13.2 Wet, Circuit, Loop & Stack Venting

13.2 Wet, Circuit, Loop & Stack Venting

Quick Answer: 248 CMR 10.16(5) wet venting is not IPC wet venting. In a bathroom with a 2-inch horizontal waste, a 2-inch extension of that drain to a lavatory may wet-vent the fixtures it serves, including double or back-to-back bathrooms. The lowest portion of the wet-vent pipe breaks the centerline of the horizontal drain (Example 11). Kitchen sinks, disposals, dishwashers, washing machines, and culinary fixtures shall not be on that wet vent. A circuit vent ties to a proper vent upstream of a 2-to-8 floor-outlet battery; a loop vent ties downstream. Stack venting on the top floor is a 3-inch independent-entry permission; on lower floors the upward water-closet waste may reduce to 2 inches and also take a kitchen sink under 10.16(6)(b).

PSI still lists this under written Venting. Candidates lose items by quoting IPC 912 bathroom-group DFU wet-vent tables, by wet-venting a kitchen, or by calling a lower-floor stack-vented kitchen a 10.16(5) wet vent. Independent OpenExamPrep teaching stays on 248 CMR 10.16 (12/8/2023). This is still plumbing venting, not NFPA 54 appliance venting.

Massachusetts wet vents: 2-inch bathrooms, drainage fittings, centerline break

248 CMR 10.03 defines a wet vent as a waste pipe that also serves as a vent, on the same floor level. 10.16(5)(a) is the only bathroom wet-vent sentence: in a bathroom having a two-inch horizontal waste, a two-inch extension of that horizontal drain connecting to a lavatory may serve as a wet vent for the fixtures it serves. Example 2 is the horizontal-extension picture. The same sentence applies to double or back-to-back bathrooms.

Because that pipe carries waste, the fittings in it are drainage-pattern fittings — sanitary tees and wyes — not vent tees or a prohibited straight T (10.05; vertical back-to-back groups still use the double sanitary drainage tee in 10.15(8)(b)). 10.16(5)(a)1 then adds the Massachusetts geometry IPC does not print this way: the lowest portion of the pipe serving as a wet vent shall break the centerline of the horizontal drain it serves. Example 11 is that break. A dry vent perched on top of the pipe without dropping through the centerline is not 10.16(5).

10.16(1)(c) still says the extension of a horizontal soil or waste pipe shall not serve as a vent, except wet venting under 10.16(5) and a fixture waste of not more than two fixture units on the vertical extension of that horizontal (Example 2). Do not invent a third exception for a kitchen island.

248 CMR 10.16(5)(b) is the stop list. Waste and vent piping that serves any of the following shall not serve as wet vents:

1. Kitchen sink
2. Garbage disposal
3. Dishwasher
4. Washing machine
5. Any fixture for culinary use

Example 10 collects miscellaneous wet-vent drawings. None of them legalize a kitchen on a 10.16(5) wet vent. Example 17 again shows wet venting next to individual and stack venting so you can see the difference, not blend them.

Scenario. A Somerville hall bath has a 2-inch horizontal carrying the water closet and bathtub, with a 2-inch lavatory waste that breaks the centerline and continues as the wet vent. That is 10.16(5). Adding the kitchen sink on the other side of the wall onto that same 2-inch wet vent fails 10.16(5)(b) even if the sink is only 2 DFU.

Circuit, loop, and relief vents on a battery

248 CMR 10.03 distinguishes the two battery vents. A circuit vent extends from the top of the horizontal soil or waste branch in front of the last fixture waste and connects to a vent stack adjacent to the upstream end. A loop vent makes the same takeoff from the battery but connects to a vent stack or stack vent adjacent to the downstream end. Example 15 sends you back to those definitions. 10.16(8)(a) then states the installation: a horizontal branch with two, but not more than eight, floor-outlet fixtures, battery-connected and discharging into the side center of the horizontal battery branch, may be vented by a circuit or loop vent installed downstream of the last fixture connection.

1. A circuit vent connects to a proper vent upstream of the horizontal battery.
2. A loop vent connects to a proper vent downstream of the horizontal battery.

Memory hook: circuit goes back toward the start of the run; loop continues to the downstream stack or stack vent.

