CPC · California Plumbing Code

When are combination waste-and-vent systems permitted and how are they sized?

A combination waste‑and‑vent system is an oversized wet‑vent used only when conventional venting can’t be installed. It must be approved by the authority, all waste pipes and traps must be two pipe sizes larger than conventional sizing, vents must provide free air circulation and have at least half the drain’s cross‑sectional area, branches over 15 feet must be separately vented, and toilets/urinals are not permitted.

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Last reviewed: July 6, 2026

What the code requires — plain English

A Combination Waste and Vent System is permitted only when ordinary venting cannot be provided because of structural constraints — and the Authority Having Jurisdiction must approve the construction documents before installation (§ 910.1, § 910.2) .
When used, the system must be vented so air can circulate above the flow line, branches longer than 15 feet need separate venting, the vent area must be at least one‑half the inside cross‑sectional area of the drain it serves, and every waste pipe and trap must be at least two pipe sizes larger than the size that would be required for a conventional (non‑combination) installation (§ 910.3, § 910.4) .
Additional limits: no vertical waste pipe is permitted except very short tailpieces (maximum 2 feet) and certain fixtures (water closets and urinals) are prohibited from connecting to these systems (§ 910.5, § 910.7) .

The single most important rule: combination systems are an oversize wet‑vent alternative used only where conventional venting is impractical — every pipe and trap must be two sizes larger and vents sized to provide free air circulation above the flow line (§ 910.1, § 910.3, § 910.4).

Requirements in detail

Where these systems are allowed and administrative steps

  • Permitted only where structural conditions preclude conventional venting (§ 910.1) .
  • Construction documents for each combination system must be approved by the AHJ before any portion is installed (§ 910.2) .
  • Appendix B provides explanatory design notes and cautions for when these systems are appropriate; consult it for guidance (non‑mandatory unless adopted) .

Venting — area, location, and branch length

  • Provide vents adequate for free circulation of air in the upper portion of the oversized drain (§ 910.3) .
  • A branch exceeding 15 feet (4572 mm) shall be separately vented in an approved manner (§ 910.3) .
  • Vent area must be at least one‑half the inside cross‑sectional area of the drainpipe it serves; the vent connection must be downstream of the uppermost fixture served (§ 910.3) .
  • Every vent for the combination system must have an accessible cleanout; limited exceptions for single‑trap wet‑vented branches are in § 910.6 (§ 910.6) .

Connections, sizes, and geometry

  • Branches serving traps must connect to the main line at an angle not exceeding 2 percent (§ 910.4) .
  • Each waste pipe and each trap in the combination system shall be not less than two pipe sizes exceeding:
    • the sizes that Chapter 7 would require for a conventional system, and
    • the size of the fixture tailpiece or connection (i.e., two sizes larger than the tailpiece) (§ 910.4) .
  • No vertical waste pipe (downward vertical waste) is allowed in the system except the fixture tailpiece/connection; those tailpieces must be as short as possible and in no case exceed 2 feet (610 mm) (§ 910.5) .
  • 45° vertical offsets on branch lines are permitted by exception to facilitate tailpiece routing (see § 910.5 exception) .

Fixtures and loads

  • No water closet or urinal may be installed on a combination waste and vent system (§ 910.7) .
  • One‑, two‑, or three‑unit fixtures that are remotely located and adjacent to the combination system may be tied in conventionally if the two‑pipe‑size increase required in § 910.4 is applied based on the total fixture unit load connected to the system (§ 910.7) .
  • Appendix B cautions that these systems are not self‑scouring (they are oversized), so grease‑producing fixtures (restaurant equipment) and large surge discharges should be excluded .

Decision‑relevant dimensions / values

Requirement Code threshold / value Code Reference
When permitted Only where structural conditions preclude conventional venting § 910.1
AHJ approval of construction documents Required before installation § 910.2
Branch separately vented when > 15 feet (4572 mm) § 910.3
Minimum vent area ≥ one‑half the inside cross‑sectional area of the drainpipe served § 910.3
Vent connection location Downstream of the uppermost fixture § 910.3
Branch connection angle Not exceeding 2 percent § 910.4
Pipe & trap sizing Not less than two pipe sizes exceeding Chapter 7 conventional sizes and tailpiece size § 910.4
Vertical waste allowed Only tailpiece/connection; max 2 ft (610 mm) § 910.5
Cleanouts Accessible cleanout in each vent for the system § 910.6
Fixtures prohibited Water closets and urinals may not be installed § 910.7

Exceptions & special cases

  • Branch lines are permitted to have 45° vertical offsets to meet tailpiece length limits (exception to § 910.5) .
  • Small single‑trap wet‑vented branches with a tailpiece or connection not less than 2 inches that provides ready access need not have a cleanout on the wet‑vented branch (see § 910.6) .
  • Appendix B notes these systems are best for extensive floor or shower drain installations and cautions against connecting grease‑producing or surge‑producing equipment (restaurants, pumps, etc.) unless engineered otherwise .

