How to Put Plumbing in for a New Basement Sink

Drainage

• If you have a choice of locations for the sink, the one nearest to an accessible sewer is the best. If you have to run the drain for a long distance, it may be difficult to keep the necessary slope, given that the sink P-trap isn't far above the sewer. In many basements, the sewer is under a concrete pad, and there's no way to tie into it without some demolition. In such cases, a solution is to locate a sump pump in a drainage box near the sink, and pump the sink waste to the nearest vertical waste stack.

Tying Into the Sewer

• Whether you run the drain on a downward slope to the sewer or you pump it up to a waste stack, you must use a "Y" fitting with its inlet sweeping toward the sewer to make the connection. From there, the drain must maintain a minimum downward slope of 1/4 inch per foot until it reaches the sink P-trap, unless you're using a pump box. It's a good idea to have the sink installed and the drain tailpiece -- which is the pipe that extends down from the drain -- in place before you install the drain pipe. That way, you'll be able to accurately measure the height difference between the sewer and the P-trap.

Water Lines

• Because you're in the basement, the most convenient water source may be the inlet and outlet pipes to the water heater. Wherever you make the connections, however, making them to a 3/4-inch main line instead of a 1/2-inch branch line prevents loss of water pressure at the sink. In most cases, you'll be running copper pipe. To tie into the existing hot or cold line, you need to turn off the water, cut the pipe with a pipe cutter and solder in a tee. From there, assemble a 1/2-inch branch line to a convenient location in the wall behind the sink.

Hooking Up the Sink

• The P-trap inlets fits over the tailpiece, and you secure it by tightening the compression nut by hand. If the outlet doesn't extend straight into a sump box, you may need a sweep elbow to connect it to the drain waste line that goes to the sewer. You need to install a shutoff valve on each of the water pipes stubbing out behind the sink. You then attach the faucet to the sink and connect each of its inlets to one of the valves with a flexible hose. You can't tighten the faucet connections by hand; you need a wrench or adjustable pliers to do it.