I have an older home that my wife and I renovating... we are starting with the kitchen and have it down to studs. Our 2nd floor bathroom is directly above the kitchen. I noticed that the tub has an old drum trap that has leaked in the past before our renovation started. What would happen is if the tub was draining while you flushed the toilet... the drum would overflow causing water to drip down into the kitchen through the light fixture I suspect the back pressure was causing this and now that the ceiling is out I feel it is time to revamp this old style trap. I have read that PVC P traps are the way to go. My question is how do I connect the new style PVC to the old cast iron and copper pipe? Can I use proflex couplings like this: http://www.fernco.com/plumbing/flexible-couplings/stock-couplings Thanks for any advice... Just want to make sure once I cut this out it is done right before hanging the ceiling.