Common mistakes

  • Assuming combination systems are “simpler”: they are an exception and require AHJ approval (missed § 910.2) .
  • Forgetting the two‑pipe‑size increase — undersizing the oversized drain/trap defeats the system’s air cushion and can cause siphonage or pressurization (required by § 910.4) .
  • Using water closets or urinals on these systems (explicitly prohibited by § 910.7) .
  • Installing vents with insufficient area (vent area must be ≥ one‑half the drain area; see § 910.3) or connecting the vent upstream of the uppermost fixture served (§ 910.3) .
  • Allowing long vertical waste runs or tailpieces > 2 feet in length (violates § 910.5) .
  • Connecting grease‑producing equipment (restaurants) despite Appendix B’s warnings — these systems are not self‑scouring and clogging is likely .

Worked example — concrete scenario with numbers

Scenario: You need a combination system for a market floor with several floor drains where conventional venting is impossible.

  1. Determine the conventional drainage size required for the total fixture load using Chapter 7 (assume — for this example only — the conventional sizing calculation indicates a 2‑inch drain would be required in a conventional system). (Note: the code requires you use Chapter 7 to find the conventional size; that table/step is not reproduced here) (Chapter 7).
  2. Apply the combination‑system rule: increase the required sizes by two pipe sizes per § 910.4. If the conventional result is 2 inches, two nominal size steps larger commonly land at 4 inches (2" → 3" → 4") — so select a 4‑inch combination waste pipe and corresponding traps sized two sizes larger than their tailpieces (§ 910.4) .
  3. Vent sizing: the vent must be at least one‑half the inside cross‑sectional area of the 4‑inch drain. Quick (approximate) check: area of a 4‑inch circle = 12.57 in², half = 6.28 in². A 3‑inch vent has area ≈ 7.07 in², which exceeds half — so a 3‑inch vent would meet the requirement in this simplified example. Verify actual inside diameters/manufacturer dimensions before final sizing; the code requires the area relationship, not a specific nominal mapping (§ 910.3) .
  4. Branch limits: ensure no branch serving traps exceeds 15 feet without separate venting; if a branch does exceed 15 ft, provide an approved separate vent per § 910.3 .
  5. Geometry: connect trap branches to the main line at ≤ 2% angle and keep fixture tailpieces ≤ 2 ft where vertical tailpieces are used; install cleanouts in each vent as required by § 910.6 .

Important caveat: the code text requires "two pipe sizes exceeding" Chapter 7 sizes but does not list the numeric progression of nominal sizes or internal diameters in the § text shown here — you must consult Chapter 7 tables and actual pipe manufacturer inside diameters to convert the Chapter 7 conventional size into the exact combination‑system nominal size to install (and confirm with the AHJ) .

Related provisions (quick references)

  • § 910.1 — Where permitted; structural limitation requirement
  • § 910.2 — AHJ approval of construction documents
  • § 910.3 — Vents: area, placement, branch length (>15 ft)
  • § 910.4 — Connections and sizing (two pipe‑size increase; connection angle)
  • § 910.5 — Vertical waste pipe and tailpiece limits (≤ 2 ft)
  • § 910.6 — Cleanout requirements
  • § 910.7 — Fixtures permitted / prohibited on combination systems
  • Appendix B — Explanatory notes and cautions on design and fixture restrictions (non‑mandatory advisory)

Code references

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

  • CPC § 910.0 High relevance — show source text

    910.0 Combination Waste and Vent Systems. 910.1 Where Permitted. Combination waste and vent systems shall be permitted where structural conditions preclude the installation of conventional systems as otherwise prescribed by this code. 910.2 Approval. Construction documents for each combination waste and vent system shall first be approved by the Authority Having Jurisdiction before a portion of such system is installed.

