13.3 Vent Terminations & Special Venting
Key Takeaways
- A through-roof plumbing vent shall be no less than 2 inches in diameter and shall extend to a point between 18 and 24 inches above the roof penetration; the increaser, when used, occurs no less than 1 foot below the roof (248 CMR 10.16 Example 4).
- Occupied roofs used for gardening, parking, or observation require the vent to extend no less than 8 feet and to increase one pipe diameter (Example 5).
- Keep terminals out from under building openings and at least 10 feet horizontally unless the terminal is at least 2 feet above the opening (Example 6); forced-air intakes need 25 feet horizontally, or 10 feet if the vent is at least 2 feet above the top of the intake (Example 7).
- Automatic vents and air-admittance valves are not permitted without Special-Permission from the Board; Product-accepted listing is not a substitute for that permission, and IPC AAV rules do not apply.
- Bow vents are the 10.16(9) permission when an individual sink or lavatory vent cannot rise 6 inches above the flood-level rim before turning horizontal; suds relief for multi-story laundries lives in 10.15(8)(d), not in an IPC island-vent article.
13.3 Vent Terminations & Special Venting
Quick Answer: Take a plumbing vent through the roof at no less than 2 inches, ending 18 to 24 inches above the penetration (248 CMR 10.16 Example 4). Put the increaser at least 1 foot below the roof. Occupied roofs (garden, parking, observation) need 8 feet of extension and one pipe diameter of increase (Example 5). Stay 10 feet from building openings unless the terminal is 2 feet above the opening (Example 6). Stay 25 feet from forced-air intakes, or 10 feet if the vent is 2 feet above the intake (Example 7). Frost-closure locations use at least 3 inches. Outside-the-building vents are remodel-only with the Inspector’s prior permission. Air-admittance valves need Board Special-Permission. Bow vents cover the sink or lavatory that cannot rise 6 inches above the flood-level rim. Suds relief is 10.15(8)(d).
This is still the PSI Venting cluster and still 248 CMR 10.16, not NFPA 54 termination of a Type B gas vent. Independent OpenExamPrep teaching uses the 12/8/2023 code.
Through-roof extensions: Examples 4 and 5
248 CMR 10.16(3)(a)1 is the ordinary roof: the vent extension through a roof shall be no less than two inches in diameter and shall extend to a point between 18 and 24 inches above the penetration. Example 4 is that picture. A 1½-inch dry vent that served a lavatory increases before it hits weather. 10.16(3)(a)3 says the change in diameter uses an increaser and occurs no less than one foot below the roof surface. Increasing in the flashing jack, or with a coupling sitting on the shingles, is not Example 4.
Occupied roofs — 10.16(3)(a)2 and Example 5. If the roof is used for gardening, a parking deck, an observation deck, or similar purposes, the vent extends no less than eight feet above the roof and is increased one pipe diameter. A 3-inch stack vent on a roof deck becomes 4 inches and rises 8 feet, not 18 inches behind a planter.
Waterproof flashings (10.16(3)(b)) make each terminal watertight with the roof. 10.16(2)(c)2 still requires vent pipes to extend undiminished in size above the roof, or to reconnect to the main soil or waste stack at least 6 inches above the flood-level rim of the highest fixture discharging into it. Vent headers (10.16(2)(e)) connect at the top of the stacks, go through the roof, and jump to 5 inches when more than two 4-inch stacks join, or 6 inches when more than four 4-inch stacks join.
| Terminal situation | Massachusetts number |
|---|---|
| Ordinary through-roof (Example 4) | Min 2 in., 18–24 in. above the penetration |
| Increaser location (Example 4) | ≥ 1 ft below the roof surface |
| Garden / parking / observation deck (Example 5) | ≥ 8 ft above the roof and +1 pipe diameter |
| Frost closure likely (10.16(3)(e)) | Through-roof extension at least 3 in. |
| Header of more than two 4-in. stacks | Roof extension 5 in. |
| Header of more than four 4-in. stacks | Roof extension 6 in. |
Scenario. A Pittsfield 1½-inch lavatory vent is 40 feet of developed length and still legal in Table 2 as 1½ inches indoors. Through the roof it is not left 1½ inches. Increase to at least 2 inches, with that increaser 12 inches or more below the roof, and stop between 18 and 24 inches above the shingles unless the roof is an occupied deck.
Distance from openings and air intakes: Examples 6 and 7
248 CMR 10.16(3)(c)1 / Example 6. No vent terminal shall be located directly beneath any building opening or within 10 feet horizontally of that opening unless it is at least 2 feet above the top of the opening. A bathroom window, a dormer, or a door to a roof deck is a building opening. Eighteen inches of roof height does not excuse a terminal sitting 4 feet beside a window sill; either move it 10 feet away or raise it 2 feet above the top of that window.
