CPC · California Plumbing Code

How far can a minimum-sized vent run horizontally?

If your vent is the minimum size allowed, the California Plumbing Code limits horizontal runs to one‑third of that vent’s permitted developed length (see **§ 904.2**). To run more horizontal distance, you must increase the vent **one pipe size for its entire length**; also remember that required vertical rise above the fixture (6 in) consumes part of the developed length. Consult **Table 703.2** for the numeric developed‑length allowed for your vent size and fixture unit load.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Plumbing Code limits the horizontal portion of a minimum-sized vent to not more than one‑third of the total permitted length (as measured from Table 703.2). The controlling rule is § 904.2 — if the vent is upsized one pipe size for its entire length, the horizontal limitation does not apply. § 904.1 defines the baseline minimum vent dimensions that feed this rule.

The single most important rule: a vent that is the minimum allowable diameter may only be installed horizontally for up to one‑third of its total permitted developed length; upsize the vent for the full run and the one‑third cap disappears. § 904.2

Requirements in detail

What is a “minimum‑sized vent”?

  • A minimum‑sized vent is a vent installed at the smallest diameter permitted by the CPC for the fixtures it serves. The CPC requires an individual vent be not less than 1 1/4 inches (32 mm) in diameter and not less than one‑half the diameter of the drain it serves (see § 904.1) — those are the baseline sizing constraints. § 904.1

How to determine the “total permitted length”

  • The phrase “total permitted length” used in § 904.2 refers to the maximum developed length for a vent pipe of that size and the fixture unit load, as set out in the vent‑sizing and length provisions (see Table 703.2 referenced by § 904.1/§ 904.2). The CPC ties permitted length to pipe diameter and fixture units; consult Table 703.2 to get the numerical developed‑length allowance for your vent size and load. § 904.1 and § 904.2
  • Note: the specific numeric values from Table 703.2 were not included in the retrieved text excerpts here, so you must look up that table in your copy of the CPC to obtain the actual developed‑length numbers for the vent sizes/fixture units you’re working with.

How the horizontal limit is applied

  • The CPC states: not more than one‑third of the total permitted length of a minimum‑sized vent may be installed in a horizontal position. In practical terms:
    • Determine the permitted developed length for the minimum vent size from Table 703.2.
    • Multiply that permitted developed length by 1/3 to get the maximum horizontal run allowed for that minimum‑sized vent.
    • Any remaining length must be vertical (or otherwise count toward the developed length).
  • If you increase the vent one pipe size for its entire developed length, the one‑third horizontal limit does not apply. § 904.2

Installation/clearance practical notes

  • Vents must rise vertically to a point not less than 6 inches (152 mm) above the fixture flood‑level rim before offsetting horizontally, unless structural conditions prohibit (see § 905.3) — this vertical rise often reduces available developed length for horizontal run. § 905.3
  • Vents must be free of sags and properly graded where required and must not receive discharge of soil/waste where prohibited (see related vent connections/grades provisions). § 905.1 – § 905.6

Decision‑relevant dimensions/values (quick reference)

Decision item Value / threshold Code Reference
Minimum allowable individual vent diameter 1 1/4 in (32 mm) § 904.1
Minimum vent relative to drain Not less than one‑half the diameter of the drain § 904.1
Maximum horizontal portion for a minimum‑sized vent Not more than one‑third of total permitted length § 904.2
Exception (removes horizontal cap) If increased one pipe size for entire length § 904.2
Required rise above fixture flood rim before horizontal offset Not less than 6 in (152 mm) § 905.3
Source of numeric developed‑length values Table 703.2 (consult table for actual lengths) § 904.1 / § 904.2

Exceptions & special cases

  • Upsized vents: If the vent is increased one pipe size for its entire developed length, the one‑third horizontal limit does not apply — you may run the larger vent horizontally to the extent allowed by its developed‑length and other applicable provisions. § 904.2
  • Where structural constraints make vertical rises impossible, CPC allows exceptions to vertical rise requirements (see § 905.3 for rise requirements and the allowance when “prohibited by structural conditions”); always document such constraints and follow any required alternative measures. § 905.3
  • Table dependence: Because the actual maximum developed length depends on Table 703.2, different fixture unit loads and vent diameters produce different permitted lengths; the one‑third rule always applies to the total permitted length provided by that table unless the upsizing exception is used. § 904.2

Common mistakes

  • Treating the one‑third rule as one‑third of the physical building run rather than one‑third of the total permitted developed length from Table 703.2. The correct base is the table value, not just measured job conditions. § 904.2 / Table 703.2
  • Forgetting to account for required vertical rise (minimum 6 in) before offsetting horizontally; that vertical segment consumes part of the developed length. § 905.3
  • Upsizing only part of the run: the exception only applies when the vent is increased one pipe size for its entire length. Upsizing a portion does not remove the one‑third limitation. § 904.2
  • Using a minimum vent diameter smaller than 1 1/4 in or smaller than half the drain diameter — both are code violations. § 904.1