    910.3 Vents. Each combination waste and vent system, as defined in Chapter 2, shall be provided with a vent or vents adequate to ensure free circulation of air. A branch exceeding 15 feet (4572 mm) in length shall be separately vented in an approved manner. The area of a vent installed in a combination waste and vent system shall be not less than one-half the inside cross-sectional area of the drainpipe served. The vent connection shall be downstream of the uppermost fixture. 910.4 Connections and Size. Branches serving traps shall connect to the main line at an angle not exceeding 2 percent. Each waste pipe and each trap in such a system shall be not less than two pipe sizes exceeding the sizes required by Chapter 7 of this code, and not less than two pipe sizes exceeding a fixture tailpiece or connection. 910.5 Vertical Waste Pipe. No vertical waste pipe shall be used in such a system, except the tailpiece or connection between the outlet of a plumbing fixture and the trap. Such tailpieces or connections shall be as short as possible, and in no case shall exceed 2 feet (610 mm).

    Exception: Branch lines shall be permitted to have 45 degree (0.79 rad) vertical offsets.

    910.6 Cleanouts. An accessible cleanout shall be installed in each vent for the combination waste and vent system. Cleanouts shall not be required on a wet-vented branch serving a single trap where the fixture tailpiece or connection is not less than 2 inches (50 mm) in diameter and provides ready access for cleaning through the trap.

    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.

  • CPC § 910.7 High 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 § 908.2.4 High relevance — show source text

    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

    fitting or an assembly of a 45 degree (0.79 rad), a 90 degree (1.57 rad), and a 45 degree (0.79 rad) elbow in the order named. Pipe sizing shall be as elsewhere required in this code. The island sink drain, upstream of the returned vent, shall serve no other fixtures. An accessible cleanout shall be installed in the vertical portion of the foot vent.

    910.0 Combination Waste and Vent Systems. 910.1 Where Permitted. Combination waste and vent systems shall be permitted where structural conditions preclude the installation of conventional systems as otherwise prescribed by this code. 910.2 Approval. Construction documents for each combination waste and vent system shall first be approved by the Authority Having Jurisdiction before a portion of such system is installed.

    910.3 Vents. Each combination waste and vent system, as defined in Chapter 2, shall be provided with a vent or vents adequate to ensure free circulation of air. A branch exceeding 15 feet (4572 mm) in length shall be separately vented in an approved manner. The area of a vent installed in a combination waste and vent system shall be not less than one-half the inside cross-sectional area of the drainpipe served. The vent connection shall be downstream of the uppermost fixture. 910.4 Connections and Size. Branches serving traps shall connect to the main line at an angle not exceeding 2 percent. Each waste pipe and each trap in such a system shall be not less than two pipe sizes exceeding the sizes required by Chapter 7 of this code, and not less than two pipe sizes exceeding a fixture tailpiece or connection. 910.5 Vertical Waste Pipe. No vertical waste pipe shall be used in such a system, except the tailpiece or connection between the outlet of a plumbing fixture and the trap. Such tailpieces or connections shall be as short as possible, and in no case shall exceed 2 feet (610 mm).

  • CPC § 1.11.0. High relevance — show source text

    This state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.0.

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    APPENDIX B

    EXPLANATORY NOTES ON COMBINATION WASTE AND VENT SYSTEMS

    (See Section 910.0 for specific limitations)

    The provisions contained in this appendix are not mandatory unless specifically adopted by a state agency, or referenced in the adopting ordinance.

    B 101.0 General.

    B 101.1 Applicability. This appendix provides general guidelines for the design and installation of a combination waste and vent system.

    B 101.2 General Requirements. Combination waste and vent systems, (which at best are merely an expedient designed to be used in locations where it would be structurally impractical to provide continuous venting of fixtures) as outlined in Section 910.0 of this code, cover the horizontal wet venting of a series of traps using a common waste and vent pipe. Pipe sizes not less than two pipe sizes larger than those required for a conventional system are designed to maintain a wetted perimeter or flow line low enough in the waste pipe to allow adequate air movement in the upper portion, thus balancing the system. One and two unit fixtures that rough in above the floor, shall be permitted to connect to a combination waste and vent system when located as required in Section 910.7.

    Combination waste and vent systems are intended primarily for extensive floor or shower drain installations where separate venting is not practical, for floor sinks in markets, demonstration or work tables in school buildings, or for similar applications where the fixtures are not adjacent to walls or partitions. Due to its oversize characteristics, such a waste system is not self-scouring and, consequently, care shall be exercised as to the type of fixtures connected to it and the location of cleanouts. Given its grease-producing potential, restaurant kitchen equipment shall not be connected to a combination waste and vent system.

