Chapter 3 of 626% of exam

Drainage, Waste and Vent (DWV)

The DWV system carries waste away by gravity while venting keeps air pressure balanced so trap seals are not lost. This chapter covers drain sizing by fixture units, slope, traps and trap seals, venting, cleanouts, and interceptors.

Drain Sizing by Drainage Fixture Units

Drain, waste, and vent piping is sized using drainage fixture units (DFU), a value that represents the discharge load of each fixture. The DFU loads are totaled and matched to a code table that gives the required pipe size for a given slope.

Drainage fixture unit (DFU)
A value representing the probable discharge of a fixture into the drainage system, used to size drains and sewers.
2022 California Plumbing Code (CPC) §702.0
Total the load
Add the DFU of all fixtures on a drain, then select the pipe size from the code table for the chosen slope.
Never reduce downstream
The size of a drainage pipe must not decrease in the direction of flow toward the sewer.
Building drain and sewer
The building drain and building sewer are sized for the total connected DFU using the drainage sizing tables.
2022 California Plumbing Code (CPC) §703.0

Slope and Grade

Horizontal drainage piping must slope so that solids are carried along by the flow; too little slope lets solids settle and too much can let liquid outrun solids. The code sets a minimum slope that depends on pipe diameter.

Minimum slope
Horizontal drainage pipe 2.5 inches and smaller commonly slopes at least 1/4 inch per foot toward the outfall.
2022 California Plumbing Code (CPC) §708.0
Larger pipe slope
Pipe 3 inches and larger may be permitted at 1/8 inch per foot where approved because the larger bore still carries solids.
Uniform grade
Grade should be uniform and continuous so there are no low spots that trap waste or create bellies in the line.
Flow velocity
Slope is chosen to keep a self-scouring velocity that carries solids without leaving the liquid behind.

Traps and Trap Seals

A trap holds a water seal that blocks sewer gases from entering the building through a fixture. The seal depth and the distance from the trap to its vent are regulated so the seal is not siphoned or blown out.

Trap seal depth
Each fixture trap must have a liquid seal generally between 2 and 4 inches deep unless a deeper seal is required.
2022 California Plumbing Code (CPC) §1008.0
One trap per fixture
Each fixture is separately trapped, and a fixture must not be double trapped because a second trap can lock the flow.
Prohibited traps
S-traps, bell traps, and traps with moving parts are prohibited because they can lose their seal or clog.
Trap arm
The trap arm is the drain between the trap weir and the vent; its length and slope are limited to protect the seal.

Trap-to-Vent Distance and Venting

Every trap must be vented so that discharge does not siphon the seal. The maximum distance from the trap to its vent depends on the size of the trap arm, and several venting methods can satisfy the requirement.

Maximum trap-to-vent distance
The developed length of the trap arm from the trap to the vent is limited by pipe size in the code table.
2022 California Plumbing Code (CPC) §1002.2
Individual vent
A vent that serves a single fixture trap and connects to the vent system or extends to open air.
Common vent
A single vent that serves two fixture traps connected at the same level, permitted within code limits.
Wet vent
A pipe that serves as both a drain for one fixture and a vent for another, allowed under specific code conditions.
Vent termination
Vents must extend through the roof and terminate a set distance above the roof and away from openings.
2022 California Plumbing Code (CPC) §906.0

Cleanouts and Interceptors

Cleanouts give access to clear stoppages in the drainage system, and interceptors separate harmful or clogging substances such as grease, oil, or sand before they reach the sewer. Both are placed where the code and the hazard require.

Cleanout access
Cleanouts must be installed and located so drainage piping can be rodded and cleared, including at changes of direction and along long runs.
2022 California Plumbing Code (CPC) §707.0
Cleanout spacing
Horizontal drains require cleanouts at intervals set by the code so the full line can be reached.
Grease interceptor
Kitchens and food service that discharge grease must install an approved grease interceptor to keep grease out of the sewer.
2022 California Plumbing Code (CPC) §1014.0
Sand and oil interceptors
Garages and industrial drains use sand or oil interceptors to catch heavy solids and petroleum before they enter the drainage system.
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Last updated: July 2026

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