13.1 Individual, Common & Dual Vents
Key Takeaways
- An individual vent under 248 CMR 10.16 is a pipe that vents one fixture drain; it is not less than 1¼ inches and not less than one-half the diameter of the drain it serves, and Table 1 still caps trap-to-vent developed length.
- A common vent is one vertical vent for two fixture traps; 10.16(7)(a) allows it when both fixture drains connect to a vertical drain at the same level.
- Dual vertical (different-level) common venting is 10.16(7)(c): same branch interval, not more than 10 inches center to center, with the vertical drain one pipe diameter larger than the upper fixture drain and not smaller than the lower fixture drain.
- Back-to-back fixtures may share one continuous vent only when each wastes separately into a double sanitary drainage tee with same-level inlets, and each trap-to-vent run stays inside Table 1.
- 248 CMR 10.16 Example 1 still prohibits a vent within two pipe diameters of the trap weir; Example 17 compares individual, wet, and stack venting as different permissions, not as interchangeable drawings.
13.1 Individual, Common & Dual Vents
Quick Answer: An individual vent under 248 CMR 10.16 (effective 12/8/2023) vents one fixture drain. Size it not less than 1¼ inches and not less than one-half the drain it serves. A common vent is one vertical vent for two traps. The textbook pair is 10.16(7)(a): both drains hit a vertical drain at the same level. Dual vertical common venting is 10.16(7)(c): same branch interval, not more than 10 inches center to center, and the vertical drain one pipe diameter larger than the upper fixture drain. Apply Table 1 from 12.3 (1½ in. = 5 ft, 2 in. = 6 ft, 3 in. = 8 ft, 4 in. = 10 ft, slope not more than 1/4 inch per foot) to every arm. Example 1 still bans a vent within two pipe diameters of the trap weir.
The PSI Candidate Information Bulletin dated October 3, 2025 puts this work in written Drain, Waste & Vent — Venting. Chapters 5 and 6 already covered Type B gas vents, draft hoods, and chimneys. Those numbers do not size a lavatory vent. 248 CMR 10.03 defines an individual vent as a pipe installed to vent a fixture drain that connects into the vent system above the fixture served or terminates above the roof, and a common vent as a vertical vent that serves two fixtures and connects in compliance with 10.16 Table 1. Independent OpenExamPrep teaching uses the 12/8/2023 code. It is not a Board publication.
Section 12.3 already walked Table 1 and the Table 2 entry method. This section applies those feet. It does not re-lecture the whole table.
Individual vents: size, takeoff, and Example 1
248 CMR 10.16(11)(a) is the individual-vent floor: diameter not less than 1¼ inches and not less than one-half the diameter of the drain to which it connects. A 3-inch water-closet drain does not get a 1¼-inch individual vent. Underground or below a basement floor, no portion of the venting system is less than two inches (10.16(11)(e)). Table 2 from 12.3 may still force a larger vent because the developed length is long; half-diameter is a minimum, not a free pass.
248 CMR 10.16(4)(b) sets the takeoff. From a horizontal soil or waste pipe, the vent comes off above the center line. For fixtures that are not floor-mounted, the vent rises vertically, or at 45° from the vertical, to at least six inches above the flood-level rim before it may offset horizontally. Floor-mounted fixtures may run the vent horizontally above the drain centerline to the nearest practical rise, but the takeoff is still not less than 45° from the horizontal. 10.16(4)(c) then requires every connection of that vent into a vent stack or stack vent, and every horizontal branch, relief, or loop vent, to sit at least six inches above the flood-level rim of the highest fixture served. Grade the dry vent so it drains back to soil or waste by gravity (10.16(4)(a) and 10.05(2)).
Example 1 (Crown Venting) is the geometry that fails first. 10.16(1)(b) says no vent shall be installed within two pipe diameters of the trap weir. On a 1½-inch trap, two diameters is three inches of pipe between weir and vent opening — not a tee on the crown of the bend. 10.08 already lists crown-vented traps as a prohibited trap type; 10.16 makes the same geometry illegal on a legal P-trap. 10.16(1)(d) and 10.16(10)(b) add that, except for water closets and similar fixtures, the vent opening shall not be below the weir. The printed exception is narrow: a flat-bottom fixture with at least 144 square inches of bottom area, Table 1 on the horizontal waste, a vertical section one pipe size larger than the trap and waste arm, and the vent opening as high and close to the fixture as possible.
Floor-outlet individual-vent trigger (10.16(10)(c)). On a floor-outlet toilet or similar fixture, vertical piping from the finish floor to the center line of the horizontal drain shall not exceed 20 inches. If it does, that fixture is individually vented. A 24-inch closet drop on a 3-inch arm is not “close enough to Table 1.”
Scenario (Table 1, not a new table lecture). A Worcester lavatory uses 1½-inch pipe. Table 1 already gave you 5 feet of developed length from weir to vent, slope not more than 1/4 inch per foot. A 4-foot-6-inch arm with the sanitary-tee takeoff 4 inches downstream of the weir (more than two 1½-inch diameters) is legal. A 6-foot arm on 1½-inch pipe is the 2-inch row, not this fixture. A vent drilled into the crown of the U-bend is Example 1, even if the arm is only 2 feet long.
Common vents at the same level
248 CMR 10.16(7)(a) is the sentence most isometric items quote: an individual vent, installed vertically, may be used as a common vent for two fixture traps when both fixture drains connect with a vertical drain at the same level. That is two lavatories, two urinals, or two similar fixtures hitting a vertical at one elevation, with one dry vent continuing up. It is not two fixtures on a long unvented horizontal with a single downstream stack pretending to be a common vent.