    B 101.3 Caution. Caution shall be exercised to exclude appurtenances delivering large quantities or surges of water (such as pumps, sand interceptors, etc.) from combination waste and vent systems so that adequate venting will be maintained. Small fixtures with a waste-producing potential of less than 7 [1] ⁄ 2 gallons per minute (gpm) (0.47 L/s) shall be permitted to be safely assigned a loading value of one unit. Long runs shall be laid at the minimum permissible slope to keep tailpieces as short as possible. Tailpieces shall not exceed 2 feet (610 mm) in length, which shall necessitate slopes up to 45 degrees (0.79 rad) (see definition of horizontal pipe) on some branches.

  • CPC § 710.11 High relevance — show source text

    Where the preceding requirements are met and the vent, after leaving the sump, is combined with vents from fixtures discharging into the sump, the size of the combined vent need not exceed that required for the total number of fixtures discharging into the sump. No vent from an air-operating sewage ejector shall combine with other vents. 710.11 Air Tanks. Air tanks shall be so proportioned as to be of equal cubical capacity to the ejectors connected in addition to that in which there shall be maintained an air pressure of not less than 2 pounds per foot (lb/ft) (3 kg/m) of height the sewage is to be raised. No water-operated ejectors shall be permitted. 710.12 Grinder Pump Ejector. Grinder pumps shall be permitted to be used. 710.12.1 Discharge Piping. The discharge piping shall be sized in accordance with the manufacturer’s installation instructions and shall be not less than 1 [1] ⁄ 4 inches (32 mm) in diameter. A check valve and fullwaytype shutoff valve shall be located on the discharge line. 710.13 Macerating Toilet Systems and Pumped Waste Systems. Fixtures shall be permitted to discharge to a macerating toilet system, or pumped waste system shall be permitted as an alternate to a sewage pump system where approved by the Authority Having Jurisdiction. Such systems shall comply with ASME A112.3.4/CSA B45.9 and shall be installed in accordance with the manufacturer’s installation

    instructions.

    710.13.1 Sumps. The sump shall be watertight and gastight. 710.13.2 Discharge Piping. The discharge piping shall be sized in accordance with manufacturer’s instructions and shall be not less than [3] ⁄ 4 of an inch (20 mm) in diameter. The developed length of the discharge piping shall not exceed the manufacturer’s instructions. A check

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    SANITARY DRAINAGE

    valve and fullway-type shutoff valve shall be located within the discharge line or internally within the device.

    710.13.3 Venting. The plumbing fixtures that discharge into the macerating device shall be vented in accordance with this code. The sump shall be vented in accordance with the manufacturer’s instructions, and such vent shall be permitted to connect to the fixture venting.

    711.0 Suds Relief.

    711.1 General. Drainage connections shall not be made into a drainage piping system within 8 feet (2438 mm) of a vertical to horizontal change of direction of a stack containing suds-producing fixtures. Bathtubs, laundries, washing machine standpipes, kitchen sinks, and dishwashers shall be considered suds-producing fixtures. Where parallel vent stacks are required, they shall connect to the drainage stack at a point 8 feet (2438 mm) above the lowest point of the drainage stack.

    Exceptions:

    (1) Single-family residences.

    (2) Stacks receiving the discharge from less than three stories of plumbing fixtures.

    712.0 Testing.

  • CPC § 2025 High relevance — show source text

    Appendix A Recommended Rules for Sizing the Water Supply System.

    Appendix A provides a method of sizing the water supply and distribution system that provides precise calculations to establish the proper pressures and flow to the system’s fixtures. The goal of sizing the system is to deliver an acceptable volume of water to the most hydraulically remote fixture during minimum pressure and maximum flow conditions; provide satisfactory water pressure to the most hydraulically remote fixture during minimum pressure and maximum flow conditions; and to prevent excessive water velocity during maximum flow conditions.

    Appendix B Explanatory Notes on Combination Waste and Vent Systems.

    Appendix B contains general guidelines for the design and installation of combination waste and vent systems. These systems are designed for waste piping and are purposely oversized to serve as both a waste and vent pipe to avoid excessive pneumatic effects at fixture drains.

    Appendix C Alternate Plumbing Systems.

    The intent of this appendix is to provide clarification of procedures for the design and approval of engineered plumbing systems, alternate materials, and equipment that are not specifically covered in other parts of the code. Alternative methods are allowed to be used where approved by the authority having jurisdiction. Approval of alternatives is based on a demonstration showing that the method or material used is at least equivalent in strength, deflection, and capacity to that provided by the prescriptive methods and materials.

    Appendix D Sizing Storm Water Drainage Systems.

    Appendix D provides general guidelines for the sizing of stormwater drainage systems. There are two pieces of information that must always be a given. They are the roof size and the rate of rainfall for the area.

