Vents

Locating Vents

Figure: Max. Distance Between Fixture Traps and Vents

Based on International Residential Code; local codes may vary.
Min. slope of trap arm: 1/4 inch per foot for traps sized 1 1/4 to 2 inches;
1/8 per foot for traps sized 3 to 4 inches.

By code, vents must be placed at least two pipe diameters from a fixture trap. In addition, the length of the trap arm is limited to the distances defined in the figure below.

Offset Vent Runs

A vent must rise at least 6 in. above the fixture flood rim before running horizontally. If necessary, a pair of 45- degree offsets may be used, as shown in the figure below.

Figure: Horizontal Offset

Individual vs. Common Vents

The most foolproof venting strategy would be to provide a separate dry vent to every fixture, just behind the trap. Such an individual vent ensures that the trap seals will never be broken by a difference in air pressure. But this would require a lot of vent pipes traveling up into the walls and attic and penetrating the roof, which would not be practical. 

Some codes allow for greater design flexibility by permitting the use of common, or wet, venting strategies. Many houses include some combination of individual and common venting strategies. 

Individual Vents

An individual vent must:

• be at least half the diameter of the drain served
• be a minimum 11/4 in. in diameter (bigger is usually better)
• slope like a drainpipe
• rise at least 6 in. vertically above the flood rim before turning horizontal or connecting to vertical vent pipe (Common Vent Configurations, below). Anything less than 45 degrees from horizontal is horizontal.
• be increased by one size if its length exceeds 40 ft.

By some codes, two fixtures can share the same individual vent as long as they are on the same floor (Common Vent Configurations). An exception allows three lavatory sinks to share a common vent, regardless of where they are. Such a common vent can be either dry or a combination of dry and wet. 

Figure: Common Vent Configurations

Common Vents

A shared, or common, vent can exist when:

• Two fixtures (or three lavatory sinks) located on the same floor can share an individual vent.
• This vent pipe comes off at the interconnection of two fixtures or further downstream.
• Drains connect at the same level, or at different levels if sized according to the table in Minimum Common Vent Sizes, below.

A common vent connection can be made either at the same level, as when you have two bathroom lavs back to back on a wall (a simple dry vent; Example A in Common Vent Configurations, above); or at different levels, as you might have if a lav and a bath share the same vent (a combination wet and dry vent; Example B in Common Vent Configurations). 

Combination wet/dry vents (for example, the section between the lav and the bath drains in Example B in Common Vent Configurations) must be sized to handle the discharge from the upper fixture and provide air to the lower fixture (see the table shown in Minimum Common Vent Sizes).

Common vents for toilets. In the case of a combination wet/dry vent, a toilet cannot be the upstream fixture because the massive amount of water coming down the pipe might cut off the lower fixture’s air and pull the trap dry. In this case, run an individual vent for the bottom fixture. 

Double sinks. A double sink connected to a single trap — as is common in bathrooms and kitchens — is treated as one fixture, not two (see Traps for Double Sinks in Traps). 

Back-to-back sinks. When putting two sinks back to back, be sure to use a special double side-outlet sanitary tee designed for such an installation. Commercial-style, pump-assisted lavs — known as “blowout” fixtures — are not allowed in a back-to-back configuration.

Figure: Minimum Common Vent Sizes

Wet Vent vs. Dry Vent

There are two basic ways to supply air to a drain line: through a dry vent or through a wet vent (if codes allow). A dry vent supplies only air to the drain line, whereas a wet vent functions as both a drain line and a vent.

Dry Vent Rules

Any dry vent must rise at least 6 in. vertically above the flood rim of the fixture before turning horizontally or connecting to a vertical vent pipe. The horizontal run must be sloped to allow any condensation that forms within the pipe to run back to the drain. No downward or upward U-bends are allowed — keep it straight. All fittings should be installed to provide smooth, unrestricted airflow: This is why a sanitary tee in a vent connection is installed upside down. 

Wet Vents

A wet vent can be used for a single branch of a bath group, or for the entire drainage system for up to two bath groups located on the same floor (Wet Vent Configurations, below). Vent air enters the system through one or more dry vents. Both horizontal and vertical pipes (limited to one-story height) can be wet vented, though some codes may not allow horizontal wet venting.

Figure: Wet Vent Configurations

Figure: Wet Vent Configurations, continued

Wet Vent Rules

• A wet vent can handle all the fixtures in two bathroom groups (a bathroom group is a lav, toilet, bidet, and tub or shower).
• To stay out of trouble with wet vents, go beyond code and use a 4-in. main horizontal waste line where possible.
• Any wet vents must be 2 in. minimum in diameter.

Wet Vent to Main Drain

Code is very specific about the angle at which a wet or dry vent may enter a horizontal drain line. Make sure you have enough room for wet vents to enter drain lines so that your drain configuration will function as a true vent (below).

Figure: Orientation of Waste and Vent Connections

Waste Stacks

These days, a waste stack is not a soil stack — it’s a graywater stack (Waste Stack Vent, below). A stack system may be used, but not with a toilet. Although individual fixtures can still drain separately into the stack, a horizontal drain containing any number of fixtures is not permitted on the stack.