Worked example — applying the rule with numbers (method)

  1. Determine the minimum‑sized vent required for the fixtures (per § 904.1). Assume the vent is minimum‑sized (1 1/4 in) for this example. § 904.1
  2. Look up the permitted developed length for a 1 1/4‑inch vent serving the given fixture unit load in Table 703.2. (Table 703.2 numeric values are not included in the retrieved text; you must consult the CPC table in your code book.) § 904.1 / Table 703.2
  3. Example calculation using an illustrative table value (ONLY an illustration — replace with the Table 703.2 number for your job):
    • Suppose Table 703.2 allowed a developed length of 30 ft for the 1 1/4‑in vent at the fixture unit load in question (this 30 ft is an example only).
    • Maximum horizontal run for the minimum‑sized vent = 1/3 × 30 ft = 10 ft.
    • Ensure at least 6 in of vertical rise above the fixture flood rim before offsetting horizontally; that vertical rise is part of the developed length. § 904.2 / § 905.3
  4. If you instead upsized the vent to the next larger diameter for the entire run (for example, from 1 1/4 in to 1 1/2 in), the one‑third cap would not apply; you would use the permitted developed length for the larger pipe (Table 703.2) without the one‑third horizontal restriction. § 904.2

Note: the numeric example used an assumed Table value (30 ft) for illustration only — consult Table 703.2 in the CPC for the exact permitted developed length that applies to your vent size and fixture unit load. § 904.2 / Table 703.2

Related provisions

  • § 904.1 — Vent pipe size requirements (minimum diameter, relation to drain size).
  • § 905.1 – § 905.3 — Vent pipe grade and the required vertical rise to 6 in above flood rim before offset.
  • § 905.2 — Requirements where vents connect to horizontal drainage pipe (invert above drainage centerline).
  • § 908.1 – § 908.2 — Wet venting and horizontal wet vent rules (where horizontal runs and wet venting interplay).
  • Table 703.2 — The table that provides the total permitted developed length and is the numeric basis for the one‑third calculation (consult the table in the CPC). § 904.1 / § 904.2

Code references

Grounded in the retrieved California Plumbing Code — click a citation to read the verbatim passage:

  • CPC § 904.2 High relevance — show source text

    904.2 Length. Not more than one-third of the total permitted length, in accordance with Table 703.2, of a minimumsized vent shall be installed in a horizontal position. Where a minimum-sized vent is increased one pipe size for its entire length, the maximum length limitation shall not apply.

    2025 CALIFORNIA PLUMBING CODE 189

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    VENTS

    905.0 Vent Pipe Grades and Connections. 905.1 Grade. Vent and branch vent pipes shall be free from drops or sags, and each such vent shall be level or shall be so graded and connected as to drip back by gravity to the drainage pipe it serves. 905.2 Horizontal Drainage Pipe. Where vents connect to a horizontal drainage pipe, each vent pipe shall have its invert taken off above the drainage centerline of such pipe downstream of the trap being served. 905.3 Vent Pipe Rise. Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than 6 inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and where two or more vent pipes converge, each such vent pipe shall rise to a point not less than 6 inches (152 mm) in height above the floodlevel rim of the plumbing fixture it serves before being connected to any other vent. Vents less than 6 inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain. 905.4 Roof Termination. Vent pipes shall extend undiminished in size above the roof, or shall be reconnected with soil or waste vent of the proper size. 905.5 Location of Opening. The vent pipe opening from soil or waste pipe shall not be below the weir of the trap.

    Exception: Water closets and similar fixtures. 905.6 Common Vertical Pipe. Two fixtures shall be permitted to be served by a common vertical pipe where each such fixture wastes separately into an approved double fitting having inlet openings at the same level.

    906.0 Vent Termination.

    906.1 Roof Termination. Each vent pipe or stack shall extend through its flashing and shall terminate vertically not less than 6 inches (152 mm) above the roof nor less than 1 foot (305 mm) from a vertical surface. ABS and PVC piping exposed to sunlight shall be protected by water based synthetic latex paints.

    906.2 Clearance. Each vent shall terminate not less than 10 feet (3048 mm) from, or not less than 3 feet (914 mm) above, an openable window, door, opening, air intake, or vent shaft, or not less than 3 feet (914 mm) in every direction from a lot line, alley and street excepted. 906.2.1 [OSHPD 1,1R, 2, 3, 4 & 5] Each vent pipe shall terminate not less than twenty-five (25) feet (7620 mm) from any air intake or vent shaft. 906.3 Use of Roof. Vent pipes shall be extended separately or combined, of full required size, not less than 6 inches (152 mm) above the roof or firewall. Flagpoling of vents shall be prohibited except where the roof is used for assembly purposes or parking.

  • CPC § 811.0. High relevance — show source text

    defined in Section 811.0.

