Unlike supply piping, drainage piping does not have a dynamic force behind it, and it is always open to the atmosphere. If you think of the old adage, "down the drain," you'll get the picture here. Gravity simply causes waste water to flow downhill (though there can be more sophisticated systems that involve mechanical assists in certain applications). Typically, you will see drainage piping included under the broad and more common category heading of "DWV." Within this category, the most common material options are plastic (typically ABS and PVC), cast iron and copper. This brings us to a point that is often confusing to newcomers in our industry. A common term used interchangeably with certain components of DWV systems is "soil pipe." Technically, soil pipe is a specific material -- cast iron -- but language habits are hard to change, and you will often hear other DWV materials being called by that name. If you're ever in doubt about what a customer is asking for, it's a good idea to clarify that point.
These are the common components of any DWV system:
Primary Drainage: In the overall piping network, the main stack and building drain serve as the heavy-duty portions. This is the piping that is used to carry waste from toilets, as well as to act as main arteries for the collection of all waste water and solids, carrying them out of the building to the connecting sewer or septic system. The portion of the overall drainage system that serves toilets is the main stack, best defined as the main vertical drainage channel. Every house or building must have least one such stack. Though there can be several horizontal channels for waste water depending on the number of floors in the building, there is only one primary horizontal drainage channel, and that is the bottom one in any building, called the "building drain" or "house drain." The building drain collects all the waste water from the building and transmits it to an outside connection with the municipal sewer or septic system. In relation to waste and vent piping, the type used for main stacks and building drains is always relatively larger because it must permit the free flow of waste from toilets without restrictions that could cause clogging.
Waste Piping: Waste piping is similar in function to the piping used in main stacks and building drains, except that it serves fixtures other than toilets; that is, it will not handle human waste. Years ago, houses were commonly built with a separate waste stack from the main stack, with the waste type emptying into an outside grease trap. Today, we continue to use "waste piping" as a separate term, but the concept has changed a bit in the sense that drainage routed through this portion of the DWV system is connected to the main stack and/or building drain, to exit the building in common with toilet drainage. Waste piping is typically smaller in diameter than the main stack and building drain piping, and it is used to carry waste water from sinks, lavatories, tubs, showers, laundry tubs and washing machines.
Vent Piping: To the average person, vent piping is probably the least understood aspect of the DWV system. Its first function is easy enough to grasp: A vent allows gases and odors that accompany waste water to leave the building through a chimney-like stack through the roof. The second part of the story perhaps will require you to jog your memory back to your high school physics class. Remember those experiments that demonstrated the function of atmospheric pressure in assisting the flow and movement of water (like the one where water wouldn't drip out of a hole in the bottom of a can until you opened the top)? That's what you're dealing with here. Strategically placed openings to atmosphere in a drainage system allow waste water to flow down easily without burping up the air being displaced. The waste flows down its channel, while the displaced air and gases rise up theirs -- and there's no gurgling interference between the two.
And most importantly, we want to keep a portion of the waste water in low-bend areas of the system called "traps," to prevent gases from entering the building. Vents, therefore, serve the important function of maintaining these water seals. Without them, a vacuum would be formed every time you sent water down the drain, emptying the trap.
Considerations for SelectionThe following are some of the common criteria for selecting the options of material and configuration within the DWV category:
Corrosion Resistance: Since there is more than just water being transmitted through the drainage system, the ability of the piping materials to resist corrosion is of greater concern than with supply piping. Human wastes, food wastes and chemicals (especially drain-cleaning types) are all common to the residential system, and it is important that only those materials that can resist attack be used.
Noise Insulation: While you normally think of restricted supply lines as the culprits in causing noisy plumbing, DWV systems can also give off unwanted signals of usage. Each basic type of DWV piping material has a different characteristic in this respect, and if the system's sound level in a house or building is of critical concern, this factor should be closely examined.
Capacity: Just as an undersized supply system can result in water delivery that is too slow (and often noisy), an undersized drainage system results in water that drains too slowly and may have a tendency to clog and/or back up. On the other hand, piping that is oversized for a particular installation runs the risk of inadequate transmission of solids.
Unobstructed Joints & Gentle Bends: Because of the solids carried through a DWV system, and because the only means of movement is gravity, this necessarily means that the system needs all the help we can give it to facilitate free flow. For this reason, DWV systems use fittings that differ significantly from supply fittings. First of all, DWV fittings are designed with unobstructed passage in relation to the mating pipe sections. While supply fittings typically create a step between the inside diameters of the pipe and fitting, such a relationship would be undesirable in DWV systems, since solids would tend to catch on the protruding shoulder of the downstream pipe end. In addition, DWV fittings always avoid sharp turns internally, to prevent solids from hanging up in the bend. A good comparative example of this is the tee configuration used with supply systems and the type used with DWV. Whereas the supply tee branches off at an abrupt angle internally, the DWV version (called a "sanitary tee") has a more gently curved internal passageway.
Local Code Requirements: All of the previous factors aside, you can't install any DWV materials that are not specifically approved by the code-governing agency in each area. Admittedly, the determinations of such approving agencies are not always made on the basis of purely objective considerations, but that's a whole different story we won't dig into here. Simply be aware that you have to deal with the realities of what is and isn't approved in your areas.