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    FORMAT OF THE UNIFORM PLUMBING CODE

    Appendix E Manufactured/Mobile Home Parks and Recreational Vehicle Parks.

    The provisions of this appendix apply to the plumbing and drainage systems of mobile home and recreational vehicle parks. These provisions also apply to the use, maintenance, and installation for supplying fuel gas, water, electricity, and disposal of sewage from accessory buildings or structures, and building components.

    Appendix F Firefighter Breathing Air Replenishment Systems.

    Appendix F provides guidance on installing firefighter breathing air replenishment systems. System components include outside fire department connection panel, interior air fill panel or station, interconnected piping distribution system and pressure monitoring switch. Fire departments access the system through an outside connection panel and are able to pump air into the system. The firefighters inside the structure access the system at fill stations that are found throughout the building. The piping distribution system is made from stainless tubing or other approved materials. It delivers compressed air to the building’s interior air-fill stations and interior air-fill panels. The tubing also acts as a conduit in the interior of the building between the outside connection panel and the air storage system. If the system becomes over-pressurized, the air monitoring system also acts as a pressure relief. A system isolation valve is placed alongside each interior air fill station and interior air fill panel to isolate the system.

    Appendix G Sizing of Venting Systems.

    Appendix G provides added information on the sizing of gas vents. This appendix is useful to the end user for the proper sizing of venting systems. A series of examples are given that show how to use the tables and other requirements of Chapter 5.

    Appendix H Private Sewage Disposal Systems.

  • CPC § 908.1 High relevance — show source text

    908.1 Vertical Wet Venting . . . . . . . . . . .190

    908.2 Horizontal Wet Venting for a Bathroom Group . . . . . . . . . . . . . .191

    909.0 Special Venting for Island Fixtures . . . . . . . . . . . . . . . . . . . . .191

    909.1 General . . . . . . . . . . . . . . . . . . . . .191

    910.0 Combination Waste and Vent

    Systems . . . . . . . . . . . . . . . . . . . .191

    910.1 Where Permitted . . . . . . . . . . . . . .191

    910.2 Approval . . . . . . . . . . . . . . . . . . . .191

    910.3 Vents . . . . . . . . . . . . . . . . . . . . . .191

    910.4 Connections and Size . . . . . . . . .191

    910.5 Vertical Waste Pipe . . . . . . . . . . .191

    910.6 Cleanouts . . . . . . . . . . . . . . . . . . .191

    910.7 Fixtures . . . . . . . . . . . . . . . . . . . . .191

    911.0 Circuit Venting . . . . . . . . . . . . . . .191

    911.1 Circuit Vent Permitted . . . . . . . . .191

    911.2 Circuit Vent Size and

    Connection . . . . . . . . . . . . . . . . . .192

    911.3 Relief Vent . . . . . . . . . . . . . . . . . .192

    911.4 Slope and Size of Horizontal Branch . . . . . . . . . . . . . . . . . . . . .192

    911.5 Additional Fixtures . . . . . . . . . . . .192

    912.0 Engineered Vent System . . . . . . .192

    912.1 General . . . . . . . . . . . . . . . . . . . . .192

    912.2 Minimum Requirements . . . . . . . .192

    CHAPTER 10 TRAPS AND

    INTERCEPTORS . . . . . . . . . . . . .193

    1001.0 General . . . . . . . . . . . . . . . . . . . . .195

    1001.1 Applicability . . . . . . . . . . . . . . . . . .195

    1001.2 Where Required . . . . . . . . . . . . . .195

    1002.0 Traps Protected by Vent Pipes . .195

    1002.1 Vent Pipes . . . . . . . . . . . . . . . . . .195

    1002.2 Fixture Traps . . . . . . . . . . . . . . . .195

    1002.3 Change of Direction . . . . . . . . . . .195

    1002.4 Vent Pipe Opening . . . . . . . . . . . .195

    1003.0 Traps – Described . . . . . . . . . . . .195

    1003.1 General Requirements . . . . . . . . .195

  • CPC § 1701.1 High relevance — show source text

    Chapter 16 Nonpotable Rainwater Catchment Systems.

    Chapter 16 regulates nonpotable rainwater catchment systems that include irrigation; toilet and urinal flushing with proper treatment; provisions where permits are required; maintenance of alternate water sources; and minimum water quality. This chapter provides guidance on how to optimize rainwater use while ensuring there is a decrease of risk to consumers from poor design, installation, and maintenance. Rainwater harvesting is the process of capturing, channeling, and storing water runoff for later use. Most systems are constructed of three principal components: the catchment area, the collection device, and the conveyance system.

