June, 2013
The Word
Inspection News and Views from the American Society of Home Inspectors

DWV Systems:


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Figure 1: Fixture drain slope

Once again The Word invites you to travel into the dark realm of subjects that are sometimes misunderstood by home inspectors. The Word hopes you will find this trip informative and maybe a little entertaining.

Our subject this month: the drain, waste and vent (DWV) system. It’s been a while since we discussed this subject. The Word hopes that a review with a different perspective will prove useful, especially for new readers.

Remember, when reading all The Word columns that we’re discussing general principles. Something you see in the field may not be wrong just because it doesn’t comply with a general principle. Local building codes, manufacturer’s instructions and engineered designs trump general principles. This discussion is based on The Word’s favorite code: the International Residential Code (IRC). For those who live in the land of the Uniform Plumbing Code (UPC), mostly out West, some rules may be different.

The Basics
All the complicated rules surrounding the DWV system boil down to two basic requirements: (1) Stuff from inside the home needs to flow out from the home smoothly and uninterrupted. (2) Stuff from outside the home needs to stay out. Okay, these statements would upset Captain Obvious, but sometimes it helps to remember the obvious. The rules help ensure that the basics occur reliably. An unreliable DWV system creates damage and unhealthy conditions; besides, an unreliable DWV system is just plain gross. That’s why plumbers make the big bucks.

Drain or Waste – Does it Matter?
The DWV system has its own lingo, and you should know it to accurately report problems. Let’s start with the D, W and V. Ever wonder what’s the difference? A drain pipe conveys any material, including solid and liquid human waste. A waste pipe conveys material that doesn’t contain solid human waste (but may contain liquid human waste from a urinal). A dry vent pipe should convey only air and whatever minor quantity of water that may result from rain entering the vent pipe at the roof termination and any condensation that may occur in the vent pipe.

The difference between drain and waste pipes may seem like an academic distinction, but recently it has taken on some practical significance. Water conservation is becoming an increasingly important concern, especially in the West. Gray water (water without human waste) may be recycled on site. Waste pipes convey gray water in the vast majority of homes. The exception is a waste pipe connected to a urinal. The Word can count on one hand the number of urinals he has seen in homes. Drain pipes do not convey gray water because every home must have at least one toilet, so drain pipes will always convey human waste. The distinction between drain and waste pipes is important when dealing with residential gray-water recycling.

The Pipes
Each drain and waste pipe has a name. Let’s start at the end of the system and work back. The building sewer starts outside near the foundation and runs to the public sewer or private sewage disposal system (usually a septic tank, but there are other types). The building drain is usually the lowest horizontal drain pipe in the home and starts at the building sewer.

A building sub-drain may exist to serve plumbing fixtures below the building drain, usually located in the basement. In this case you need a sewage ejector to pump the material up to the building drain. Sewage ejectors are sometimes incorrectly called sump pumps. A sump pump deals with water originating outside the DWV system, such as rain water and ground water, so you should use the term sewage ejector for pumps in the DWV system.

Branch drains and stacks connect to the building drain. A branch drain is mostly horizontal and may have some vertical sections. A stack is mostly vertical and conveys material between stories.

A dry vent lets air into the system to protect the water seal in the trap from siphoning and from blowout. Other types of venting systems, such as wet vents, are larger drain and waste pipes. The increased pipe size allows enough air into the system to protect traps. Air admittance valves (sometimes referred to by the trade name Studor™ vents) also let air into the system. These valves are useful when connecting to a vent pipe is impractical or expensive.

Inspecting drain, waste and vent pipes is usually easy, when you can see them. If the pipes are properly sloped and supported, and if there’s no visible deterioration or leaking, you’re usually good to go.

Horizontal pipes should uniformly slope toward the building sewer. Drain and waste pipes smaller than 3 inches diameter should slope at ¼ inch per foot. Drain and waste pipes 3 inches diameter and larger may slope at ⅛ inch per foot. The slope shouldn’t be less than the minimum, but it shouldn’t be too much more either. Horizontal pipes with too great a slope may leave the solids behind when the water flows too quickly. There is no rule for vent pipe slope other than the general rule that it should slope toward the building sewer. The attic is a good place to look for vent pipe slope problems. Sometimes the roof termination pipe is pressed by the roofers forcing any horizontal part of the vent pipe into an improper slope.

Pipes need to be supported to maintain slope and to keep them from sagging over time. PVC and ABS drain and waste pipes should be supported every 4 feet horizontally. Cast iron drain and waste pipes should be supported every 10 feet horizontally when using 10 foot pieces; otherwise, they should be supported every 5 feet horizontally. Pipe support material should be compatible with the pipe. Avoid corrosion caused by galvanic reaction by not mixing metals when supporting pipes. Copper shouldn’t support galvanized and cast iron pipe. Galvanized material shouldn’t support copper.

