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The Drainage System: Drain Waste-Vent System From "Blueprint for Home Building" episode DBHB-202

Figure A

There are two important things involved with plumbing: drain waste and vent piping. In this segment host Jeff Wilson takes a look at the system that takes the water away from your home.

The distribution piping is the system that brought the water to the toilet bowl and the drain waste-vent system (figure A) transports all the used water and waste from the house to the sewer or septic system. A house's sewer system basically works on gravity. Everything goes down in other words. Today the piping is generally PVC as opposed to galvanized caste iron, which clogged much easier, in the past.

When the waste water leaves the bowl, it descends down the drain and enters the waste line. To allow gravity to move the waste water, there are certain drainage specifications devised to go with the flow. Supply lines are fitted with 45- or 90-degree angles. Waste fittings, however, require larger angles with more sweeping bends.

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	Figure B
	Figure C
	Figure D

An important component of drainage is a piping system called the "venting system," which consists of pipes (figure B) that allow gasses to escape. The venting system also relieves air pressure so waste water can drain properly. If you've noticed several pipes sticking up through the roofs of houses, those are vent pipes on the waste lines, required by code to prevent drawing the water out the traps that are under the sinks. These vents are a component of the drainage system.

Note: There are products on the market such as air-admittance vents (figure C) that will fit under your sink, which will draw a bit of air in from under the sink while preventing any sewer gasses from coming out. These vents are great for two purposes: 1) they reduce the amount of waste line vent stacks that you run, which saves money, and 2) they reduce the number of penetrations through your roof.

At this point you've followed the drainage system from the faucets down through the house, and now the waste water pipes are ready to be tied into the sewer line. The plumber installs the "house trap," which keeps the waste and odors flowing out of the home, and the "lateral line" (figure D), which ties the house to the municipal sewer lines. Every house has a lateral that goes into a sewer system that goes down the street, collects through a main distribution line and goes to a sewer processing plant.

Note: Sewage moves by gravity, which means the sewage lines coming into the house need to be higher than where they go to connect to the city sewer line. To make the connection from your home to the public sewer, you need to find out from your civil engineer where the sewers are. You'll want to have at least a minimum of two or three percent fall or you're going to be in a pump situation. The amount of fall in your sewer system is going to depend upon what your elevation of the home is and what the elevation of the city connection location is. The fall for the drain waste-vent system is 1/4-inch per foot, which carries fluids through the sewage system. The lateral should be two degrees, which means you have a four-inch pipe with a 1/4-inch fall, going into a six-inch sewer lateral that results in a two-degree fall into the main line.

In the final segment you'll find out what to do if you're new home is in an area where municipalities aren't available. The alternative is a septic system.

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Michael Morse
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Pedicone Plumbing
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Building Wisdom
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