CPC · California Plumbing Code

When are combination waste-and-vent systems permitted and what are the limits?

A combination waste-and-vent system uses an oversized waste pipe that also serves as the vent; the CPC allows it only when normal venting is structurally impractical and only if vents, pipe sizes (each trap/waste increased two sizes), connection angles, and branch lengths meet the specific limits in §§ 910.1, 910.3 and 910.4, with AHJ approval required.

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

A Combination Waste and Vent System is a specially designed, oversized waste pipe that also functions as a vent for the fixtures it serves. These systems are permitted only where structural conditions preclude the installation of conventional venting, per § 910.1. They must provide vents adequate for free air circulation, meet specific sizing requirements (pipes and vents), and follow connection/angle limits described in § 910.3 and § 910.4.

The most important rule: combination waste-and-vent systems are an exception — allowed only when normal venting cannot be installed, and then only if vents, oversized piping, and connections meet the precise limits in § 910.3 and § 910.4.

Requirements in detail

Where permitted

Allowed only when structural conditions preclude conventional venting — § 910.1.
Construction documents for each system must be approved by the Authority Having Jurisdiction before installation (see § 910.2).

Venting and air circulation

The system must be provided with a vent or vents adequate to ensure free circulation of air — § 910.3.
A branch exceeding 15 feet (4572 mm) shall be separately vented in an approved manner — § 910.3.
The vent area installed in the combination system must be not less than one-half the inside cross-sectional area of the drainpipe served — § 910.3.
The vent connection must be downstream of the uppermost fixture — § 910.3.

Connections, angles, and sizing

Branches serving traps shall connect to the main line at an angle not exceeding 2 percent — § 910.4.
Each waste pipe and each trap in the system shall be not less than two pipe sizes larger than the sizes required by Chapter 7, and not less than two pipe sizes larger than the fixture tailpiece or connection — § 910.4.

Supplemental guidance from Appendix B (design notes)

These systems are intentionally oversized so the flow line stays low and air can circulate above it; because they are oversized they may not be self‑scouring and are unsuitable for grease-producing equipment (e.g., restaurant kitchen equipment) — Appendix B.
Long mains should have relief vents at intervals not exceeding 100 feet (30 480 mm); each such relief vent shall be not less than one-half the inside cross-sectional area of the drainpipe served — Appendix B guidance.
Tailpieces should be as short as possible; Appendix B reiterates the 2‑ft (610 mm) maximum tailpiece length for vertical drops (see also § 910.5) and gives an example of increasing a 2‑inch trap/waste two sizes to 3 inches in a combination system.

Quick decision table (decision‑relevant dimensions/values)

What to check	Required value / limit	Code reference
Where allowed	Only when conventional venting is impracticable (structural constraints)	§ 910.1
Branch length before separate vent required	> 15 ft (4572 mm) → separately vent	§ 910.3
Vent pipe minimum area	≥ 1/2 the inside cross‑sectional area of the drain served	§ 910.3
Vent connection location	Downstream of the uppermost fixture	§ 910.3
Branch connection angle	Not to exceed 2%	§ 910.4
Waste/trap sizing increase	+2 pipe sizes over Chapter 7 requirement and over fixture tailpiece	§ 910.4
Tailpiece max vertical length	≤ 2 ft (610 mm) (see § 910.5 / Appendix B)	§ 910.5 and Appendix B
Relief vents for long mains	Guidance: every 100 ft; vent area ≥ 1/2 drain area (Appendix B)	Appendix B

Exceptions & special cases

No water closet or urinal shall be installed on a combination waste-and-vent system; other small fixtures (one, two, or three unit fixtures) may connect if located and sized per § 910.7 and the two‑pipe‑size increase rule is based on total fixture unit load.
Appendix B explicitly warns against connecting grease‑producing equipment (e.g., restaurant kitchen equipment) and other appurtenances that deliver large surges of water (pumps, sand interceptors) because the pneumatic balance of the system can be upset.
Construction documents must be approved by the Authority Having Jurisdiction before installing any portion of the system — § 910.2.