    Chapter 17 Referenced Standards.

    Chapter 17 provides two comprehensive tables with referenced standards. The standards listed in Table 1701.1 are applied as indicated in the applicable reference section(s). A list of additional approved standards, publications, practices, and guides that are not referenced in specific sections appear in Table 1701.2.

    Referenced standards set forth specific details of accepted practices, materials specifications, or test methods in many specialized applications. Standards provide an efficient method of conveying complex information and specifications on the performance requirements for materials, products, systems, application, and installation. The manner and purpose for a standard’s use and, in turn, code compliance, must be definitive in all references to the standard. If the standard is intended to be a requirement for judging code compliance, the code must state its intent for use. The standard should adequately address a defined need and at the same time specify the minimum performance requirements, technical characteristics and methods of testing, and required test results.

    The referenced standards tables are organized in a manner that makes it easy to find specific standards in alphabetical order, and by acronym of the publishing agency of the standard. The tables list the title of the standard, the edition, and any addenda. Contact information for each publishing agency is provided at the end of the chapter.

    Appendix A Recommended Rules for Sizing the Water Supply System.

    Appendix A provides a method of sizing the water supply and distribution system that provides precise calculations to establish the proper pressures and flow to the system’s fixtures. The goal of sizing the system is to deliver an acceptable volume of water to the most hydraulically remote fixture during minimum pressure and maximum flow conditions; provide satisfactory water pressure to the most hydraulically remote fixture during minimum pressure and maximum flow conditions; and to prevent excessive water velocity during maximum flow conditions.

    Appendix B Explanatory Notes on Combination Waste and Vent Systems.

    Appendix B contains general guidelines for the design and installation of combination waste and vent systems. These systems are designed for waste piping and are purposely oversized to serve as both a waste and vent pipe to avoid excessive pneumatic effects at fixture drains.

    Appendix C Alternate Plumbing Systems.

    The intent of this appendix is to provide clarification of procedures for the design and approval of engineered plumbing systems, alternate materials, and equipment that are not specifically covered in other parts of the code. Alternative methods are allowed to be used where approved by the authority having jurisdiction. Approval of alternatives is based on a demonstration showing that the method or material used is at least equivalent in strength, deflection, and capacity to that provided by the prescriptive methods and materials.

    Appendix D Sizing Storm Water Drainage Systems.

    Appendix D provides general guidelines for the sizing of stormwater drainage systems. There are two pieces of information that must always be a given. They are the roof size and the rate of rainfall f

  • CPC § 2025 High relevance — show source text

    Chapter 7 Sanitary Drainage.

    This chapter regulates the design and installation of sanitary drainage systems to ensure they will work as intended. Drainage piping should not be oversized nor undersized, and constructed of approved materials to guard against fouling, deposit of solids, clogging, and with cleanouts so arranged that the pipes may be readily cleaned. The purpose of the sanitary drainage system is to remove effluent discharged from plumbing fixtures and other equipment to an approved point of disposal, such as a public sanitary system or private sewage disposal system.

    The basics of a sanitary drainage system include public and private sewage disposal; selection of materials; installation of the building drain and sewer; joining methods for pipe and fittings; drainage fixture units for sizing the drainage system; sumps and ejectors; vent sizing and length of vents; and testing.

    Chapter 8 Indirect Wastes.

    Chapter 8 regulates indirect waste connections that are required for plumbing fixtures and plumbing appliances dealing with food preparation, dishwashing, potable liquids, and similar equipment. An indirect connection prevents sewage from backing up into a fixture or appliance, thus providing protection against potential health hazards. The waste pipe discharges through an air gap or air break into a waste receptor or standpipe. The protection in the form of an air gap is necessary when the contamination is a potential health hazard or cross connection with the potable water system. Where there is no possibility of contaminating the potable water (nonpotable discharge), the indirect waste pipe may connect in the form of an air break. This method is often preferred to prevent splashing. In addition, health care facilities and special wastes must be protected from contamination that may result from the connection to the drainage system. The waste must be treated to prevent any damage to the piping or sewage treatment process. Waste receptors are sized and designed to prevent splashing and allow for peak discharge conditions.

    Chapter 9 Vents.