Lower floors need a relief vent (10.16(8)(b), Example 14). Lower-floor batteries get a relief vent installed vertically downstream of the first fixture in the battery, connecting back to the circuit or loop vent. Two published modifiers:

• Only two battery-wasted fixtures on the same branch: that relief vent is not required.
• Batteries of more than eight fixtures may be installed if an additional vent is installed for each eight or fewer fixtures connected.

Non-floor-outlet fixtures may also hit the battery drainage system, but they shall be individually or common vented (10.16(8)(c)). A lavatory hanging on a water-closet battery does not ride the circuit vent for free.

Dual (parallel) branches (10.16(8)(d)). Parallel horizontal batteries still follow 10.16(8), except fixtures on each parallel branch are sized on 50 percent of the fixture units permitted on a horizontal branch for a battery drainage system. Vent connections come off at a vertical angle above the centerline of the drain or from the top of the horizontal branch.

Size (10.16(11)(b)). A relief vent on battery-connected fixtures is not less than one-half the diameter of the soil or waste branch. Table 2 from 12.3 still sizes vents that are not given a specific diameter elsewhere (10.16(11)(d)).

Yoke / offset relief (10.16(2)(d) and (2)(f)). In buildings five or more stories, a soil or waste offset more than 45° from vertical is vented as two stacks: a relief as a vertical continuation of the lower section or a yoke vent between the offset and the next lower fixture or branch, plus a yoke on the upper section, each not smaller than the smaller of the main vent or the stack (Example 3). Stacks with more than ten branch intervals get a relief vent at each tenth interval, counting from the top down, sized equal to the vent stack, with the lower wye below the floor’s horizontal branch and the upper wye not less than 3 feet above the floor.

Stack venting: top floor versus lower floors

10.03 defines stack venting as venting a fixture or fixtures through a soil or waste stack, and a stack vent as the portion of that stack six inches above the highest flood-level rim of the highest fixture on it.

Top branch interval — 10.16(6)(a). Fixtures at the highest level may enter a 3-inch soil or waste stack. Continuation of that 3-inch stack, vented through the roof or reconnected above the highest fixture, is accepted when all four are true:

• every such fixture enters the stack independently;
• every waste pitches not more than 1/4 inch per foot;
• the toilet and the bathtub or shower connect at the same floor level;
• every trap stays inside Table 1.

Example 12 shows top-floor stack venting, top-floor stack venting with Estabrook, and stack venting on lower floors. Use the drawing the stem names. Do not invent Estabrook dimensions the code does not print in text.

Lower floors — 10.16(6)(b). Provided the building already has the soil or waste stack required by 10.16(2)(a) (full-size main stack vent or a vent stack no less than 3 inches), the continuation upward of the vertical waste for a toilet may be reduced to 2 inches and serve as the vent for the toilet and the waste for a lavatory, bathtub or shower stall, and a kitchen sink. That kitchen sink is stack venting, not 10.16(5) wet venting. 10.16(6)(c) allows back-to-back bathroom groups when they meet 10.16(6)(a).

Combination waste and vent — 10.16(1)(a). A combination waste-and-vent system is prohibited without Special-Permission by the Board. Even with that permission it is limited to floor drains and sinks, the fixture drains are not individually vented, and every drainage pipe is not less than two pipe sizes larger than 10.15 would require. 10.03 defines that system as horizontal wet venting of fixtures or floor drains through a common waste-and-vent pipe sized for free air above the flow line, and it sends you back to 10.16(1)(a). It is not a substitute for 10.16(5) bathrooms and it is not IPC combination-waste-and-vent as a default method.

Scenario. A three-story Springfield walk-up has a 3-inch soil stack. Top-floor bath: closet, tub, and lavatory enter independently, 1/4-inch pitch, Table 1 arms — 10.16(6)(a). Second-floor kitchen plus bath: the closet’s vertical waste continues up at 2 inches and takes the lavatory, shower, and kitchen — 10.16(6)(b). Do not label that 2-inch kitchen run a 10.16(5) wet vent.

Official resources

• 248 CMR 10.16(5)–(8) (Cornell LII) — wet, stack, circuit, loop, Examples 10–15 and 17
• 248 CMR 10.16(1)(a) Combination waste and vent — Special-Permission, two pipe sizes larger
• 248 CMR 10.03 Circuit Vent / Loop Vent / Wet Vent / Stack Venting
• 248 CMR 10.00 (Mass.gov) — 12/8/2023 official posting