I want to run a new 2"kitchen drain. In the joist bay the first thing I come to before the stack is the tiolet drain. About 18" from the main stack. It would be a lot easier to just cut into the tiolet drain. Is there any reason I shouln't do this?

Hi August

Good question.

As nice as it would seem, you shouldn't cut into the toilet drain pipe as you have described because it would effectively block the toilet vent which could allow sewer gasses into the house every time you empty a sink load of dishwater, run the disposal or the dishwasher empties. Here, the waste water from the sink travels past the toilet drain and because the toilet is now "unvented" siphoning of the drain can happen resulting in a burping toilet and the result as explained above.

Cut the kitchen sink drain in below the toilet connection at the stack if possible and don't forget to vent your kitchen sink or you could have similar issues as mentioned for the unvented toilet at your sink, OK?

More questions? Let me know...

Mark