    Chapter 9 regulates the material, design, and installation of vents. A vent system is a pipe or pipes installed in a drainage system that provide a flow of air to and from the system to ventilate it, provide a circulation of air to eliminate trap siphonage, and reduce back-pressure and vacuum surge. In addition, vents provide the rapid and silent flow of waste without exposing occupants of the building to any sewer gases. Proper installation of vents is crucial, as a telltale sign that there is a problem in the drain and vent system is related to the elevation of the horizontal portion of the venting. Venting is not limited to sanitary drainage systems. Venting methods are applicable to other drainage systems such as those for chemical waste, graywater waste, and clear water waste. Sizing the venting system is directly tied to the design of the drainage system. For example, the velocities in the drainage system and its peak flow rates affect the diameters in the venting system. Where the vertical distance between a fixture outlet and trap is excessive, velocities in the entire drainage system will be greater than those in the vent sizing table. All venting methods in this chapter are categorized as either dry vents or wet vents. Vent stacks, stack vents, branch vents, island vents, relief vents, and individual vents are dry vents. Wet vents (horizontal or vertical), circuit vents, combination drain and vents are versions of “wet venting” in which the vent is wetted by drainage flow.

    2025 CALIFORNIA PLUMBING CODE xv

  • CPC § 911.2.1 High relevance — show source text

    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.

    911.4.1 Multiple Circuit-Vented Branches. Circuitvented horizontal branches are permitted to be connected together. Each group of a maximum of eight fixtures shall be considered a separate circuit vent and shall be in accordance with Section 911.4.1.1 and Section 911.4.1.2.

    911.4.1.1 Size of Parallel Horizontal

    Branches. Parallel horizontal circuit vented branches shall be permitted to connect on the same floor level. Each separate circuit-vented horizontal branch that is interconnected shall be sized independently in accordance with Section 911.4.

    911.4.1.2 Size of Continuous Horizontal Branches. Two or more circuit vented systems continuous on the same horizontal branch shall be uniformly sized for the total discharge into the branch.

    911.5 Additional Fixtures. Fixtures, other than the circuitvented fixtures, are permitted to discharge to the horizontal branch drain. Such fixtures shall be located on the same floor as the circuit-vented fixtures and shall be either individually or common vented.

    912.0 Engineered Vent System. 912.1 General. The design and sizing of a vent system shall be permitted to be determined by accepted engineering practices. The system shall be designed by a registered design professional and approved in accordance with Section 301.5. 912.2 Minimum Requirements. An engineered vent system shall provide protection of the trap seal in accordance with Section 901.3.

    192 2025 CALIFORNIA PLUMBING CODE

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

    CALIFORNIA PLUMBING CODE – MATRIX ADOPTION TABLE

    CHAPTER 10 - TRAPS AND INTERCEPTORS

    (Matrix Adoption Tables are non-regulatory, intended only as an aid to the code user. See Chapter 1 for state agency authority and building applications.)

  • CPC § 2025 High relevance — show source text

    The waste must be treated to prevent any damage to the piping or sewage treatment process. Waste receptors are sized and designed to prevent splashing and allow for peak discharge conditions.

    Chapter 9 Vents.

    Chapter 9 regulates the material, design, and installation of vents. A vent system is a pipe or pipes installed in a drainage system that provide a flow of air to and from the system to ventilate it, provide a circulation of air to eliminate trap siphonage, and reduce back-pressure and vacuum surge. In addition, vents provide the rapid and silent flow of waste without exposing occupants of the building to any sewer gases. Proper installation of vents is crucial, as a telltale sign that there is a problem in the drain and vent system is related to the elevation of the horizontal portion of the venting. Venting is not limited to sanitary drainage systems. Venting methods are applicable to other drainage systems such as those for chemical waste, graywater waste, and clear water waste. Sizing the venting system is directly tied to the design of the drainage system. For example, the velocities in the drainage system and its peak flow rates affect the diameters in the venting system. Where the vertical distance between a fixture outlet and trap is excessive, velocities in the entire drainage system will be greater than those in the vent sizing table. All venting methods in this chapter are categorized as either dry vents or wet vents. Vent stacks, stack vents, branch vents, island vents, relief vents, and individual vents are dry vents. Wet vents (horizontal or vertical), circuit vents, combination drain and vents are versions of “wet venting” in which the vent is wetted by drainage flow.

    2025 CALIFORNIA PLUMBING CODE xv

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

    FORMAT OF THE UNIFORM PLUMBING CODE

    Chapter 10 Traps and Interceptors.

