Tying into cast iron soil stack by drilling hole

[gadolphus32]

I am guessing I'm going to be told this is a horrible idea, but I have to ask...

I am putting a bathroom in my attic. The soil stack pipe that comes up through the attic floor and continue through the roof for venting is 4 inches and cast iron. The soil pipe for my new bathroom is 3 inch PVC.

I'm thinking of tying the PVC into the cast iron by drilling a hole in the cast iron with a 3-1/4 inch hole saw, then sticking the PVC into that.

Part of the reason I thought of doing it this way is that by eliminating the need to cut out a vertical section of the cast iron pipe, I won't have to worry about the top portion of the pipe not being properly supported/falling through the roof. The other reason is that I already have a 3-1/4 inch metal hole saw, I am cheap, and this would save me from having to buy a special reciprocating blade saw to cut the cast iron, or rent a pipe snapper. Also, because of access issues, drilling straight in seems easier than sawing or trying to wrap a snapper chain around the pipe.

Any thoughts on whether this is doable?

 

[hj]

I don't know how many ways there are to say that this is a "dumb idea", but ALL of the would apply to it. Starting with your hole saw would NOT be long enough to make the hole in the first place, and second 3 1/4" is NOT big enough for the pipe. Won't even bother with numbers 3, 4, 5, ,,,,,,,,,,,,,,,,,,,,,,,.

 

[Sponsor]

 

[Master Plumber Mark]

That is pretty dumb, not to mention how were going to seal up this hole after you
stuck a pipe into it>>> silicone or something.....

Really the idea is not up to code in the first place because you are going to tie in a bathroom
into the stack vent for the bath under it... Being 4 inch it would probably work but the down
stairs bathroom will have water passing through the stack with no re-vent for that lower bath

It will probably work , just don't get caught and hope if you ever sell the house that a home
inspector does not catch this little short cut...

 

[Jadnashua]

You can use that for your vent, but you cannot use it for your drain line. You'd want to run the drain down and connect it where the pipe is actually a drain.

 

[gadolphus32]

Thanks for the responses. I'm convinced it's worth the effort to do this one the right way. I just thought I'd ask first in case my shortcut was more common than I thought. Clearly it's not.

The bathroom below this one has a vent that comes up into the attic separately and ties into the main 4-inch stack above where I'd be tying in the drain line. As far as I understand, that makes it OK to tie in the new drain where I am. (I'll vent the new bathroom separately.)

 

[hj]

It only "makes it OK" if the downstairs was piped so it WILL BE okay, and that is not a certainly, since it would be unusual for someone to anticipate you putting a bathroom up there.

 

[Master Plumber Mark]

If you actually have a revent for the downstairs bathroom being tied in above in the attic
then really all you have to do is snap off the cast low enough to install your pipe with decent fall

Being a 4 inch cast stack going out the roof with a revent for the bottom bath
it would be passable

 

[Jadnashua]

How many stories, and what else could that pipe be venting? Say the attic is above the second floor, and that revent comes from the second floor. What about stuff on the first floor or maybe the basement? Essentially, once a pipe becomes a vent, it cannot be used as a drain. Older houses often won't meet current codes, but unless you make changes, would be grandfathered...but, adding new, the parts you touch must be to current codes.

 

[Toptradesman]

Hello, there is some interesting information on this subject here https://www.bbsbuilding.co.uk/how-to-connect-a-plastic-pipe-to-a-cast-iron-soil-stack/

 

[Mr tee]

The approach shown in the above link might be acceptable if you moved your house to the UK. But, ain't gonna fly in NY.

 

[Terry]

The approach shown in the above link might be acceptable if you moved your house to the UK. But, ain't gonna fly in NY.

Yep, maybe UK, but not here.
I did do something like that in a Seattle hotel for condensate while we were adding air conditioning to the Mayflower Park Hotel. That was in the 70's, and I couldn't talk the inspector into crawling between the lobby floor and the second floor to look at my work. The slither space was 14" high with nails pointing down. If you weren't low enough, it would embed nails into your back. I was in that crawl space for six weeks.

Above the ceiling is where I spent a lot of time plumbing condensate lines.