Section 10.16 - Vents and Venting(1)Prohibited Venting. (a)Combination Waste and Vent. A combination waste and vent system shall be prohibited without Special-permission by the Board and shall comply with the following: 1. A combination waste-and-vent system is limited to the installation of floor drains and sinks.2. A combination waste-and-vent system consists of a wet vented installation of waste piping in which fixture drains are not individually vented.3. Every drainage pipe in a combination waste-and-vent system shall be not less than two pipe sizes larger than the size required in 248 CMR 10.15.(b)Crown Venting. No vent shall be installed within two pipe diameters of the trap weir. See 10.16: Example 1.Example 1 - Crown Venting
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(c)Extension of Horizontal Drain. The extension or continuation of a horizontal soil or waste pipe shall not serve as a vent. Exception:
1. When permitted under wet venting 248 CMR 10.16(5); and2. When a fixture waste of not more than two fixture units is connected to the vertical extension of the extended horizontal piping. See 10.16: Example 2.Example 2 - Wet Vent Extension of a Horizontal Drain
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(d)Below Trap Weir. The vent pipe opening from a soil or waste pipe, except for water closets and similar fixtures shall not be below the weir of the trap.(e)Automatic Vent Fittings and Air Admittance Valves: Automatic vents and air admittance valves are not permitted without Special-Permission from the Board.(2)Vent Stacks and Stack Vents. (a) All building drains within a structure shall, at minimum have at least one full size main stack vent or a vent stack no less than three inches in diameter. (See248 CMR 10.16 (11): Table 2: Size and Lengths of Vents for fixture unit values to determine the appropriate stack vent or vent stack size). Buildings with three or more branch intervals shall have at least one main vent stack properly sized in accordance with 10.16: Table 2 from the building drain through to the open air above the roof or connect back to a main stack vent six inches above the flood level rim of the highest fixture being served.(b) A vent stack or a main vent shall be installed with a soil or waste stack whenever individual vents, relief vents, or other branch vents are required.(c)Connections at Base and Top. 1. All main vents or vent stacks shall connect full size at their base to the drainage of the building or to the main soil or waste pipe, at or below the lowest fixture branch.2. All vent pipes shall extend undiminished in size above the roof, or shall be reconnected with the main soil or waste stack a minimum of six inches above the flood level rim of the highest fixture discharging into it.(d)Offsets in Buildings Five or More Stories in Height. 1. Except as provided in 248 CMR 10.15, offsets of more than 45 ° from the vertical in a soil or waste stack may be vented: a. as two separate soil or waste stacks; i. installing a relief vent as a vertical continuation of the lower section of the stack; orii. as a yoke vent connected to the lower section between the offset and the next lower fixture or horizontal branch.iii. The upper section of the offset shall be provided with a yoke vent.iv. The diameter of the vents shall not be less than the diameter of the main vent or of the soil and waste stack, whichever is the smaller. See 10.16: Example 310.16: Example 3 - Offsets of More Than 45°
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(e)Vent Headers. 1. Where vent stacks and stack vents connect to a vent header, the connections shall be made at the top of the stacks.2. The vent header shall connect to a vent extension through the roof.3. When more than two four-inch soil or waste stacks are connected, the vent header extension through the roof shall be five inches in diameter.4. When more than four four-inch stacks are connected, the diameter shall be six inches in diameter.(f)Relief Vents for Vents in Buildings with More than Ten Branch Intervals.1. Soil and waste stacks in buildings having more than ten branch intervals shall be provided with a relief vent at each tenth interval installed, beginning from the top floor down.2. The size of the relief vent shall be equal to the size of the vent stack to which it connects.3. The lower end of each relief vent shall connect to the soil or waste stack through a wye below the horizontal branch serving the floor and the upper end shall connect to the vent stack through a wye not less than three feet above the floor level.(3)Vent Terminals. (a)Extension Above Roof. 1. 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 through the roof. See 10.16: Example 4.2. If the roof area is used for gardening, a parking deck, observation deck or similar purposes, the vent shall extend no less than eight feet above the roof and be increased one pipe diameter. See 10.16: Example 5.3.Increaser. The change in the diameter of a vent terminal shall be made using an increaser; and occur no less than one foot below the roof surface. See 10.16: Example 4.Example 4 - Vent Extension Through Roof
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Example 5 - Vent Extension Through Observation Deck or Similar
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(b)Waterproof Flashings. Each vent terminal shall be made watertight with the roof by proper flashing.(c)Location of Vent Terminal. 1. No vent terminal shall be located directly beneath any building opening or within ten feet horizontally of the opening unless it is at least two feet above the top of said opening. See248 CMR 10.16: Example 6.2. Plumbing vent terminals shall be located no less than 25 feet horizontally from forced air intakes. Vents terminating no less than two feet above the top of the forced air intake may be located as close as ten feet from the forced air intake. 248 CMR 10.16(3)(a) does not apply in this case. See248 CMR 10.16: Example 7.Example 6 - Location of Vent Terminal
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Example 7 - Vent Terminal from Forced Air Inlet
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(d)Vent Extensions Outside of the Building. All soil, waste or vent pipe extensions shall be installed inside the building. Exception: For remodeling and alteration work only, vents may be installed outside the building when all other means of venting have been eliminated or are not practical and with prior permission of the Inspector.