    903.2.3 Marking. Copper or copper alloy tubing, in addition to the required incised marking, shall be marked in accordance with either ASTM B306 or ASTM B88. The colors shall be Type K, green; Type L, blue; Type M, red; and Type DWV, yellow. 903.3 Changes in Direction. Changes in the direction of vent piping shall be made by the appropriate use of approved fittings, and no such pipe shall be strained or bent. Burred ends shall be reamed to the full bore of the pipe.

    904.0 Size of Vents.

    904.1 Size. The size of vent piping shall be determined from its length and the total number of fixture units connected thereto, in accordance with Table 703.2. The diameter of an individual vent shall be not less than 1 [1] ⁄ 4 inches (32 mm) nor less than one-half the diameter of the drain to which it is connected. In addition, the drainage piping of each building and each connection to a public sewer or a private sewage disposal system shall be vented by means of one or more vent pipes, the aggregate cross-sectional area of which shall be not less than that of the largest required building sewer as determined from Table 703.2. Vent pipes from fixtures located upstream from pumps, ejectors, backwater valves, or other devices that obstruct the free flow of air and other gases between the building sewer and the outside atmosphere shall not be used for meeting the cross-sectional area venting requirements of this section. Exception: Where connected to a common building sewer, the drainage piping of two or more buildings located on the same lot and under one ownership shall be permitted to be vented by means of piping sized in accordance with Table 703.2, provided the aggregate cross-sectional area of vents is not less than that of the largest required common building sewer. 904.2 Length. Not more than one-third of the total permitted length, in accordance with Table 703.2, of a minimumsized vent shall be installed in a horizontal position. Where a minimum-sized vent is increased one pipe size for its entire length, the maximum length limitation shall not apply.

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    VENTS

    905.0 Vent Pipe Grades and Connections. 905.1 Grade. Vent and branch vent pipes shall be free from drops or sags, and each such vent shall be level or shall be so graded and connected as to drip back by gravity to the drainage pipe it serves. 905.2 Horizontal Drainage Pipe. Where vents connect to a horizontal drainage pipe, each vent pipe shall have its invert taken off above the drainage centerline of such pipe downstream of the trap being served. 905.3 Vent Pipe Rise. Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than 6 inches (152 mm) above the flood-level rim of the fixture served before offsetting horizontally, and where two or more vent pipes converge, each such vent pipe shall rise to a point not less than 6 inches (152 mm) in height above the floodlevel rim of the plumbing fixture it serves before being connected to any other vent. Vents less than 6 inches (152 mm) above the flood-level rim of the fixture shall be installed with approved drainage fittings, material, and grade to the drain.

  • CPC § 12.7.4.1 High relevance — show source text

    (4) For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.

    (5) Engineering methods. [NFPA 54:12.7.4.1] 509.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 509.6.2.1(3) or Section 509.6.2.1(4) shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60 degree (1.05 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 509.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions. [NFPA 54:12.7.4.3] 509.6.2.4 Sizing. Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods. [NFPA 54:12.7.4.4] 509.6.3 Gas Vents Serving Appliances on More than One Floor. Where a common vent is installed in a multistory installation to vent Category I appliances located on more than one floor level, the venting system shall be designed and installed in accordance with engineering methods. Crawl spaces, basements, and attics shall be considered as floor levels. [NFPA 54:12.7.5.1]

    509.6.3.1 Occupiable Space. All appliances connected to the common vent shall be located in rooms separated from occupiable space. Each of these rooms shall have provisions for an adequate supply of combustion, ventilation, and dilution air that is not supplied from occupiable space. [NFPA 54:12.7.5.2] (See Figure 509.6.3.1)

    FIGURE 509.6.3.1

    PLAN VIEW OF PRACTICAL SEPARATION

    METHOD FOR MULTISTORY GAS VENTING

    [NFPA 54: FIGURE A.12.7.5.2]

    509.6.3.2 Multistory Venting System. The size of the connectors and common segments of multistory venting systems for appliances listed for use with a Type B double-wall gas vent shall be in accordance with Table 510.2(1), provided all of the following apply:

    (1) The available total height ( H ) for each segment of a multistory venting system is the vertical distance between the level of the highest draft hood outlet or flue collar on that floor and the centerline of the next highest interconnection

    tee.

  • CPC § 25.4 Medium relevance — show source text

    For SI Units: 1 inch = 25.4 mm, 1 foot = 304.8 mm

    509.6.1.1 Insulation Protection Shield. Where a vent passes through an insulated assembly, an approved metal shield constructed of steel having a thickness of not less than 26 gauge (0.0179 inch) (0.45 mm) (No. 26 gauge) shall be installed between the vent and insulation. The shield shall extend not less than 2 inches (51 mm) above the insulation and be secured to the structure in accordance with the

    manufacturer’s installation instructions.

    509.6.2 Size of Gas Vents. Venting systems shall be sized and constructed in accordance with Section

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    WATER HEATERS

    509.6.2.1 through Section 509.6.2.3 and the appliance manufacturer’s instructions. [NFPA 54:12.7.4] 509.6.2.1 Category I Appliances. The sizing of natural draft venting systems serving one or more listed appliances equipped with a draft hood or appliances listed for use with a Type B gas vent, installed in a single story of a building, shall be in accordance with one of the following:

    (1) The provisions of Section 510.0.

