I have a basement I want to finish were the plumbing is roughed in. They have 4 drain pipes in a row. One is 3 inches, two are two inches and one is one and a half inch. I figure one is for a shower , one is for a sink one is for the toilet, but what is the one and a half inch pipe for?

I have a basement I want to finish were the plumbing is roughed in. They have 4 drain pipes in a row. One is 3 inches, two are two inches and one is one and a half inch. I figure one is for a shower , one is for a sink one is for the toilet, but what is the one and a half inch pipe for?

Good question. Can you contact the plumbing company that did the job and ask them? I would say it's some sort of a vent except that we use the lavatory vent to service the group. But perhaps the plumber didn't have a choice so until you hear different I'm going to go along with a vent. Regards, tom