10.16(7)(b) covers side-by-side or back-to-back bathtubs (or similar flat-bottom fixtures): a common vent may be used in a vertical position for both. Keep each trap-to-vent developed length inside Table 1. A 1½-inch tub arm still has 5 feet, not the 6-foot 2-inch row.
10.16(7)(d) is the back-to-back continuous vent. Two fixtures installed back-to-back within Table 1 may share one continuous vent if each fixture wastes separately into a double sanitary drainage tee whose inlet openings are at the same level. The cross-reference is 248 CMR 10.15(8)(b): vertical back-to-back uses double sanitary drainage-pattern tee fittings (horizontal back-to-back uses double sanitary wyes to stop throw-over). A double straight T is not a drainage fitting under 10.05, and it is not this permission.
| Pattern (10.16(7)) | What must be true |
|---|---|
| (a) Individual as common | One vertical vent; both drains hit a vertical drain at the same level |
| (b) Side-by-side / back-to-back tubs | Flat-bottom fixtures; common vent stays vertical |
| (c) Dual vertical (different levels) | Same branch interval; ≤ 10 inches center to center; vertical drain one size larger than the upper fixture drain and not smaller than the lower |
| (d) Back-to-back continuous | Each trap inside Table 1; double sanitary drainage tee; inlets at the same level |
| (e) Two lavatories on a 2-inch branch | Adjacent or back-to-back; within 6 feet of a main vented stack, proper wet vent, or continuous waste-and-vent; branch 2 inches throughout; wastes into the side center |
Scenario. Two Quincy lavatories sit back-to-back on a 1½-inch double sanitary tee. Each developed length to the continuous vent is 4 feet 8 inches (inside 5 feet). Inlets are at the same level. That is 10.16(7)(d). Sliding one lavatory 12 inches higher on the stack without enlarging the vertical drain is not (7)(d) and is not yet (7)(c).
Dual vertical vents and the 2-inch lavatory branch
248 CMR 10.16(7)(c) is the dual vertical (different-level) rule. A vertical vent may serve two fixtures that are in the same branch interval but connect to the stack at different levels, not exceeding 10 inches center to center, provided:
- The vertical drain is one pipe diameter larger than the upper fixture drain, and is not smaller than the lower fixture drain, whichever is larger.
- Both wastes stay inside Table 1. Example 13, Drawing 3 is the Board picture.
Worked sizes. Upper fixture is a 1½-inch lavatory. Lower fixture is a 1½-inch bathtub. The vertical drain cannot remain 1½ inches. It becomes 2 inches (one size larger than the upper). Each trap-to-vent run still uses its Table 1 row: 1½-inch arms 5 feet, not 6. If the lower fixture were a 3-inch water closet, the vertical drain cannot be smaller than that 3-inch lower drain, so the “one size larger than the upper” sentence does not shrink a closet bend. Ten inches is a center-to-center cap, not a “close enough” slogan. Twelve inches of offset between inlets fails (7)(c) even if both arms are short.
10.16(7)(e) is the other two-lavatory shortcut. Two lavatories or similar fixtures, adjacent or back-to-back, within 6 feet of a main vented stack, a proper wet vent, or a continuous waste and vent, may sit on a 2-inch horizontal waste branch without an independent vent when (i) that branch is not less than 2 inches throughout and (ii) the fixture wastes connect into the side center. Example 13, Drawing 1 is that picture. Drawing 2 is the opposite reminder: when Table 1 is exceeded, a relief vent is necessary. Do not stretch a 7-foot 1½-inch arm and call it (7)(e). The branch is 2 inches, the distance is 6 feet, and the connection is side center, not the invert.
Wet-vent preview only. 10.03 defines a wet vent as a waste pipe that also serves as a vent, on the same floor level. 10.16 Example 17 is titled Individual Venting – Wet Venting – Stack venting. That one drawing is a comparison, not a license to mix the three. Individual venting is this section. Wet venting and stack venting are 13.2. A kitchen sink is not a 10.16(5) wet-vent fixture; do not preview it as one.
Keep Type B appliance vents in Chapters 5–6. On this section, the scored ideas are 1¼-inch / half-diameter individual vents, same-level common vents, 10-inch dual vertical with the enlarged vertical drain, double sanitary tees for back-to-back, Table 1 feet applied to each arm, and Example 1 crown-vent geometry.
Official resources
- 248 CMR 10.16 Vents and Venting (Cornell LII) — (7) common vents, Table 1 application, Examples 1 and 17
- 248 CMR 10.03 Definitions (Cornell LII) — individual vent, common vent, continuous vent
- 248 CMR 10.15(8)(b) Back-to-back fittings (Cornell LII) — double sanitary drainage tees and wyes
- 248 CMR 10.00 (Mass.gov) — 12/8/2023 official posting
What is the minimum diameter of an individual vent under 248 CMR 10.16(11)(a)?
When may one vertical individual vent serve as a common vent for two fixture traps under 248 CMR 10.16(7)(a)?
Two fixtures in the same branch interval connect to a stack 8 inches apart vertically. What does 248 CMR 10.16(7)(c) require of the vertical drain?
A 1½-inch lavatory trap has a vent opening 1 inch downstream of the weir, and the developed length to that opening is 4 feet. Which statement is correct?