    (2) Vents serving fan-assisted combustion system appliances, or combinations of fan-assisted combustion system and draft hood-equipped appliances, shall be sized in accordance with Section 510.0 or other engineering methods.

    (3) For sizing an individual gas vent for a single, draft hood-equipped appliance, the effective area of the vent connector and the gas vent shall be not less than the area of the appliance draft hood outlet or greater than seven times the draft hood outlet area.

    (4) For sizing a gas vent connected to two appliances with draft hoods, the effective area of the vent shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.

    (5) Engineering methods. [NFPA 54:12.7.4.1] 509.6.2.2 Vent Offsets. Type B and Type L vents sized in accordance with Section 509.6.2.1(3) or Section 509.6.2.1(4) shall extend in a generally vertical direction with offsets not exceeding 45 degrees (0.79 rad), except that a vent system having not more than one 60 degree (1.05 rad) offset shall be permitted. Any angle greater than 45 degrees (0.79 rad) from the vertical is considered horizontal. The total horizontal distance of a vent plus the horizontal vent connector serving draft hood-equipped appliances shall not be greater than 75 percent of the vertical height of the vent. [NFPA 54:12.7.4.2] 509.6.2.3 Category II, Category III, and Cate- gory IV Appliances. The sizing of gas vents for Category II, Category III, and Category IV appliances shall be in accordance with the appliance manufacturers’ instructions. The sizing of plastic pipe specified by the appliance manufacturer as a venting material for Category II, III, and IV appliances shall be in accordance with the appliance manufacturers’ instructions.

  • CPC § 910.7 Medium relevance — show source text

    910.7 Fixtures. No water closet or urinal shall be installed on such a system. Other one, two, or three unit fixtures remotely located from the sanitary system and adjacent to a combination waste and vent system shall be permitted to be connected to such system in the conventional manner by means of waste and vent pipes of regular sizes, providing that the two pipe size increase required in Section 910.4 is based on the total fixture unit load connected to the system.

    See Appendix B of this code for explanatory notes on the design of combination waste and vent systems.

    911.0 Circuit Venting. 911.1 Circuit Vent Permitted. A maximum of eight flooroutlet water closets, showers, bathtubs, or floor drains con

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    VENTS

    nected to a horizontal branch shall be permitted to be circuit vented. Each trap arm shall connect horizontally to the horizontal branch being circuit vented in accordance with Table 1002.2. The horizontal branch shall be classified as a drain and a vent from the most downstream trap arm connection to the most upstream trap arm connection to the horizontal branch.

    Exception: Back-outlet and wall-hung water closets shall be permitted to be circuit vented provided that no floor-outlet fixtures are connected to the same horizontal branch. Backoutlet and wall-hung water closets shall connect horizontally to the horizontal circuit vented drain.

    911.2 Circuit Vent Size and Connection. The circuit vent size shall be in accordance with Table 703.2 according to the number of circuit vented fixtures connected to the horizontal branch but shall be not less than 2 inches (50 mm) in diameter. The vent shall connect to the horizontal branch on the vertical between the two most upstream trap arms. The circuit vent pipe shall not receive the discharge of soil or

    waste.

    911.2.1 Multiple Circuit Vents. When multiple circuit vents are interconnected according to Section 911.4.1, each individual circuit vent shall be sized according to Section 911.2. The vent pipe connecting each circuit vent shall be sized according to Table 703.2.

    911.3 Relief Vent. A 2 inch (50 mm) relief vent shall be provided for circuit-vented horizontal branches receiving the discharge of four or more water closets when connecting to a drainage stack that receives the discharge of soil or waste from upper horizontal branches.

    911.3.1 Connection and Installation. The relief vent

    shall connect to the horizontal branch between the stack and the most downstream trap arm of the circuit vent. The relief vent shall be installed on the vertical to the

    horizontal branch.

    911.3.2 Fixture Drain. The relief vent is permitted to serve as a fixture drain. Fixtures discharging to a relief vent shall be one or two fixture unit fixtures but shall not

    exceed a total of 4 fixture units.

    911.4 Slope and Size of Horizontal Branch. The vented section of the horizontal branch shall be uniformly sloped and not more than 1 inch per foot (83.3 mm/m). The entire length of the vented section of the horizontal branch shall be sized for the total drainage discharge to the branch according to Table 703.2.

