Can a bathroom sink on two floors (one above the other) share a 1-1/2 inch drain pipe?

Yes, if both are trapped and vented. Good luck, Tom

Like Tom said, yes, two sinks on different floors can use the same drain line, BUT the sink on the top floor can't just connect into the vent from the sink below and that may complicate things for you... ;)

Why are you asking this question... installing a sink upstairs or what?

Back to you...

Mark

Thanks to Tom and Mark for your answers.
Mark, I'm adding a bathroom in the basement which includes a sink. I can use another available vent for the sink in the basement and will use a proper trap as suggested by Tom.

Thanks again!

I can use another available vent for the sink in the basement
How far away from the sink is this vent? Is this "vent" a dry vent or is some other fixture draining into it from above? Let me know Tom

How far away from the sink is this vent? Is this "vent" a dry vent or is some other fixture draining into it from above? Let me know Tom

Turns out the 1-1/2 in pipe is a dry vent that leads to a 2" drain pipe in the floor below. The sink above uses a different drain pipe.

This existing line should be perfect for the basement sink.

Thanks again for your advise.

How far away from the sink is this vent? Let me know, Tom