(e)Frost Closure. Where frost closure is likely to occur, each vent extension through a roof shall be at least three inches in diameter.(4)Vent Grades and Connections.(a)Vent Grade. All vents shall be uniformly graded in accordance with 248 CMR 10.05(2) and connected as to drain back to a soil or waste pipe by gravity.(b)Vertical Rise. 1. Where vent pipes connect to a horizontal soil or waste pipe: a. The vent shall be taken off above the center line of the soil or waste pipe drain.b. For other than floor mounted fixtures, the vent pipe shall rise vertically, or at an angle of 45 ° from the vertical, to a point at least six inches above the flood-level rim of the fixture it is venting, before it may offset horizontally.c. For floor mounted fixtures, the vent may be extended horizontally above the centerline of the drain of the fixture to the nearest practical location where it can rise vertically. The vent shall come off the soil or waste pipe above the centerline of the drain not less than 45° from the horizontal before running in a horizontal position.(c)Height Above Fixtures. 1. All connection between a vent pipe and a vent stack or stack-vent shall be made at least six inches above the flood-level rim of the highest fixture served by the vent.2. Horizontal vent pipes forming branch vents, relief vents, or loop vents shall be installed at least six inches above the flood-level rim of the highest fixture served.(5)Wet Venting.(a)Bathroom Wet Vent. In a bathroom having a two-inch horizontal waste, a two-inch extension of the horizontal drain connecting to a lavatory may serve as a wet vent for the fixtures it serves. See 10.16: Example 2. This would also apply to double or back-to-back bathrooms. The lowest portion of the pipe serving as a wet vent shall break the centerline of the horizontal drain it serves. See 10.16: Example 11.
(b)Piping Not to Serve as Wet Vents. Waste and vent piping that serves: (6)Stack Venting. (a)Plumbing Fixtures at the Top Branch Interval of a Stack.1. Plumbing fixtures at the highest level may enter a three-inch soil or waste stack.2. The continuation of a three-inch soil or waste stack vented through the roof or reconnected to the venting system above the highest fixture shall be accepted, provided: a. all such fixtures shall enter said stack independently;b. the waste pipe from all fixtures shall have a pitch of not more than one quarter inch pitch per foot;c. the toilet and bathtub or shower drain connect to the stack at the same floor level; andd. the traps from all fixtures shall be placed in compliance with 248 CMR 10.16(10): Table 1: Distance of Fixture Trap from Vent. See 10.16: Example 12 for Miscellaneous Stack Venting. Example 12 - Miscellaneous Stack Venting
Stack Venting Top Floor
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Stack Venting Top Floor with Estabrook
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Stack Venting Lower Floors
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(b)Stack Venting. Provided there is a soil and/or waste stack in a building as required under 248 CMR 10.16(2)(a), the continuation in an upwards direction of the vertical waste for a toilet may be reduced to two inch and serve as the vent for the toilet and the waste for a lavatory, bathtub, or shower stall, and a kitchen sink. See 10.16: Example 12, Stack Venting on Lower Floors.(c)Back to Back Installation (Stack Vented). Bathroom groups installed back-to-back shall be permissible provided they comply with the provisions of 248 CMR 10.16(6)(a).(7)Common Vents. (a)Individual Vent as Common Vent. 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.(b)Side by Side. If two bathtubs or similar flat bottom fixtures are installed back to back or side by side, a common vent may be used in a vertical position to serve as the vent for both fixtures.(c)Different Levels. A vertical vent may be used for two fixtures that are in the same branch interval but connected to the stack at different levels, not exceeding ten inches center to center, provided: 1. The vertical drain is one pipe diameter larger than the upper fixture drain but is not smaller than the lower fixture drain, whichever is the larger.2. That both wastes for said fixtures conform to 248 CMR 10.16(10): Table 1: Distance of Fixture Trap from Vent. See 10.16: Example 13, Drawing 3.(d)Back-to-back Fixtures. Two fixtures installed back-to-back within the distance allowed between the trap and vent as stated in 248 CMR 10.16(10): Table 1, may have one continuous vent provided each fixture wastes separately into a double sanitary drainage tee fitting having inlet openings at the same level. See248 CMR 10.15(8)(b).(e)Horizontal Waste Branch. 1. Two lavatories or similar fixtures installed adjacent or back-to-back within six feet of a main vented stack, proper wet vent, or continuous waste and vent, may be installed on a two-inch horizontal waste branch without installation of an independent vent, provided: a. the horizontal waste branch is not less than two inches throughout its entire length; andb. the fixture wastes are connected into the side center of the branch. See 10.16: Example 13, Drawing 1. Example 13 - Miscellaneous Types Venting
Drawing 1.