  • CPC § 0.228 Medium relevance — show source text

    657|0.228|0.096|0.025| |19.77|2.750|0.699|0.243|0.102|0.026| |20.48|2.925|0.744|0.258|0.109|0.028| |21.19|3.106|0.790|0.274|0.115|0.030| |24.72|4.074|1.034|0.358|0.151|0.039| |28.25|–|1.307|0.452|0.190|0.049| |31.78|–|1.608|0.556|0.234|0.060| |35.31|–|1.936|0.669|0.281|0.072| |38.84|–|2.291|0.791|0.332|0.085| |42.37|–|2.672|0.922|0.387|0.099| |45.90|–|3.078|1.062|0.446|0.113| |49.43|–|3.510|1.211|0.508|0.129| |52.97|–|3.969|1.368|0.574|0.146| |56.50|–|4.450|1.534|0.643|0.163| |63.56|–|–|1.890|0.792|0.201| |70.62|–|–|2.278|0.954|0.242| |77.68|–|–|2.699|1.130|0.286| |84.74|–|–|3.151|1.318|0.334| |91.81|–|–|3.634|1.520|0.385| |98.87|–|–|4.148|1.734|0.439| |105.93|–|–|4.691|1.961|0.496| |112.99|–|–|–|2.200|0.556|

    For SI units: 1 standard cubic foot per minute = 28.32 SLPM, 1 inch = 25 mm, 1 foot = 304.8 mm, 1 inch of mercury = 3.386 kPa Notes: 1 Based on the pressure of 14.7 psig (101 kPa) at 68°F (20°C). 2 Based on the pressure of 19 inches of mercury gauge vacuum (64 kPa) at 68°F (20°C).

    1323.3 Location of Piping. Piping risers shall be permitted to be installed in pipe shafts if protected from physical damage, effects of excessive heat, corrosion, or contact with oil. [NFPA 99:5.1.10.11.3.1] 1323.3.1 Prohibited Locations. Piping shall not be installed in kitchens, stairwells, elevator shafts, elevator machine rooms, areas with open flames, electrical service equipment over 600 volts, and areas prohibited Cal- ifornia Electrical Code except for the following locations:

    (1) Room locations for medical air compressor supply systems and medical-surgical vacuum pump supply systems

    (2) Room locations for secondary distribution circuit panels and breakers having a maximum voltage rating of 600 volts [NFPA 99:5.1.10.11.3.2] **1323.3.2 Approved Locations.

  • CPC § 501.6 Medium relevance — show source text

    ** Water supply fixture units shall be determined by the manufacturer’s instructions.

    C 501.6 Materials. Materials used for water distribution pipe and fittings shall be in accordance with Table 604.1. Materials used for aboveground drainage shall be in accordance with Table 701.2 and shall have a smooth bore, and be constructed of non-porous material. C 501.7 Traps and Cleanouts. Traps and cleanouts shall be installed in accordance with Chapter 7 and Chapter 10. C 501.8 Testing. The entire vacuum waste system shall be subjected to a vacuum test of 29 inches of mercury (98 kPa) or not less than the working pressure of the system for 30 minutes. The system shall be gastight and watertight at all points. Verification of test results shall be submitted to the Authority Having Jurisdiction.

    C 501.9 Manufacturer’s Instructions. Manufacturer’s instructions shall be provided to provide information regarding safe and proper operating instructions whether or not as part of the condition of listing to determine compliance. Such instructions shall be submitted and approved by the Authority Having Jurisdiction.

    C 601.0 Single-Stack Vent System. C 601.1 Where Permitted. Single-stack venting shall be designed by a registered design professional as an engineered design. A drainage stack shall be permitted to serve as a single-stack vent system where sized and installed in accordance with Section C 601.2 through Section C 601.10. The drainage stack and branch piping in a single-stack vent system shall provide for the flow of liquids, solids, and air without exceeding the pressure differential described in Section 901.3. C 601.2 Stack Size. Drainage stacks shall be sized in accordance with Table C 601.2. Not more than two water closets shall be permitted to discharge to a 3 inch (80 mm) stack. Stacks shall be uniformly sized based on the total connected drainage fixture unit load, with no reductions in size. C 601.2.1 Stack Vent. The drainage stack vent shall have a stack vent of the same size terminating to the outdoors. C 601.3 Branch Size. Horizontal branches connecting to a single-stack vent system shall be sized in accordance with Table 703.2.

    Exceptions:

    (1) Not more than one water closet within 18 inches (457 mm) of the stack horizontally shall be permitted on a 3 inch (80 mm) horizontal branch.

    (2) A water closet within 18 inches (457 mm) of a stack horizontally and one other fixture with up to 1 [1] ⁄ 2 inch (40 mm) fixture drain size shall be permitted on a 3 inch (80 mm) horizontal branch where connected to the stack through a sanitary tee. C 601.4 Length of Horizontal Branches. Water closets shall be not more than 4 feet (1219 mm) horizontally from the stack.

    Exception: Water closets shall be permitted to be up to 8 feet (2438 mm) horizontally from the stack where connected to the stack through a sanitary tee.

    C 601.4.1 Other Fixtures. Fixtures other than water closets shall be not more than 12 feet (3658 mm) horizontally from the stack. C 601.4.2 Length of Vertical Piping. The length of vertical piping from a fixture trap to a horizontal branch shall not be considered in computing the fixture’s horizontal distance from the stack.