Common mistakes

Failing to increase trap and waste pipe sizes by two pipe sizes — undersizing defeats the “air space above flow” principle and violates § 910.4.
Using vertical waste piping (other than short fixture tailpieces) instead of horizontal oversized piping — vertical waste is generally prohibited in these systems (see § 910.5) and will likely be rejected.
Not separately venting branches that exceed 15 ft — leads to poor air circulation, trap siphonage risk, and noncompliance with § 910.3.
Connecting water closets/urinals or grease‑producing fixtures (restaurant equipment) — expressly prohibited or cautioned against in the code and Appendix B.
Omitting Authority Having Jurisdiction approval or layout drawings for complicated systems — § 910.2 and Appendix B require prior approval and layout submittals.

Worked example — concrete scenario

Scenario: You have a line of floor drains in a market stall area where wall venting is structurally impossible. A typical floor drain normally uses a 2‑inch trap and waste.

Step 1 — Permission: Confirm structural impracticability and obtain Authority approval per § 910.1 and § 910.2.
Step 2 — Increase sizes: Because each trap/waste must be two pipe sizes larger, the normal 2‑inch trap/waste is increased to a 3‑inch trap/waste on the combination system (example provided in Appendix B). The floor drain’s tailpiece remains 2‑inch (normal) to ensure the trap partially fills.
Step 3 — Venting: If any branch run from the most upstream fixture to the most downstream exceeds 15 ft, provide separate venting for that branch; size the vent(s) to be not less than one‑half the inside cross-sectional area of the drain served and connect downstream of the uppermost fixture.
Step 4 — Connections & slope: Connect branch serving each trap to the main at an angle not exceeding 2%, and locate relief vents at intervals per Appendix guidance if mains are very long (Appendix B suggests relief vents every 100 ft).

Result: The floor drains may be served by a combination waste-and-vent main sized and vented per code, provided the AHJ approves the construction documents and the system meets the vent area, spacing, and sizing rules.

Related provisions

§ 910.1 — Where permitted (controlling allowance).
§ 910.3 — Vents: minimum vent area, branch length venting, vent connection location.
§ 910.4 — Connections and size: branch angle and two‑size increase requirement.
§ 910.2 — Approval: AHJ approval of construction documents.
§ 910.5 — Vertical waste pipe limitations (tailpiece max length).
§ 910.6 — Cleanout requirements for vents and exceptions.
§ 910.7 — Fixture limitations (no water closets/urinals; small fixtures allowed with conditions).
Appendix B — Explanatory notes on design, relief vents, cautions (grease, surge appliances), and an example of sizing.
Definition of Combination Waste and Vent System in the code’s definitions (Chapter 2/definition section).

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 § 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.

2025 CALIFORNIA PLUMBING CODE 381

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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 § 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 § 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 § 101.3 High relevance — show source text
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.

B 101.4 Pneumatics. It is essential that the pneumatics of such a system be properly engineered, as the air pressure within the line shall at all times balance that of outside atmosphere to prevent either trap seal loss or air locking between traps. Long mains shall be provided with additional relief vents located at intervals not exceeding 100 feet (30 480 mm). Each such relief vent shall equal not less than one-half of the inside cross-sectional area of the drainpipe served.

B 101.5 Trap Sizes. Trap sizes are required to be equivalent to the branches they serve (two pipe sizes larger than normal), and tailpieces between fixtures or floor drains and such traps shall be reduced to normal size.

B 101.6 Layout Drawings. Duplicate layout drawings of each such proposed piping system shall be presented to the Authority Having Jurisdiction and approval obtained before an installation is made. Complicated layouts shall be checked by qualified personnel. B 101.6.1 Example of Sizing. A floor drain normally requires a 2 inch (50 mm) trap and waste. On a combination waste and vent system, both trap and waste shall be increased two pipe sizes (through 2 [1] ⁄ 2 inches and 3 inches) (65 mm and 80 mm), which would make the trap 3 inches (80 mm). Pipe sizes recognized for this purpose are 2 inches, 2 [1] ⁄ 2 inches, 3 inches, 3 [1] ⁄ 2 inches, 4 inches, 4 [1] ⁄ 2 inches, 5 inches, 6 inches, etc. (50 mm, 65 mm, 80 mm, 90 mm, 100 mm, 115 mm, 125 mm, 150 mm, etc.). The tailpiece between the floor drain and its trap shall be 2 inches (50 mm) (or normal size) to ensure that the amount of wastewater entering the trap partially fills the waste branch.
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 § 905.1 High relevance — show source text
905.1 Grade . . . . . . . . . . . . . . . . . . . . . .190

