Plumbing – How to connect PVC plumbing for two sinks to old cast iron stack with two threaded tees

No, there should not ever be two traps on a single drain line.

The point of the trap is to create a water barrier which prevents sewer gasses from coming up the empty pipe and into the house.

When you have two traps on the same line, you end up creating a kind of vapor block in the line which prevents it from draining properly.

What do you do?

Get yourself some 1 1/2" PVC (not CPVC, which is yellow, PVC which is white) with a 90 deg elbow, and also get two Fernco couplings (black plastic couplings with straps on each end that can be tightened) that will fit the pvc and iron drain pipe.

Use a metal blade on a sawz-all and cut out the p-trap in your basement. (Have a bucket handy!) Leave room to connect the Fernco coupling on both sides. I'd cut the right hand side out just past the coupling.

Measure and cut the PVC so that with the 90 elbow the ends line up with the newly cut drain lines. Glue the PVC together and confirm the fit, then connect to the old drains with the Fernco.

Viola - a proper drain line that will drain smoothly w/ no vapor lock.

Practically speaking, what you want to do is fine. The minimum horizontal slope is 1/4 inch per foot and the max is 3 inches per foot unless vertical. the inlet of the washer drain must be above the height of the washer. This is usually 36 to 38 inches minimum for floor seated washers. Obviously, if your washer is set on a platform, you will need to adjust the height of the drain inlet to be above the washer. I like to see at least a foot to 18 inches higher than the washer so that the drain can keep up with the washer pump and not overflow. I caution you however, if the diverter valve slows the drain flow at the current drain diameter, you may need to increase the pipe/valve diameter size to avoid back-up and overflow.