Every habitable dwelling, and most commercial buildings have at least two plumbing systems:  A potable water supply, and a drainage waste system (also called a drain, waste, and vent system).  The potable water supply system is used to deliver water to a building, and to the fixtures within that building.

Once that water is used, and sent down the drain, the drainage and waste system is in charge of disposing it; usually via the municipal sewage treatment system, or a septic system.  In this article, we will be giving an overview of the drainage and waste system, and how it works.

The Basic Behind a Waste System

So, we already covered that the drain, waste, and vent system (DWV) is how the waste water is disposed of.  The easiest way to understand how it does this to start at the sewer main.  Let’s say that you have a simple 1-bedroom home, with a bathroom at the rear of the house.

Your sewer main will start at that back toilet and run (as straight as possible) all the way to the front of the house, where it exits the foundation, continues through your front yard (underground of course), and into the street, until it ties into the city main.  The city main takes it, and send it to the sewage treatment plant.  Of course, most homes are much more complex, but they work on the same basic principal.

The toilet is the only fixture that ties pretty much directly to the main.  Every other fixture in your home will have a smaller drain line that goes from that fixture, to the main.  If you were to picture a tree, with a toilet at the top, and a sink, or tub, or shower at the tip of each branch, you would not be far from the truth.  These fixture lines are called secondary lines, as they are “secondary” to the main.

Each fixture line will also have a trap, and a vent.  We cover traps and vents in our P-Trap article, but we will touch on them as well here.

Traps, Vents, and Grade, Oh My!

There are a LOT of code regulations pertaining to DWV systems.  For the sake of simplicity, however, we are going to say that there are three basic components that are needed for a waste system to perform properly:  Traps, vents, and grade.


Every fixture in your home that accepts waste water must have a p-trap.  The only exception to this rule is the toilet which has its own internal trap, built in.  The trap’s job is to hold water.  It does this to keep the gases from the sewer system out of your home.  Without traps, not only would your home smell like sewage, but it would accumulate Methane Gas… Not good!


Each fixture must also have a vent, and there are code requirements that state what sized vents you need for which fixtures.  The will often tie together within the walls, so that there only a couple sticking out from your roof (yes… that is what those pipes are for).  Vents serve three purposes.

1 – To keep the water from syphoning out of the traps.  With the proper air flow ventilating the traps, the water will stay in them, and not drain out.

2 – To allow the water to flow easily down the drain.  If you ever had a straw in liquid, and held your finger over the end, you found that when you lifted the straw, the water stayed in it.  Release your finger, and the water drains.  Without a vent, the water in your drain lines will stay in them, just like the liquid in the straw.

3 – Vents give the sewer gases somewhere to go so they do not build pressure in your pipes.


Okay, so we need traps, and we need vents… what is “grade”?  Grade is the first rule of plumbing… “Poop flows downhill”.  That means that if we want to make sure it flows downhill, we need to make sure all the pipes flow that direction.

“Grade” (at least in this instance) is the measure of downhill slope of a drainage pipe. For example, saying that a sewer line must have a 2% grade, what we are really saying is that the pipe most have a downward slope of at least 2%.

There you have the basics of a drainage waste system.  For more information on this, or any other plumbing topic under the sun, give Allstar a call.  Our plumbers can help you with any DWV issues you may have, and will do so with a smile.   🙂