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All effective plumbing drains have four key features: a tailpiece and a curved pipe segment called a “trap” located below each sink, shower and tub; solid anchoring of the pipe; a consistent downward pipe slope of a quarter inch per foot of horizontal travel; and a system of air vents (called a vent stack) that allows wastewater to drain smoothly.

All renovations are unique, but most plumbers begin drain installations at the fixture and work back to the main drainpipe. Toilets are sealed from the sewer line by the standing water in the bowl, but all other fixtures require a curved drainpipe trap directly underneath the drain, which retains water and creates a liquid seal that prevents gases in the sewer line from wafting into your bathroom.

Cutting and fastening plastic piping is simple. Saw the length of pipe you need, swab a generous coat of cement on the pipe and the connector you’re assembling, then push the parts together. In a couple of seconds, the cement bond is permanent and waterproof.

As you cut and fit piping to build your drain system, be sure you don’t forget the all-important quarter-inch slope downward for every foot of horizontal pipe travel. Water won’t drain properly in piping with less slope. But more slope also can be a problem, because wastewater can drain too quickly, possibly leaving behind solids that can clog the drainpipe over time.

In addition to establishing a good drainpipe slope, you also need to install drainpipe venting. As wastewater travels down a drainpipe, it creates suction behind it. And without something to relieve this suction, you’ll hear loud gurgling noises in your sink, shower or tub. Even more serious, water can be drawn out of the drain trap, eliminating that all-important seal between the sewer line and your home.

This is why all drains have a secondary system of pipes that work as air vents to relieve suction. Ideally this should be installed within a few feet downstream of every connection to the drain. These secondary vent pipes connect to the vent stack, which extends vertically through your roof to the outdoors. In this illustration, the drainpipe connects directly to the vent stack.

Sometimes, it’s not easy to connect a vent pipe to the drain you’re working on and have it run back to the vent stack. In cases like this, you can install a one-way vent valve made especially for this purpose, called an air admittance valve. This valve must be installed as close as possible to the drain opening under a sink, countertop or shower drain, and connected to the drainpipe with its own short plastic pipe. A spring automatically seals the air admittance valve when no suction is present in the pipe, which prevents sewer gases from seeping into the bathroom. But when water travels down the drain and pressure drops within the pipe, the valve automatically opens and admits room air into the pipe, relieving the suction.

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