I am installing a new washer & dryer on second floor outside of a half bath. Can I tie into vanity drain or should I run a separate linepast vanity & tie into 3" line going thorough the roof? Do I need to run a vent line?The washing machine will be about 5ft. away from 3" line.

Hi Fishgepp...

Not a good idea to connect into the lavatory drain and plumbing code definitely doesn't allow it... OK?

You want to connect into the 3" drain line and install a 3"x2" wye and if possible run a 2" drain line over to the new washing machine location. And yes, unfortunately, you will need to run an 1.5" vent. This vent can connect into any other vent as long as it connects at a minimum of 42-48" off the finish floor.

Check out the diagram below:below... ;)

Back to you...

MARK