The more common alternative, particularly for a two-story house, is to use a stack with all the fixtures discharging into it, but to vent every fixture individually.

Figure: Waste Stack Vent

Waste Stack Vent Rules

• A vertical waste stack can handle the discharge of all fixtures except toilets and urinals.
• The fixtures must connect individually to the waste stack.
• No offsets of any kind are allowed.
• The vent for a waste stack, which continues as an extension at the top, must be the same size as the waste stack.
• See the table below for sizing.

Figure: Minimum Waste Stack Vent Sizes

Circuit Vents

Similar to wet venting, a circuit vent can accommodate eight fixtures on a drain line (Circuit Vent Configurations, below). The basic rules are that the vent pipe — sized to at least half the size of the drain line — must be installed between the two most upstream fixtures, and all the fixture trap arms must be within the code length limits (Max. Distance Between Fixture Traps and Vents, above). If an individual fixture is beyond the code-allowed distance, it can still discharge into the pipe, but that fixture must be individually vented. 

Sizing a circuit vent. For a circuit vent to work, the drains must be large enough for air to circulate above the wastewater. To stay out of trouble using a circuit vent system, use 4-in. pipe as the main drain, even if 3-in. is allowed, or use 2-in. pipe to individual fixtures, even if 11/2-in. is allowed. Typically, this means that the dry vent to the roof will be 2 to 3 in.

Circuit Vent Rules

• A circuit vent can drain up to eight individual fixtures into a main horizontal waste line.
• The dry vent connection must be made between the two most upstream fixtures in the group.
• Multiple circuit-vented branches can be connected together, as long as the most downstream branch is sized to handle its fixtures plus all upstream fixtures and branches.
• Each fixture must connect individually (back-to-back lavs need their own individual dry vent).
• If there are more than eight fixtures on the line, group the upper eight and individually vent the lower fixtures.
• If the uppermost fixture is beyond the trap arm limit, run an individual vent for that fixture and move the
  circuit-vent system down to the next eight.
• An upstream fixture refers to fixtures that are frequently used under normal conditions — a floor drain would not be counted.

Figure: Circuit Vent Configurations

Figure: Circuit Vent Configurations, continued

Combination Waste and Vent

For laundry rooms, an oversized horizontal drain system off the main drain is typical (Combination Drain and Vent, below). This line can serve sinks, lavs, washing machine standpipes, and floor drains. The only vertical pipes allowed are the drain lines from individual fixtures, and they must be limited to a total rise of 8 ft. This system allows only graywater — no toilets. 

Figure: Combination Drain and Vent

Combination Waste and Vent Rules

• A combination drain and vent can serve floor drains, clothes washer standpipes, and sinks and lavs.
• A kitchen sink on a combination drain and vent cannot have a garbage disposal.
• Toilets are not allowed on a combination drain and vent.
• Minimum size is 2 in.

Figure: Size of Combination Drain and Vent Pipe

Island Vents

There are a number of possible approaches to draining and venting an island sink:

Automatic Vents

The easiest solution is to install an automatic vent — a little unit about the size of a tin can that screws into the drain line several inches above the trap weir. These devices, called air admittance valves, sense any negative pressure and allow air to come into the lines when needed. They must be 4” above the drain being vented.

Oversized Vent

Another solution that an inspector may allow is to increase  the size of the pipe into which the trap dumps by one or two sizes, and run a dry vent to the drain line as close by as possible (below). Essentially, this treats the island drain like a combination drain and vent. It’s a good idea to oversize a kitchen sink drain anyway because a standard 11/2-inch drain frequently clogs up.

Figure: Kitchen Island Oversized Vent

Bow Vent

This vent gets its name from the loop of drainpipe that runs above the entry point of the sink outlet. The loop must extend above the outlet of the sink. A bow vent can be located ahead of the vent stack or behind the vent stack (below). Both arrangements require a cleanout. A bow vent gobbles up considerable cabinet space, and when a large multi-bowl sink is placed in a small base cabinet, the only alternative may be to install the piping in an adjacent cabinet. 

Figure: Kitchen Island Bow Vent

Island Vent Rules

• Island venting is permitted only for lavs and sinks (including connected dishwashers and food grinders).
• The island vent pipe must rise to a level above the sink outlet before turning horizontally.
• Cleanouts must be provided for island vents.

Vent Termination

• The distance the vent extends above the roof varies by locale, depending on local snowfall. The minimum is 6 in., while 1 to 3 ft. is the norm in cold regions.
• In cold regions, where the vent is prone to frost up, the termination must be at least 3 in. in diameter.
• Vent terminations must be not less than 4 ft. below an opening, or 10 ft. horizontally away (unless more than 3 feet above such opening).
• The end of the vent often looks better if it’s cut at the same pitch as the roof and painted to match the color of the roof. 

Side-Wall Terminations

Some codes permit the vent to exit the side of the house as opposed to the roof. This is sometimes done to keep the cost down during a renovation. If this is done, use a fitting at the end of the pipe to point it downward. If allowed, it’s also a good idea to increase the size of the fitting by one size and put a screen over it to keep birds out. A problem with going through the wall as opposed to the roof is that the gas will have less chance to disperse, and any window under the pipe, even if it is 10 ft. away, is likely to pick up the odor.