    **C 601.5 Maximum Vertical Drops from Fixtures.

  • CPC § 25.4 Medium relevance — show source text

    DISTANCE FROM COMBUSTIBLE MATERIAL (inches)**|MINIMUM DISTANCE FROM COMBUSTIBLE MATERIAL (inches)|MINIMUM DISTANCE FROM COMBUSTIBLE MATERIAL (inches)|MINIMUM DISTANCE FROM COMBUSTIBLE MATERIAL (inches)|MINIMUM DISTANCE FROM COMBUSTIBLE MATERIAL (inches)| |APPLIANCE|LISTED TYPE B GAS
    VENT MATERIAL|LISTED TYPE L VENT
    MATERIAL|SINGLE-WALL METAL
    PIPE|FACTORY-BUILT
    CHIMNEY SECTIONS| |Listed appliances with draft hoods and appli-
    ances listed for use with Type B gas vents|As listed|As listed|6|As listed| |Residential boilers and furnaces with listed gas
    conversion burner and with draft hood|6|6|9|As listed| |Residential appliances listed for use with
    Type L vents|Not permitted|As listed|9|As listed| |Unlisted residential appliances with draft hood|Not permitted|6|9|As listed| |Residential and low-heat appliances other than
    those above|Not permitted|9|18|As listed| |Medium-heat appliances|Not permitted|Not permitted|36|As listed|

    For SI units: 1 inch = 25.4 mm

    • These clearances shall apply unless the installation instructions of a listed appliance or connector specify different clearances, in which case the listed clearances shall apply.

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    WATER HEATERS

    Notes: 1 A – Equals the clearance with no protection specified in Table 509.7.3.4(1) and Table 509.7.3.4(2) and in the sections applying to various types of equipment. 2 B – Equals the reduced clearance permitted in accordance with Table 509.7.3.4(2). 3 The protection applied to the construction using combustible material shall extend far enough in each direction to make C equal to A.

    FIGURE 509.7.3.4(1) [1, 2, 3] EXTENT OF PROTECTION NECESSARY TO REDUCE CLEAR-

    ANCES FROM GAS APPLIANCE OR VENT CONNECTORS

    [NFPA 54: FIGURE 10.3.3.3(a)]

    For SI units: 1 inch = 25.4 mm

    Note: Masonry walls shall be attached to combustible walls using wall ties. Spacers shall not be used directly behind appliance or connector.

    FIGURE 509.7.3.4(2) WALL PROTECTOR CLEARANCE REDUCTION SYSTEM

    [NFPA 54: FIGURE 10.3.3.3(b)]

    For SI units: 1 inch = 25.4 mm FIGURE 509.7.3.4(3) MASONRY CLEARANCE REDUCTION SYSTEM

    [NFPA 54: FIGURE 10.3.3.3(c)]

    (3) For residential and low-heat appliances, the thimble shall be a minimum of 12 inches (300 mm) larger in diameter than the metal pipe.

  • CPC § 601.2.1 Medium relevance — show source text

    Not more than two water closets shall be permitted to discharge to a 3 inch (80 mm) stack. Stacks shall be uniformly sized based on the total connected drainage fixture unit load, with no reductions in size. C 601.2.1 Stack Vent. The drainage stack vent shall have a stack vent of the same size terminating to the outdoors. C 601.3 Branch Size. Horizontal branches connecting to a single-stack vent system shall be sized in accordance with Table 703.2.

    Exceptions:

    (1) Not more than one water closet within 18 inches (457 mm) of the stack horizontally shall be permitted on a 3 inch (80 mm) horizontal branch.

    (2) A water closet within 18 inches (457 mm) of a stack horizontally and one other fixture with up to 1 [1] ⁄ 2 inch (40 mm) fixture drain size shall be permitted on a 3 inch (80 mm) horizontal branch where connected to the stack through a sanitary tee. C 601.4 Length of Horizontal Branches. Water closets shall be not more than 4 feet (1219 mm) horizontally from the stack.

    Exception: Water closets shall be permitted to be up to 8 feet (2438 mm) horizontally from the stack where connected to the stack through a sanitary tee.

    C 601.4.1 Other Fixtures. Fixtures other than water closets shall be not more than 12 feet (3658 mm) horizontally from the stack. C 601.4.2 Length of Vertical Piping. The length of vertical piping from a fixture trap to a horizontal branch shall not be considered in computing the fixture’s horizontal distance from the stack.

    C 601.5 Maximum Vertical Drops from Fixtures. Vertical drops from fixture traps to horizontal branch piping shall be one size larger than the trap size, but not less than 2 inches (50 mm) in diameter. Vertical drops shall be 4 feet (1219 mm) maximum length. Fixture drains that are not increased in size, or have a vertical drop exceeding 4 feet (1219 mm) shall be individually vented. C 601.6 Additional Venting Required. Additional venting shall be provided where more than one water closet is on a horizontal branch and where the distance from a fixture trap to the stack exceeds the limits in Section C 601.4. Where additional venting is required, the fixture(s) shall be vented by one of the methods described in Section 908.0 through Section 911.5. The dry vent extensions for the additional venting shall connect to a branch vent, vent stack, stack vent, or be extended outdoors and terminate to the open air.

