Quick question: We are remodeling the master bathroom and need to move the toilet, sink and shower. Right now the toilet is on a dedicated drain line and the sink and shower share one. I'm will put in a P trap to use the toilet drain as the new shower drain. Can I combine the new sink drain and the toilet into one shared drain?

Quick question: We are remodeling the master bathroom and need to move the toilet, sink and shower. Right now the toilet is on a dedicated drain line and the sink and shower share one. I'm will put in a P trap to use the toilet drain as the new shower drain. Can I combine the new sink drain and the toilet into one shared drain?

Nope! Not the way you explain it. Your toilet's on a 3 or 4 inch branch. I guess you could reduce that down to 2 inches to pick up the shower but I don't hear any mnention about the toilet having a dedicated vent so how's the shower going to vent? Since you're going to tear up the plumbing any way why not do it right the first time. Let me show you how.
Most bathroom groups are roughed in like this.
Toilet connects to sewer main or the stack vent. Lavatory connects to toilet drain and runs a vent off the top the stubout tee out the roof or revents back into a dry vent in the attic.. The toilet wet vents through the lavatory vent and the tub/shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by local and state codes and also The Standard Plumbing Code Book in 90 percent of the country. Check your local codes to make sure you're not in the excluded 10 percent.. The vent off the lavatory may be run out the roof or revented back into a dry vent in the attic or if you're reventing back into a fixtures dry vent you must make your connection at least 6 inches over that fixtures flood rim. Good luck and thank you for rating my answer. Tom

OK First thanks for the reply.
My problem is I don't know yet what I have under the flooring to the toilet and already had moved the shower and sink plumbing and sealed up the floor then my wife didn't like where the shower was going to be AFTER I framed it out. UGH!

I do know the existing toilet drain is vented to the roof and the other line which shares the sink and shower has it's own vent. Both are (I'm 99% sure) dedicated drains to the sewer tree in the basement. If I'm reading this right my main issue is the actual size of the pipe especially since the sink an shower are 2 1/2"
Thanks for the help!

You're going to hafta to reopen the floor and tell me what you have. A picture would be nice. Regards, Tom

Ok thanks for your help. I think I have it now. What it looks like I can do is connect to the 3" line from the outflow then next add on the sink and wet vent through that using the existing vent to the roof. The new shower location I will just connect to the existing 3" (old toilet drain) which is already vented to the roof . Thanks again.
Steve

The new shower location I will just connect to the existing 3" (old toilet drain) which is already vented to the roof . I can find no fault with that as long as the toilet's not on the same line. Good luck, Tom