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Drawing 2.
Relief Vent Necessary Per 10.16 Table 1
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Drawing 3.
See248 CMR 10.16(7)(c)
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(8)Venting of a Battery Drainage System.(a) A horizontal branch drain may be vented by either a circuit or loop vent installed downstream of the last fixture connection of the battery provided the branch drain has two, but not more than eight floor outlet fixtures that are connected in a battery and discharged into the side center of the horizontal battery branch drain. 1. A circuit vent shall be connected to a proper vent upstream of the horizontal battery drainage system.2. A loop vent shall be connected to a proper vent downstream of the horizontal battery drainage system.(b) In addition, lower floor branches serving fixtures as described above shall be provided with a relief vent installed vertically downstream of the first fixture in the battery and shall connect back to the circuit or loop vent. See 10:16: Example 14. 1. Where only two fixtures that are battery waste and vented are installed on the same branch, a relief vent as described above shall not be required.2. Batteries of more than eight fixtures may be installed providing an additional vent is installed for each eight or less of the fixtures connected.(c) Fixtures other than the floor outlet type may also connect to the battery drainage system but shall be either individual or common vented.(d)Dual Branches. When parallel branches are installed, all the provisions and requirements of 248 CMR 10.16(8) shall prevail. Exception: Fixtures connecting to each parallel horizontal branch shall be sized based on 50% of the fixture units permitted on a horizontal branch for a battery drainage system.
(c)Vent Connections. Vents installed in a battery drainage system shall be taken off at a vertical angle above the centerline of the drain or from the top of the horizontal branch. Example 14 - Battery Venting Installations
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Example 15 - Circuit and Loop VentingSee 10.03: Definition of Circuit Vent and Loop Vent
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(9)Bow Vents. (a) Bow vents are permitted when a vent for an individual sink or lavatory cannot rise six inches above the flood level rim of the fixture before turning horizontal.(b) The bow vent shall be sized in accordance with 248 CMR 10.16(11): Table 2: Size and Lengths of Vents. Example 16 - Typical Bow Vent Installations
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(10)Fixture Vents. (a)Distance of Trap from Vent. Each fixture trap shall have a vent with a developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in 248 CMR 10.16(12): Table 1: Distance of Fixture Trap from Vent. Click to view image
Table 1
Distance of Fixture Trap from Vent
Diameter of Pipe | Maximum Developed Length of the Pipe |
11/2 | 5" |
2 | 6' |
3 | 8' |
4 | 10' |
Slope not to exceed 1/4" per foot |
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(b)Venting of Fixture Drain Below Trap. 1. The vent pipe openings from a soil or waste pipe, except for toilets and similar fixtures, shall not be below the top weir of the trap.2. An exception to 248 CMR 10.16(10)(b) shall be permitted if the following requirements are satisfied:a. The fixture has a flat bottom with a minimum area of 144 square inches.b. The horizontal section of the fixture waste must comply with 248 CMR 10.16(10): Table 1: Distance of Fixture Trap from Vent and the vertical section shall be at least one pipe size larger than the fixture trap and waste arm.c. The vent opening shall be as high and close to the fixture as possible.(c)Floor-mounted Fixture Outlet.1. When installing the piping for a floor outlet type toilet or similar fixture, the vertical piping distance shall not exceed 20 inches from the finish floor of the fixture served to the center line of the horizontal drain serving such fixture.2. If the vertical distance exceeds 20 inches the fixture shall be individually vented.(11)Size and Length of Vents.(a)Size of Individual Vents. The minimum diameter of an individual vent shall be not less than 11/4 inch or less than 1/2 the diameter of the drain to which it connects.(b)Size of Circuit, Loop and Relief Vents. The diameter of a relief vent shall be not less than 1/2 the diameter of the soil or waste branch to which it connects when fixtures are battery connected.