OK to grind coupling flange on stack?

[Tony Paille]

I am doing a partial remodel of a bathroom for a friend. Replacing toilet and sink, and drywall behind them. I am not a plumber. We have removed the old drywall, and I have found furring behind only a portion of the wall behind the toilet. This is to work around the fact that the flange for the coupling of the stack pipes protrudes beyond the studs by about 1/4" (I may not have exactly the right terminology). The stack is 4" black pipe. I have a similar type of stack at my place that only goes halfway up, so I can see the inside of that coupling flange, and it looks like there is about 1/2" of metal on that flange. I am wondering if it is acceptable to grind down that flange so it no longer protrudes from the studs? Practically, this will mean an up-to 1/4" removal of metal across a width that is less than 1 inch. The stack is fairly plumb, but this is a two-story house, and the bathroom in question is on the main floor, so there is a lot of pipe above that location. I mention plumb because it does not seem like there would be much horizontal load on the coupling. I realize that grinding pipe is not ideal, but the alternative is to furr out the entire wall, and I would rather avoid that if I can. I believe, if I have my terminology correct, that this is a soil stack, as it continues down into the basement floor.

 

[Ars Glonalin]

I would not grind the coupling flange. That you asked the question is a good indication of the doubt you reasonably have. When it starts to leak it will begin as a seep that may never be a large volume, but, as the place settles more, it could begin to leak sewer gas too. You could taper the furring as you get further from the spot. Also could carve out the drywall a little where it covers the flange

 

[JG plumbing]

I came upon this situation recently with my new house. I did what was said above. Carved out the back of the drywall. It's really the best option. It's fairly common.

 

[Ars Glonalin]

One other try - in a bathroom I did I put up two layers of greenboard under the new tile. Around the tub it had cement board with light construction adhesive over a layer of greenboard. (I did this largely for soundproofing the bath which was in the middle of a ranch house, and it helped a lot.) There was a little furring in a few spots, but the riser pipe was pvc and joints within the wall fit in pretty well due to having older (1940's) studs, which are larger, though I had to carve the greenboard at a y-joint a little. With 2 layers of wallboard/greenboard, gaining a 1/4" is even easier, and there's probably no need to fir it out.

In your installation, is the cast iron so well supported above and below that you could replace the pipe beside the bath? Be careful as that system is designed to transfer the weight of the verticals, at least, downd the column....

 

[Tony Paille]

I would not grind the coupling flange. That you asked the question is a good indication of the doubt you reasonably have. When it starts to leak it will begin as a seep that may never be a large volume, but, as the place settles more, it could begin to leak sewer gas too. You could taper the furring as you get further from the spot. Also could carve out the drywall a little where it covers the flange

Good advice there, in terms of creating a thin hollow in the drywall for the protrusion. I think it would have been OK to grind it, but I am sure it would not pass code, and it is the poorest option really. Turns out the black pipe only goes to the ceiling of the first floor, and looks to be ABS above that, so the weight wouldn't really have been an issue. But, as suggested, not modifying the stack is the best option. Thanks for everyone's input. Seems to be clear sailing going forward.