905.2 Horizontal Drainage Pipe . . . . . . .190

905.3 Vent Pipe Rise . . . . . . . . . . . . . . .190

905.4 Roof Termination . . . . . . . . . . . . .190

905.5 Location of Opening . . . . . . . . . . .190

905.6 Common Vertical Pipe . . . . . . . . .190

906.0 Vent Termination . . . . . . . . . . . . .190

906.1 Roof Termination . . . . . . . . . . . . .190

906.2 Clearance . . . . . . . . . . . . . . . . . . .190

906.3 Use of Roof . . . . . . . . . . . . . . . . .190

906.4 Outdoor Installations . . . . . . . . . . .190

906.5 Joints . . . . . . . . . . . . . . . . . . . . . .190

906.6 Lead . . . . . . . . . . . . . . . . . . . . . . .190

906.7 Frost or Snow Closure . . . . . . . . .190

907.0 Vent Stacks and Relief Vents . . . .190

907.1 Drainage Stack . . . . . . . . . . . . . . .190

907.2 Yoke Vent . . . . . . . . . . . . . . . . . . .190

908.0 Wet Venting . . . . . . . . . . . . . . . . .190

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
CPC § 363 Medium relevance — show source text
APPENDICES TABLE OF CONTENTS . . . . . . . .363

Appendix A Recommended Rules for Sizing the Water Supply System . . . . . . . . . . . . . . . . . . . . .365

Appendix B Explanatory Notes on Combination Waste and Vent

Systems . . . . . . . . . . . . . . . . . . . .381

Appendix C Alternate Plumbing Systems . . . .385

Appendix D Sizing Storm Water Drainage Systems . . . . . . . . . . . . . . . . . . . .395

Appendix E Manufactured/Mobile Home Parks and Recreational

Vehicle Parks . . . . . . . . . . . . . . . .405

Appendix F Firefighter Breathing Air Replenishment Systems . . . . . . . .417

Appendix G Sizing of Venting Systems . . . . . .423

Appendix H Private Sewage Disposal Systems . . . . . . . . . . . . . . . . . . . .435

Appendix I Installation Standards . . . . . . . . . .447

Appendix J Combination of Indoor and Outdoor Combustion and

Ventilation Opening Design . . . . .487

Appendix K Potable Rainwater Catchment Systems . . . . . . . . . . .491

Appendix L Sustainable Practices . . . . . . . . . .497

Appendix M Peak Water Demand Calculator . . . . . . . . . . . . . . . . . . .517

Appendix N Impact Of Water Temperature On The Potential for Scalding and Legionella Growth . . . . . . . . .525

Appendix O Non-Sewered Sanitation Systems . . . . . . . . . . . . . . . . . . . .531

Appendix P Professional Qualifications . . . . . .535

Appendix Q Indoor Horticultural Facilities . . . .539

Appendix R Tiny Houses . . . . . . . . . . . . . . . . .543

Appendix S Onsite Stormwater Treatment Systems . . . . . . . . . . . . . . . . . . . .547

USEFUL TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . .553

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . .563

HISTORY NOTE

APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . .589

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CALIFORNIA PLUMBING CODE – MATRIX ADOPTION TABLE

CHAPTER 1 - ADMINISTRATION

(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.)

|Adopting
CPC § 205.0 Medium relevance — show source text
Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0

