I am building a laundry room adjacent to an existing bathroom. Gas, water, and electric are no problem but the main stack does not have an easy access point in the 14″ area between the floor and ceiling. There is already a toilet branch and another branch that leads to my neighbors wash (its a condo with a shared stack) My question is, can I connect my washing machine pipe to the 1 1/2″ Copper drain pipe of my bathtub with a wye-branch. I’m not concerned with whether or not it’s to code but I’m wondering if I run the risk of backup while taking a shower and doing wash at the same time.