    Chapter 10 regulates the material, design, and installation of traps, interceptors, and separators. Traps are required on drainage type plumbing fixtures and must be self-scouring without interior partitions. Interceptors, on the other hand, are designed to control what goes down a drain. Interceptors are used to keep harmful substances from entering the sanitary drainage system, such as grease, sand, oil and other materials. The retained materials need periodic removal to maintain efficiency and function of the separating device. The capacity of an interceptor is based on retention and flow rate. There are many types of interceptors that are used at beauty salons, hospitals, meat, fish or foul packaging, refineries, repair garages, gas stations, car washing facilities, various plants, factories, and processing sites. The designer of the building is responsible for locating interceptors with the expectation for the frequency of maintenance, ease of cleaning and floor space for equipment.

    Chapter 11 Storm Drainage.

    Chapter 11 regulates the removal of stormwater from roofs, yards, paved areas, and similar areas. The objective of storm drainage systems is to provide a conduit or channel through which runoff will be carried from a point of collection to a point of disposal; this protects the property and the public from the uncontrolled flow of runoff and ensures that drains and inlets are adequately sized to receive the volume of runoff that flows to the drains. For the purpose of system design, it’s necessary to specify the duration of a selected storm.

  • CPC § 10-8 High relevance — show source text

    000|10-8|12-6|9-8|10-0|9-0|8-2|7-7|6-4|6-2| |24|2-#4
    1-#6|60,000|12-11|15-2|11-9|12-2|11-0|9-11|9-3|7-8|7-6| |24|2-#5|40,000|15-2|17-9|13-9|14-3|12-10|11-7|10-10|9-0|8-9| |24|2-#5|60,000|18-4|21-6|16-7|17-3|15-6|14-0|13-1|10-4|10-0| |24|2-#6|40,000|18-0|21-1|16-4|16-11|14-10|12-9|11-8|9-2|8-11| |24|2-#6|60,000|21-7|25-4|19-2|20-4|17-2|14-9|13-4|10-4|10-0| |24|Center distance_A_k, l|Center distance_A_k, l|4-6|6-2|3-8|4-0|3-3|2-8|2-3|1-7|1-6| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound per square inch = 6.895 kPa, 1 pound per square foot = 0.0479 kPa, Grade 40 = 280 MPa, Grade 60 = 420 MPa.
    a. See Table R608.3 for tolerances permitted from nominal thickness.
    b. Table values are based on concrete with a minimum specified compressive strength of 2,500 psi. See Note j.
    c. Table values are based on uniform loading. See Section R608.8.2 for lintels supporting concentrated loads.
    d. Deflection criterion is_L_/240, where_L_ is the clear span of the lintel in inches, or1/2 inch, whichever is less.
    e. Linear interpolation is permitted between ground snow loads and between lintel depths.
    f. DR indicates design required.
    g. Lintel depth,D, is permitted to include the available height of wall located directly above the lintel, provided that the increased lintel depth spans the entire length of the
    lintel.
    h. Stirrups shall be fabricated from reinforcing bars with the same yield strength as that used for the main longitudinal reinforcement.
    i. Allowable clear span without stirrups applicable to all lintels of the same depth,D. Top and bottom reinforcement for lintels without stirrups shall be not less than the least
    amount of reinforcement required for a lintel of the same depth and loading condition with stirrups. All other spans require stirrups spaced at not more than_d_/2.
    j. Where concrete with a minimum specified compressive strength of 3,000 psi is used, clear spans for lintels without stirrups shall be permitted to be multiplied by 1.05.

Frequently asked questions

When can I choose a combination waste and vent system instead of standard venting?

Only when structural conditions make conventional venting impractical — and the system design and construction documents must be approved by the Authority Having Jurisdiction (§ 910.1, § 910.2).

How much larger must I size the drain and traps?

Each waste pipe and each trap must be not less than two pipe sizes larger than the sizes required by Chapter 7 for a conventional design and two sizes larger than the fixture tailpiece or connection (§ 910.4).

How do I size the vent for a combination system?

The vent's cross‑sectional area must be at least one‑half the inside area of the drain it serves; the vent must provide free air circulation and connect downstream of the uppermost fixture (§ 910.3).

Can I run a toilet on a combination waste and vent system?

No. Water closets and urinals are expressly prohibited from being installed on such systems (§ 910.7).

What if a branch serving traps is very long?

Any branch serving traps that exceeds 15 feet must be separately vented in an approved manner (§ 910.3).

Are these systems suitable for restaurant kitchen equipment?

Appendix B cautions against connecting grease‑producing or surge‑producing equipment to combination systems because the oversize pipe is not self‑scouring; avoid restaurant kitchen connections unless an engineered alternative is approved.

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Circuit venting — permitted configurations, sizing, and relief vent rules

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Venting, combination waste-and-vent, and special venting methods

California Plumbing Code