2021 IRC Chapter 31
Lesson 10 covered the individual vent, the baseline method where one vent protects one trap. Real houses rarely run that clean: a hall bathroom with three fixtures on one wall would need three separate pipes through the roof if individual venting were the only option. The methods in this lesson exist to protect the same trap seals with less pipe, by sharing one vent among several fixtures or by using a drain pipe itself as part of the air path. Every one of these methods still answers to the same rule from Lesson 10: keep an air path open behind the discharge so no trap siphons.
A common vent is one vent that protects two fixture traps instead of one. The textbook case is two lavatories in adjoining bathrooms, back to back on opposite sides of a shared wall, with both fixture drains connecting at the same level to one vertical drain. A single vent taken off at or downstream of that junction protects both traps, because the air path above the connection serves either fixture equally. P3107.1 requires both fixtures to be on the same floor level. The IRC also allows common venting for two fixtures that connect at different levels on the same vertical drain (P3107.3), under added size limits: the upper fixture cannot be a water closet, and Table P3107.3 caps its load by pipe size. Either way, a common vent removes one roof penetration without weakening the protection either trap gets, because the vent is still sized and positioned by the same rules covered in Lesson 10.
Wet venting is a method GCHS field staff will see often, because it is a common way to plumb hall and primary bathrooms. In a wet vent, one horizontal pipe serves as the drain for some fixtures and as the vent for others, at the same time. P3108.1 limits a horizontal wet vent to any combination of fixtures within two bathroom groups located on the same floor level, and a bathroom group is a water closet, lavatory, and bathtub or shower, with or without a bidet. Only those fixtures may connect to the wet-vented branch; any additional fixture, a kitchen sink or a laundry standpipe for example, has to discharge downstream of the wet vent. The wet vent still needs a dry vent: under P3108.2.1 the dry-vent connection is an individual or common vent for any fixture in the bathroom group except an emergency floor drain, and where it connects to a water closet's drain, that drain must connect horizontally to the wet vent. Not more than one wet-vented fixture drain may discharge upstream of the dry-vented fixture drain connection. That dry vent is what actually reaches outdoor air; the wet-vented fixtures downstream rely on the pipe itself to keep an air path open above the flow. Because the pipe is doing two jobs, Table P3108.3 sizes it by the fixture unit load it carries: 1-1/2 inch for 1 d.f.u., 2 inch for 4, 2-1/2 inch for 6, 3 inch for 12, and 4 inch for 32. Each wet-vented fixture drain also has to stay within the trap arm lengths of Table P3105.1 (P3108.5).
A waste stack vent is a different trick for the same problem: instead of adding a separate vent pipe for each fixture, the waste stack itself is treated as the vent for the fixtures connected to it (P3109.1). This only works under narrow conditions. P3109.2 says the stack cannot receive the discharge of water closets or urinals, because their surges would fill the pipe and defeat the air space the method depends on. The stack must be vertical, with no horizontal or vertical offsets between the lowest and highest fixture drain connections, and every fixture drain connects separately to it. A stack vent, at least as large as the stack, continues above the highest connection to the outdoors (P3109.3), and Table P3109.4 caps the fixture units the stack can carry. Where those conditions hold, a properly sized waste stack lets a run of lighter fixtures, such as a kitchen sink and a couple of lavatories stacked floor to floor in a multi-story addition, share one pipe with no separate dry vent for each. Add a water closet to that stack later, during a remodel, and the waste-stack-vent arrangement no longer applies; the stack has to be re-evaluated and typically re-vented conventionally.
Circuit venting protects a group of fixtures on one horizontal branch with a single vent. P3110.1 allows not more than eight fixtures on a circuit-vented branch, each fixture drain connecting horizontally to the branch, and the branch itself counts as the vent from the most downstream fixture connection to the most upstream one. The vent connection goes between the two most upstream fixture drains (P3110.2), and the vent section of the branch is sloped not more than 1 unit in 12 (P3110.3). Circuit venting and wet venting solve a similar problem from different directions: wet venting uses a drain pipe as the vent for bathroom-group fixtures, while circuit venting uses one vent placed at the upstream end of a branch of fixtures. Residential work sees circuit venting less often than wet venting.
Some fixtures have no practical way to run a conventional vent up above their flood level rim, such as a floor drain in the middle of a garage slab with no nearby wall. The combination waste and vent method solves this by deliberately oversizing the drain pipe so a substantial air space remains above the flow at all times, and that air space performs the venting function. P3111.1 allows it to serve only floor drains, sinks, lavatories, and drinking fountains, never water closets. The only vertical pipe in the system is the connection from a fixture drain to the horizontal pipe, and it may not be longer than 8 feet (P3111.2); the horizontal pipe is sloped not more than 1/2 unit in 12, and the system needs a dry vent or a connection to a horizontal drain serving vented fixtures on the same floor. Table P3111.3 sets the size, for example 2 inch for 3 d.f.u. or 3 inch for 12 d.f.u. on a horizontal branch. It is a special-case method, not a general substitute for venting: an ordinary drain sized for its fixture unit load has little air space to spare, while a combination waste and vent pipe is sized well beyond that load specifically to leave room for air.
