawlspace)?
    – wallyk
    Jan 10, 2015 at 5:40
  • @wallyk It is on the ground - there is just dirt under there. The house is quite old.
    – Steven
    Jan 10, 2015 at 5:48

1 Answer 1


This is what I ended up doing after some research and opinion asking:

Chisel all the loose concrete edges so there is an undercut. This prevents the patch in the slab from rising. Backfill with sand. As you put the sand in moisten it and tamp it down. Make sure the piping is well supported and that the closet flange is level. Keep adding sand until it matches just below the bottom of the existing slab. Mix concrete and pour into the hole, backing it under the undercut and ensuring no voids exist. Keep filling until it is slightly higher than the hole and then screed it off.

So far it looks okay although I think my concrete could have been a little wetter.

  • That is about how it is done for the most part. I would perhaps drilled a few steel pins into the edge of the cut before I poured the concrete. Then trowel it to get a smooth top after the initial screed.
    – Jack
    Jan 10, 2015 at 23:13
  • @Jack I thought about the pins but read that thinner slabs might be weakened by them so opted to skip. Trowelled three times at various stages and now it looks ready for subfloor and tile.
    – Steven
    Jan 11, 2015 at 0:23
  • Sounds like you got it, thumbs up!
    – Jack
    Jan 11, 2015 at 2:56

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