(c)Length and Size of Vent Stacks. The length and size of the vent stack or main vent shall be determined by the developed length from the lowest connect of the vent to the sanitary drainage system to the open air. See248 CMR 10.16(11): Table 2.(d)Size of Vents. The vent pipe sizes shall be determined from their developed length and the total number of fixture units connected thereto, as listed in 248 CMR 10.16(11): Table 2: Size and Lengths of Vents. This table shall be used to size all vents, except for those vents that are specifically sized elsewhere in 248 CMR 3.00 through 10.00.(e)Size of Underground Vent Piping. No portion of the venting system installed underground or below a basement floor, shall be less than two inches in diameter. Table 2
Size and Lengths of Vents
Diameter of Soil or Waste Stack or Branch in Inches | Total Fixture Units Connected to Stack or Branch in Fixture Units | |
11/4 | 11/2 | 2 | 21/2 | 3 | 4 | 5 | 6 | 8 | 10 |
11/2 | 4 | 50 | 150 |
2 | 10 | 25 | 50 | 150 |
21/2 | 28 | 30 | 100 | 300 |
3 | 7 | 42 | 150 | 360 | 1040 |
3 | 21 | 32 | 110 | 270 | 810 |
3 | 53 | 27 | 94 | 230 | 680 |
3 | 102 | 25 | 86 | 210 | 620 |
4 | 43 | 35 | 85 | 250 | 980 |
4 | 140 | 27 | 65 | 200 | 750 |
4 | 320 | 23 | 55 | 170 | 640 |
4 | 530 | 21 | 50 | 150 | 580 |
5 | 190 | 28 | 82 | 320 | 990 |
5 | 490 | 21 | 63 | 250 | 760 |
5 | 940 | 18 | 53 | 210 | 640 |
5 | 1,400 | 16 | 49 | 190 | 590 |
6 | 500 | 33 | 130 | 400 | 1,000 |
6 | 1,100 | 26 | 100 | 310 | 780 |
6 | 2,000 | 22 | 84 | 260 | 660 |
6 | 2,900 | 20 | 77 | 240 | 600 |
8 | 1,800 | 31 | 95 | 240 | 940 |
8 | 3,400 | 24 | 73 | 190 | 720 |
8 | 5,600 | 20 | 62 | 160 | 610 |
8 | 7,600 | 18 | 56 | 140 | 560 |
10 | 4,000 | 31 | 78 | 310 | 960 |
10 | 7,200 | 24 | 60 | 240 | 740 |
10 | 11,000 | 20 | 51 | 200 | 630 |
10 | 15,000 | 18 | 46 | 180 | 570 |
Note 1: Table 2 shall also apply to the sizing of vents for branch soil and waste lines.
To determine size of vent, use the following procedure:
a. Compute total number of fixture units, using 248 CMR 10.15(7): Table 1: Fixture Unit Values for Various Plumbing Fixtures and 248 CMR 10.15(2)(b).b. Knowing total fixture unit load, refer to 248 CMR 10.15(7): Table 3: Maximum Loads in Fixture Units for Soil and Waste Stacks Having One or Two Branch Intervals or 248 CMR 10.15(7): Table 4: Maximum Loads in Fixture Units for any One Branch Interval on Multistory Soil and Waste Stacks depending on number of intervals, to determine size of stack.c. With selected stack size and total fixture unit load refer to 248 CMR 10.16 (11): Table 2: Size and Lengths of Vents and determine size of vent. Follow same procedure to determine size of vents for branch soil and waste lines.(12)Future Venting. (a) In the lowest level of any building there shall be an accessible future vent connection.(b) Buildings that require a main vent stack shall have a future vent connection full size of the vent stack. In all other buildings (including residential) there shall be a minimum of a two-inch future vent connection.(c) All future vent connections shall be drip connected identified and labeled "Future Vent".(13)Venting of Ejector Sumps. (a)Size of Vents. The size and length of all vent pipes serving building sanitary ejector sumps shall be determined from, and in accordance with 248 CMR 10.16 (14) Table 3: Size and Lengths of Sump Vents.(b)Pneumatic Ejector. The air pressure relief pipe from a pneumatic ejector shall not be connected to the regular venting system but shall be vented independently to the atmosphere through the roof. Table 3
Size and Lengths of Sump Vents2
Diameter of Drain to Sump1 | Diameter of Vent (inches) |
1A | 1A | 2 | 2A | 3 | 4 | 5 | 6 | 8 | 10 | 12 |
2 | 23 | 52 | 290 |
2A | 5 | 13 | 89 | 290 |
3 | 2 | 30 | 110 | 290 |
4 | 1 | 17 | 57 | 280 |
5 | 10 | 80 | 280 |
6 | 20 | 97 | 280 |
8 | 3 | 41 | 270 |
10 | 1 | 53 | 250 |
12 | 61 | 230 |
Note 1: Where more than one drain connects to the sump, size vent on the basis of a drain diameter having a cross sectional area equal to the sum of the areas of the multiple drains.
Note 2: The above values provide for a maximum of one-inch pressure drop in the system.
Example 17
Individual Venting - Wet Venting - Stack venting
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Amended by Mass Register Issue 1331, eff. 1/27/2017.Amended by Mass Register Issue 1510, eff. 12/8/2023.