Forced-air intakes — 10.16(3)(c)2 / Example 7. Plumbing vent terminals shall be no less than 25 feet horizontally from forced-air intakes. Vents that terminate no less than 2 feet above the top of the forced-air intake may be as close as 10 feet. The code then says 10.16(3)(a) does not apply in this case — do not use the 18-to-24-inch roof-height sentence as a substitute for the intake clearance. Intake clearance is its own pair of numbers: 25 feet, or 10 feet plus 2 feet of height above the intake.
These are plumbing vent terminals. A Category I draft-hood connector from Chapters 5–6 is a different code.
Frost, exterior vents, grades, and AAVs
Frost closure (10.16(3)(e)). Where frost closure is likely to occur, each vent extension through a roof shall be at least 3 inches in diameter. Massachusetts winters make this a live item. A 2-inch Example 4 terminal is the ordinary minimum; frost-closure sites step to 3 inches.
Outside the building (10.16(3)(d)). All soil, waste, or vent pipe extensions shall be installed inside the building. The only exception is remodeling and alteration work, and only when all other means of venting have been eliminated or are not practical, and with prior permission of the Inspector. New construction does not get a painted-on exterior 2-inch vent because the carpenter boxed out the chase. “The inspector might allow it” is not a plan-review sentence until that prior permission exists.
Grades (10.16(4)). Dry vents are uniformly graded per 10.05(2) and drain back to soil or waste by gravity. The 6-inch-above-flood-rim rise before a horizontal offset, already used in 13.1, is why island counters fail ordinary individual venting and why bow vents exist.
Automatic vents and air-admittance valves — 10.16(1)(e). Automatic vents and air-admittance valves are not permitted without Special-Permission from the Board. That is the whole Massachusetts AAV rule. Product-accepted under 248 CMR 3.04 does not, by itself, let you terminate a lavatory into an AAV instead of 10.16. IPC / IRC Chapter 31 AAV locations, ASSE 1051 “individual and branch-type” permissions, and “AAVs are allowed if they are listed” are not 248 CMR 10.16. If a stem offers an AAV as the default island or remodel vent, the Massachusetts answer is still Special-Permission, not an IPC installation checklist.
Island fixtures, bow vents, and suds relief
248 CMR 10.16 has no IPC-style island-vent article. An island kitchen sink or island lavatory still owes a trap seal protected by 10.16. If you can rise vertically (or at 45° from vertical) to 6 inches above the flood-level rim before turning horizontal, you have an ordinary individual vent (10.16(4)(b)). If you cannot get that rise — the usual island-counter problem — 10.16(9) permits a bow vent for an individual sink or lavatory. Size the bow vent from 10.16(11) Table 2. Example 16 is the typical bow-vent drawing. A bow vent is not a 10.16(5) wet vent, and a kitchen sink still cannot use 10.16(5) wet venting. Do not import an IRC P3112 island-vent loop or an AAV island kit as if 10.16 printed it.
Suds relief is 10.15(8)(d), not 10.16. In buildings where laundries are installed on more than three branch intervals, the waste connects to an independent laundry stack. That stack hits an independent laundry main drain. That main drain connects to the building drain a minimum of 40 pipe diameters upstream and downstream of any soil or waste stack. A suds relief vent connects to the laundry main drain a minimum of 40 pipe diameters downstream from the base of the laundry stack, and that relief vent connects to a vent a minimum of two branch intervals above the base of the laundry stack. Example 4 under 10.15 prints the laundry-stack drawing and the suds-pressure-relief-vent size table (waste size versus relief-vent size). Read those printed pairs on the exam; do not paste an IPC suds table. The Inspector may permit a variation when conditions will not allow compliance — that is an inspection permission, not a design default.
Worked geometry. A 3-inch laundry stack: 40 pipe diameters is 40 × 3 inches = 120 inches = 10 feet. Put the building-drain connections at least that far from other soil or waste stacks, put the suds relief at least that far downstream of the laundry-stack base, and land the relief into a vent at least two branch intervals above that base. A 4-inch laundry stack would be 40 × 4 inches = 13 feet 4 inches of separation.
Do not terminate a plumbing vent like a gas vent, do not treat AAVs as a Massachusetts standard method, and do not use an IPC island chapter in place of 10.16(9) bow vents.
Official resources
- 248 CMR 10.16(3) Vent Terminals (Cornell LII) — Examples 4–7, frost, exterior vents, AAVs at 10.16(1)(e)
- 248 CMR 10.16(9) Bow Vents — Example 16, Table 2 sizing
- 248 CMR 10.15(8)(d) Laundry stacks and suds relief — 40 pipe diameters, two branch intervals
- 248 CMR 10.00 (Mass.gov) — 12/8/2023 official posting
What through-roof size and height does 248 CMR 10.16(3)(a) require for an ordinary plumbing vent, and where is the increaser placed?
How far must a plumbing vent terminal be from a building window and from a forced-air intake under 248 CMR 10.16(3)(c)?
May a Massachusetts journeyman use a Product-accepted air-admittance valve, or run a vent up the outside of the building, without further permission?
An island lavatory cannot rise 6 inches above the flood-level rim before turning horizontal, and a four-story building has laundry rooms on every floor. Which pair of Massachusetts rules applies?