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    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    APPENDIX C

  • CPC § 4.5 Medium relevance — show source text

    0|4.5|4.1|4.1| |Each Additional1⁄2 Bath|0.5|0.1|0.4|0.4| |Each Additional Bathroom Group|1.0|0.5|0.8|0.8| |Other Groups of Fixtures|Other Groups of Fixtures|Other Groups of Fixtures|Other Groups of Fixtures|Other Groups of Fixtures| |Bathroom Group (1.6 GPF Flushometer Value)|6.0|2.5|4.0|1.7| |Kitchen Group (Sink and Dishwasher)|2.0|2.0|1.5|1.5| |Laundry Group (Sink and Clothes Washer)|5.0|5.0|3.0|3.0|

    Notes: 1 A bathroom group, for this table, consists of one water closet, up to two lavatories, and either one bathtub or one shower. 2 A half-bath or powder room, for this table, consists of one water closet and one lavatory. 3 Multi-unit dwellings with individual water heaters use the same WSFU as for individual dwellings.

    C 304.2 Size of Building Drain and Building Sewer. The maximum number of drainage fixture units allowed on the building drain or building sewer of a given size shall be in accordance with Table C 304.2. The size of a building drain or building sewer serving a water closet shall be not less than 3 inches (80 mm).

    C 304.3 Size of Horizontal Branch or Vertical Stack. The maximum number of drainage fixture units allowed on a horizontal branch or vertical soil or waste stack of a given size shall be in accordance with Table C 304.3. Stacks shall

    be sized based on the total accumulated connected load at each story or branch interval.

    C 304.3.1 Horizontal Stack Offsets. Horizontal

    stack offsets shall be sized in accordance with Table C 304.2 as required for building drains.

    C 304.3.2 Vertical Stack Offsets. Vertical stack off sets shall be sized in accordance with Table C 304.3 as required for stacks.

    C 304.4 Horizontal Stack Offset and Horizontal Branch

    Connections. Horizontal branch connections shall not connect to a horizontal stack offset or within 2 feet (610 mm) above or below the offset where such horizontal offset is located more than four branch intervals below the top of the stack.

    C 401.0 Vent System Sizing. C 401.1 Size of Vents. The size of vent piping shall be determined from the developed length and the total number of drainage fixture units connected in accordance with Table C 401.1. Vents shall be not less than one-half the required size of the drainage pipe size served as determined by Table C 304.3 for horizontal fixture branches and stacks nor less than 1 [1] ⁄ 4 inches

    (32 mm) in diameter. The drainage system shall be vented by not less than one vent pipe which shall be not less than one-half the size of the required building drain and which shall extend from the building drain or extension of building drain to the outdoors. Vents shall be installed in accordance with Chapter 9. C 401.2 Vent Stack. A vent stack shall be required for a drainage stack that extends five or more branch intervals above the building drain or horizontal branch. The developed length of the vent stack shall be measured from the lowest connection of a branch vent to the termination outdoors.

  • CPC § 904.0. Medium relevance — show source text

    VENTS

    smaller than the minimum vent pipe size required for a fixture served, or by Section 904.0. 908.2 Horizontal Wet Venting for a Bathroom Group. A bathroom group located on the same floor level shall be permitted to be vented by a horizontal wet vent where all of the conditions of Section 908.2.1 through Section 908.2.5 are met.

    908.2.1 Vent Connection. The dry vent connection to the wet vent shall be an individual vent for the bidet, shower, or bathtub. One or two vented lavatory(s) shall be permitted to serve as a wet vent for a bathroom group. Only one wet-vented fixture drain or trap arm shall discharge upstream of the dry-vented fixture drain connection. Dry vent connections to the horizontal wet vent shall be in accordance with Section 905.2 and Section

    905.3.

    908.2.2 Size. The wet vent shall be sized based on the fixture unit discharge into the wet vent. The wet vent shall be not less than 2 inches (50 mm) in diameter for 4 drainage fixture units (dfu) or less, and not less than 3 inches (80 mm) in diameter for 5 dfu or more. The dry vent shall be sized in accordance with Table 702.1 and Table 703.2 based on the total fixture units discharging into the wet vent.

    908.2.3 Trap Arm. The length of the trap arm shall not exceed the limits in Table 1002.2. The trap size shall be in accordance with Section 1003.3. The vent pipe opening from the horizontal wet vent, except for water closets and similar fixtures, shall not be below the weir of the trap.

    908.2.4 Water Closet. The water closet fixture drain or trap arm connection to the wet vent shall be downstream of fixture drain or trap arm connections to the horizontal wet vent.

    908.2.5 Additional Fixtures. Additional fixtures shall discharge downstream of the wet vent system and be conventionally vented. Only the fixtures within the bathroom group shall connect to the wet-vented horizontal branch.