An island sink, most often a kitchen island sink, has no adjacent wall to carry a vent upward. P3112 allows island fixture venting only for sinks and lavatories, and a kitchen sink with a dishwasher waste connection, a food waste disposer, or both qualifies. The vent connects to the fixture drain like an individual or common vent, rises vertically to above the fixture's drainage outlet, and only then offsets horizontally or downward. The portion of that vent below the fixture's flood level rim is installed like drainage piping, its lowest point connects full size to a vertical drain or the top half of a horizontal drain, and cleanouts are required so all vent piping below the flood level rim can be rodded (P3112.3). That low point collects condensate and debris, which is exactly why the cleanouts matter. A looped island vent installed without a cleanout, or with the cleanout buried behind a finished cabinet base, is a common defect in kitchen remodels: it looks fine on the day of rough-in inspection and becomes unreachable the day the cabinets go in. A listed air admittance valve under the cabinet, where the adopted code and the local jurisdiction allow it, is the simpler modern alternative to the loop vent for the same island-sink problem.
An air admittance valve is a one-way mechanical device: it opens to let air in when a fixture drains and creates negative pressure, and it seals closed the rest of the time, so sewer gas never passes through it. That makes it useful anywhere a conventional vent cannot be run, an island sink, an addition far from the existing stack, a fixture in an interior wall with no path to the roof, but it also means an AAV can never be the building's only vent. Under P3114.7 every plumbing system still needs at least one open vent stack or stack vent that reaches outdoor air, because only an open vent can relieve the positive pressure a falling column of water creates elsewhere in the system; a one-way valve has nothing to offer on the pressure side. An individual or branch-type AAV has to sit at least 4 inches above the horizontal branch or fixture drain it vents, clear of the splash zone where waste could reach and foul the sealing mechanism, and, where it is installed in an attic, at least 6 inches above insulation materials, since a valve packed in batt or blown insulation is starved of the air it needs to open. Individual and branch-type valves may vent only fixtures on the same floor level that connect to a horizontal branch drain, the valve must sit within the maximum developed length allowed for that vent, and it goes in only after the drain-waste-vent pressure test is done (P3114.2, P3114.3, P3114.4). A stack-type AAV, the kind serving an entire vent stack rather than one fixture, has to sit at least 6 inches above the flood level rim of the highest fixture it protects, the same logic that sets the height of an ordinary dry vent connection. P3114.5 also requires access to every AAV and a location that lets air enter it, because it is a mechanical part with a service life, not a permanent fitting; one sealed behind a finished wall or a closed soffit with no access panel cannot be serviced when its seal eventually hardens or sticks.
In the field
A common rehab finding is a hall bathroom where a previous owner added a half bath below grade and vented the new toilet with an air admittance valve tucked into a sealed base cabinet, with no other vent connection anywhere in the new work. The AAV itself may be a listed, correctly installed device, but if that half bath has no path back to an existing vent stack or its own vent to outdoor air, the building's positive-pressure relief depends entirely on whatever vent already existed for the rest of the house, and GCHS inspectors check that connection specifically rather than assuming the AAV covers it. The fix, where the AAV alone is the only venting for that fixture group, is confirming (or adding) the open vent connection the code still requires, not adding a second AAV.
A worked example: a hall bathroom wet vent serves a water closet (3 d.f.u.) and a bathtub (2 d.f.u.), with the lavatory's individual vent providing the dry vent connection for the group. The wet vent carries 5 d.f.u., so Table P3108.3 calls for at least a 2-1/2 inch pipe, because a 2 inch pipe is rated for only 4 d.f.u. The bathtub's trap arm is 1-1/2 inch and runs 5 feet from the trap weir to the wet vent. Checking that distance against the trap arm table from Lesson 9, the maximum allowed run for a 1-1/2 inch trap arm is 6 feet, so a 5-foot run passes with a foot to spare. If the tub were moved 2 feet farther out during a layout change, the run would reach 7 feet, exceed the 6-foot limit, and fail, because P3108.5 holds wet-vented fixture drains to the same Table P3105.1 lengths. The water closet is treated differently: as a self-siphoning fixture it is not held to the trap arm length limit.
1. A common vent serves:
2. Wet venting in a dwelling is limited to:
3. In a horizontal wet-vent system, the dry vent connection is made through:
4. Which fixture is NOT permitted on a wet-vented bathroom group's drain?
5. A waste stack vent arrangement is invalidated if:
6. Circuit venting protects the fixtures on a horizontal branch (up to eight) through a vent connected:
7. A combination waste and vent system is permitted to serve only:
8. Island fixture venting requires accessible cleanouts (P3112.3) because:
9. Which statement about air admittance valves is correct?
10. An individual air admittance valve serving a lavatory must be located at least how far above the horizontal branch drain it vents?
11. An air admittance valve installed in an attic must be at least how far above insulation materials?
12. An inspector finds an air admittance valve installed inside a sealed wall cavity with no access panel. The correct finding is:
13. An inspector finds a basement half bath added by a previous owner, with the new water closet vented only by an air admittance valve and no other vent connection anywhere in the new work. The finding should note:
14. A stack-type air admittance valve serving several upper-floor fixtures must be located at least how far above the flood level rim of the highest fixture it vents?