Where the Flow Starts
Components that we see most often include fixture drains, traps, tail pieces, fixtures and receptors. Fixture drains (a.k.a. trap arms) connect traps to drain and waste pipes and to vents. Fixture drain length is limited to avoid draining the trap. A fixture drain that is too long will terminate below the trap, thus providing a way to siphon the trap. The maximum fixture drain length depends on the size of the trap. The easy way to remember the maximum fixture drain length for pipes 2 inches and smaller is to divide the trap size by ¼ (the ideal slope). For example, a 1 ½ inch trap may have a maximum fixture drain length of 6 feet (1 ½ / ¼). The fixture drain length may be less than the maximum and still be a problem if the fixture drain slope is greater than ¼ inch. (See Figure 1.) By the way, there is no maximum fixture drain length for toilets.

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Figure 2: Flow direction and change

Traps provide the water seal that keeps sewer gasses in the DWV system and outside the home. Traps should be installed level to keep the water seal from draining. The minimum and maximum depth of a trap water seal is 2 inches and 4 inches respectively. A shallow water seal may allow the water seal to evaporate. A deep water seal may not clear during use, and the trap may have a tendency to clog. Trap water seal depth usually isn’t a problem with commercially made traps that are properly installed. Sometimes people create traps using other fittings, and sometimes they install a vertical extension on the trap outlet to get around an obstruction or to reach a fitting
that’s already installed. These traps may not be correct, so look for them.

The maximum vertical distance between the fixture outlet and the trap is 24 inches. Water from the fixture that falls farther than this may blow out the trap water seal. Look for this in basements and in crawlspaces, especially under pedestal sinks. For aesthetic reasons, people will install the trap under the floor and too far below the sink outlet.

Tail pieces are the vertical pipes between a fixture and its trap. Amateur plumbers, including some who get paid for their work, sometimes use flexible, corrugated tubular tail pieces. These tail pieces don’t have the smooth interior surface required of drainage pipes and fittings. These components were not allowed under earlier editions of the International Residential Code. Unfortunately, IRC 2012 carves out a specific exemption for these components, allowing their use if they are installed vertically between the fixture outlet and the trap inlet. The Word will continue to report these components as a deficiency because of their unsanitary nature, and because of their propensity to clog. The UPC does not allow these components. Way to go, UPC!

Fixtures include components like sinks, lavatories (the fancy name for a bathroom sink), water closets (the fancy name for toilets), bidets and bathtubs. Receptors include components such as shower pans, floor drains and washing machine standpipes.

Fittings connect all these other components together and are necessary to change flow direction. Fittings are the most interesting DWV system components because many fittings have specific allowed uses. Use the wrong fitting in the wrong place and the DWV system basics won’t work reliably.

There are two primary rules for installing fittings. Install fittings so that they provide a gradual curve in the direction of flow. Install fittings so that slope is maintained. These rules help prevent clogs.

Perhaps the most misused fitting is the sanitary tee. This fitting has one purpose in life: to change flow direction from horizontal to vertical using the curved fitting inlet. That’s all, except when it’s used to connect a dry vent to the system (see Figure 2). Using a sanitary tee to change flow direction from vertical to horizontal and from horizontal, to horizontal, is always wrong.

Wye and tee wye fittings are the generalists of the fittings world. These fittings, when properly installed, connect multiple pipes and change the flow in any direction. (See Figure 3.)

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Figure 3: Fittings are nexessary to change the flow and direction

The final type of fitting is the bend. These fittings are also called elbows, although the term bend helps distinguish these DWV fittings from their water supply cousins. Bends are described by the fraction of a circle that they sweep. A ¼ bend changes direction 90˚ (¼ of 360˚), a ⅛ bend changes direction 45˚, and so on. Bends may change flow in any direction except for ¼ bends, which have some restrictions.

Quarter bends come in regular (short) bends and long bends. Long bends may change flow in any pipe and in any direction. Regular bends may change flow direction from vertical to horizontal and from horizontal to horizontal only in fixture drains 2 inches and smaller and in all pipes 3 inches and larger.

The Bottom Line

We can usually miss minor problems without clients getting too upset. Missing a DWV problem that causes a backup of gross stuff into the home will usually create some very unhappy campers. You now have some additional tools to help keep everything flowing as it should.

Memo to Poseidon: The Word does not reside on Mt. Olympus (just at its base) and welcomes other viewpoints. Send your lightning bolts or emails to Bruce@DreamHomeConsultants.com. The thoughts contained herein are those of The Word. They are not ASHI standards or policies.

Bruce Barker operates Dream
Home Consultants. He has been building and inspecting homes since 1987. He is the author of “Everybody’s Building Code” and currently serves as chair of the ASHI Standards Committee. To read more of Barker’s articles,
go to www.dreamhomeconsultants.com.