    909.0 Special Venting for Island Fixtures. 909.1 General. Traps for island sinks and similar equipment shall be roughed in above the floor and shall be permitted to be vented by extending the vent as high as possible, but not less than the drainboard height and then returning it downward and connecting it to the horizontal sink drain immediately downstream from the vertical fixture drain. The return vent shall be connected to the horizontal drain through a wyebranch fitting and shall, in addition, be provided with a foot vent taken off the vertical fixture vent by means of a wye branch immediately below the floor and extending to the nearest partition and then through the roof to the open air, or shall be permitted to be connected to other vents at a point not less than 6 inches (152 mm) above the flood-level rim of the fixtures served. Drainage fittings shall be used on the vent below the floor level, and a slope of not less than [1] ⁄ 4 inch per foot (20.8 mm/m) back to the drain shall be maintained. The return bend used under the drainboard shall be a one-piece

  • CPC § 304.3.1 Medium relevance — show source text

    be sized based on the total accumulated connected load at each story or branch interval.

    C 304.3.1 Horizontal Stack Offsets. Horizontal

    stack offsets shall be sized in accordance with Table C 304.2 as required for building drains.

    C 304.3.2 Vertical Stack Offsets. Vertical stack off sets shall be sized in accordance with Table C 304.3 as required for stacks.

    C 304.4 Horizontal Stack Offset and Horizontal Branch

    Connections. Horizontal branch connections shall not connect to a horizontal stack offset or within 2 feet (610 mm) above or below the offset where such horizontal offset is located more than four branch intervals below the top of the stack.

    C 401.0 Vent System Sizing. C 401.1 Size of Vents. The size of vent piping shall be determined from the developed length and the total number of drainage fixture units connected in accordance with Table C 401.1. Vents shall be not less than one-half the required size of the drainage pipe size served as determined by Table C 304.3 for horizontal fixture branches and stacks nor less than 1 [1] ⁄ 4 inches

    (32 mm) in diameter. The drainage system shall be vented by not less than one vent pipe which shall be not less than one-half the size of the required building drain and which shall extend from the building drain or extension of building drain to the outdoors. Vents shall be installed in accordance with Chapter 9. C 401.2 Vent Stack. A vent stack shall be required for a drainage stack that extends five or more branch intervals above the building drain or horizontal branch. The developed length of the vent stack shall be measured from the lowest connection of a branch vent to the termination outdoors.

    C 401.3 Branch Vents. Where branch vents exceed 40 feet (12 192 mm) in developed length, such vent shall be increased by one pipe size for the entire developed length of the vent pipe. C 401.4 Venting Horizontal Offsets. Drainage stacks with horizontal offsets shall be vented where five or more branch intervals are located above the offset. The upper and lower section of the horizontal offset shall be vented in accordance

    with Section C 401.4.1 and Section C 401.4.2.

    C 401.4.1 Venting Upper Section. The vent for the upper section of the stack shall be vented as a separate stack with a vent stack connection installed at the base of the drainage stack. Such vent stack shall connect below the lowest horizontal branch or building drain. Where vent stack connects to the building drain, the connection shall be located downstream of the drainage stack and within a distance of 10 times the diameter of the drainage stack. C 401.4.2 Venting Lower Section. The vent for the lower section of the stack shall be vented by a yoke vent connecting between the offset and the next lower horizontal branch using a wye-branch fitting. The size of the yoke vent and connection shall be not less in diameter

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    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    APPENDIX C

    TABLE C 303.3 DRAINAGE FIXTURE UNIT VALUES (DFU) FOR BATHROOM GROUPS [1, 2]

Frequently asked questions

Can I run a minimum‑sized vent entirely horizontally if I keep it short?

No. For a minimum‑sized vent, the CPC limits horizontal installation to no more than one‑third of the vent’s total permitted developed length from Table 703.2. To run more horizontally, you must increase the vent one pipe size for its entire length, which removes the one‑third cap. § 904.2

How do I know the numeric length I can run horizontally?

You first determine the total permitted developed length for the vent size and fixture units from Table 703.2, then take one‑third of that value for a minimum‑sized vent. The CPC text references Table 703.2 but the table’s numeric entries must be read directly from the code. § 904.2 / Table 703.2

If I upsize the vent halfway only, does the one‑third rule go away?

No. The exception applies only when the vent is increased one pipe size for its entire length. Partial upsizing does not remove the one‑third horizontal limitation. § 904.2

Does the 6‑inch rise requirement affect the available horizontal length?

Yes. The required vertical rise of not less than 6 in (152 mm) above the flood‑level rim before any horizontal offset consumes part of the vent’s developed length and therefore reduces the remaining allowable horizontal portion. § 905.3

Where can I find the exact developed‑length numbers to use?

Consult Table 703.2 in the CPC for developed‑length limits by vent diameter and fixture unit load. The code text in § 904.1/§ 904.2 points you to that table for the numeric basis. § 904.1 / § 904.2

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