Replacing a single sink vanity with a double sink requires an entire new drain system between the sinks and the house drain pipe coming from the wall. This is because the house drain most often does not line up with either of the sink drains. Installing offset drain pipes for bathroom sinks requires a T fitting that joins the pipes connecting each drain. The T fitting attaches to a single P-trap to direct water from each sink to the house drain pipe. Purchase PVC drain pipe as it is more user-friendly than metal.
Things You'll Need
2 PVC pop-up drains
2 PVC sink drain J-elbows
1 PVC sink drain 90-degree tee
1 PVC P-trap
1 PVC drain pipe 2-feet long
7 PVC slip nuts with plastic spacers
Thread the pop-up drain assemblies onto the threads of each drain under the sinks. The pop-up drains have holes on the side for the linkage assembly. Thread each one until they are hand tight and the holes for the linkage are pointing to the back of the sink. Plumbers tape is not necessary when using PVC pipe.
Measure from the house wall drain pipe to the center of the sink drains with a tape measure. Measure the length of the P-trap. Subtract the second measurement from the first and add 4 inches. Cut the straight piece of PVC drain pipe to the revised measurement with a hacksaw. Sand the cut end of the pipe with fine grit sandpaper to remove the burrs.
Slide a slip nut and space washer over each end of the straight drain pipe. Insert one end into the house wall drain pipe. Install the P-trap on the opposite end of the pipe. The end going into the P-trap should penetrate the P-trap by at least 2 inches. Thread the slip nut onto the threads of the P-trap just until it is hand tight. Do not thread the slip nut on the house wall pipe at this time.
Place a slip nut and spacer washer on the bottom end of the tee fitting. The tee fitting looks like a "T" and installs with the two side openings pointing to each sink. Insert the bottom of the tee fitting into the top of the P-trap. Do not thread the slip nut onto the P-trap at this time.
Measure from the center of the pop-up drain pipe to the indention on the same side tee fitting opening. Transfer the measurement to one of the J-elbows. The J-elbows look like a "J" and the short end connects to the pop-up drain pipe. Cut the J-elbow to the measurement and sand off the burrs.
Slip a slip nut and spacer washer over each end of the J-elbow. Insert the long end of the J-elbow into the tee fitting and the short end over the pop-up drain. Thread the slip nuts onto the threads until they are hand tight. Repeat the process for the opposite sink.
Tighten all slip nuts until each is hand tight. No joint should show any signs of binding. Install the pop-up drain linkage assemblies according to the directions that come with your faucet. Once you have the faucet and pop-up assembly in place, run water in both sinks and look for leaks at the drain pipe connections. Turn any leaking connections another 1/4-turn.
The sink drain PVC pipe is in the sink plumbing section at home improvement centers. There are also double sink drain kits available.