Supply, Drain, Waste, and Vent

Two systems

Every house has two plumbing systems that must never mix ¹:

• Supply brings clean water in under pressure, from the city main or a well pressure tank, through a meter and main shutoff, to a water heater and every fixture. Because it’s pressurized, supply pipe can run up, down, or sideways.
• Drain-waste-vent (DWV) carries used water and waste out by gravity, through traps and sloped pipes down to the building drain and the sewer or septic system, with vents letting air in so it all flows.

Wisconsin’s code starts from these principles: fixtures get enough water at enough pressure to work, drains carry waste away efficiently with cleanouts for clearing clogs, the drain system breathes so trap seals are never sucked out or blown out, and backflow never contaminates food or water ¹.

Every dwelling on a sewer or septic system needs at least one toilet, one wash basin, one kitchen sink, and one tub or shower ¹.

Supply

The water service enters below the frost line, usually in the basement, and passes a main shutoff valve and the meter (city water) or a pressure tank and switch (well). From there, cold water branches off to fixtures and to the water heater, and hot water leaves the heater on its own set of pipes. Each fixture should have its own small shutoff valve so it can be repaired without shutting down the house.

Know where your main shutoff is before you need it. A burst pipe dumps gallons a minute.

Drain and waste

Each fixture drains through a trap, a dip that holds water (the next lesson), into a fixture drain, then into branch drains, then down a drain stack to the building drain under the floor or in the basement, and out through the building sewer.

Gravity does all the work, so every horizontal pipe slopes. Wisconsin’s minimums for horizontal branch drains ¹:

• 2 inches and smaller: 1/4 inch per foot.
• Larger than 2 inches: 1/8 inch per foot.

Too flat and solids settle; the code sets the minimum, and the building drain and sewer have their own table. Cleanouts, screw-capped openings, let you rod out a clog without taking pipe apart ¹.

Vent

Water rushing down a pipe pushes air ahead of it and pulls a vacuum behind it. Without a way for air to move, drains glug, flow slowly, and suck the water out of traps. Vents are pipes that rise from the drains and connect to a stack that exits through the roof, letting air in and out ¹.

Wisconsin requires every gravity building sewer to be served by at least one main stack that’s at least 3 inches in diameter all the way from the building drain to the vent terminal, and every vent terminal to be at least 2 inches in diameter so frost can’t close it in a Wisconsin winter ¹. If you’ve seen a vent pipe frosted shut on a January morning, you’ve seen why.

Sizing by fixture units

How big does a drain need to be? The code converts each fixture into drainage fixture units (dfu), a measure of how much and how fast it discharges ¹:

Add up everything draining through a pipe and check the code’s table: a 3-inch horizontal drain (other than the building drain) may carry up to 20 dfu, a 4-inch up to 160, and a 2-inch only 6 ¹. That’s why a toilet always gets 3-inch drain and a whole house gets at least a 3-inch, usually 4-inch, building drain.

What to take from this

Supply is pressurized and can go anywhere; drain-waste-vent works by gravity, so it slopes (1/4 inch per foot for 2-inch and smaller branches, 1/8 inch for larger) and must breathe through vents. Wisconsin requires a 3-inch main stack to the roof and vent terminals at least 2 inches for frost. Pipes are sized by drainage fixture units: a toilet is 4, a bathroom group 6, a kitchen sink 2. Find your main shutoff now.