Residential gray water . . . .1503.1.1, 1503.2.2 Exception

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INDEX

CODE, DEFINITION . . . . . . . . . . . . . . . . . . . . . . . . . . .205.0

COLLECTION CENTERS AND

FACILITIES . . . . . . . . . . . . . . . . . . . . . .601.6, 725.0, 1010.3

COMBINATION WASTE AND

VENT SYSTEMS . . . . . . . . . . . . .205.0, 910.0, Appendix B

COMBUSTIBLE

MATERIAL, DEFINITION . . . . . . . . . . . . . . . . . . . . .205.0

COMBUSTIBLE PIPING

INSTALLATIONS . . . . . . . . . . . . . . . . . . . . . . . . . .1404.0

COMBUSTION AIR . . . . . . . . . . . . . . . . . . . . . . . . . . .506.0

COMMISSARIES SERVING MOBILE

FOOD PREPARATION UNITS . . . . . . . . . . . . . . . . .422.10

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0

COMMON VENT, FUEL GAS . . . . . .509.6.3, 509.10.2.2,

509.10.2.3, 509.10.3.1,

509.10.8.2, 510.2.3, 510.2.9,

510.2.15, 510.2.16

COMPLEX SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . .1503.1.3

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0

COMPRESSION JOINTS . . . . . . . . . . .605.7.1.1, 605.10.1.1

705.2.2, 719.6, 1208.5.10.3

CONCEALED SLIP JOINTS . . . . . . . . . . . . . . . . . . .402.10

CONCEALING OF WORK PROHIBITED

BEFORE INSPECTION . . . . .105.1, 105.2.1.1, 105.2.1.2,

503.2, 1203.2

CONDENSATE WASTE
CPC § 2025 Medium 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.

2025 CALIFORNIA PLUMBING CODE xvii

), 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

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 § 707.8 Medium relevance — show source text
Cleanouts . . . . . . . . . . . . . . . . . . . . . . . . . .707.8, 707.9

Fuel gas vents . . . . . . . . . .509.7.3.4, Table 509.7.3.4(1),

Table 509.7.3.4(2)

Water heaters . . . . . . . . . . . . . . . . . . . .504.3.1, 504.3.2

CLEAR WATER WASTE

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .205.0

Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .812.1

CLINICAL SINKS . . . . . . . . . . . . . . . . .205.0, 413.1, 1303.7

CLOSET BENDS . . . . . . . . . . . . . . . . . . . . . .310.3, 402.6.1

CLOSET RINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . .402.6.1

CLOSET SCREWS . . . . . . . . . . . . . . . . . . . . . . . . . .402.6.2

CLOTHES WASHER SYSTEM

Commercial gray water discharge . . . . . . . .1503.8.2.3

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.0

Residential gray water . . . .1503.1.1, 1503.2.2 Exception

2025 CALIFORNIA PLUMBING CODE 565

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

INDEX

CODE, DEFINITION . . . . . . . . . . . . . . . . . . . . . . . . . . .205.0

COLLECTION CENTERS AND

FACILITIES . . . . . . . . . . . . . . . . . . . . . .601.6, 725.0, 1010.3

COMBINATION WASTE AND

VENT SYSTEMS . . . . . . . . . . . . .205.0, 910.0, Appendix B

COMBUSTIBLE

MATERIAL, DEFINITION . . . . . . . . . . . . . . . . . . . . .205.0

COMBUSTIBLE PIPING

INSTALLATIONS . . . . . . . . . . . . . . . . . . . . . . . . . .1404.0

COMBUSTION AIR . . . . . . . . . . . . . . . . . . . . . . . . . . .506.0

COMMISSARIES SERVING MOBILE
CPC § 2025 Medium 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

Frequently asked questions

Can I put a toilet or urinal on a combination waste-and-vent system?

No. Water closets and urinals are not permitted on combination waste-and-vent systems — see § 910.7.

How much larger must I size the trap and waste piping?

Each waste pipe and trap must be not less than two pipe sizes larger than the sizes required by Chapter 7 and two sizes larger than the fixture tailpiece — § 910.4.

When do I need a separate vent on a branch?

A branch exceeding 15 feet (4572 mm) in length must be separately vented — § 910.3.

What minimum vent area is required?

A vent in a combination system must have an area not less than one‑half the inside cross‑sectional area of the drain it serves — § 910.3.

Do I need AHJ approval before installing one of these systems?

Yes. Construction documents must first be approved by the Authority Having Jurisdiction before any portion of a combination waste-and-vent system is installed — § 910.2.

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