学习材料

This is the largest chapter on the exam — about a quarter of the whole test — and for good reason: the DWV system is where gravity, sizing math, and public health all meet. The drainage system carries waste away by gravity, and the vent system keeps the air pressure inside the pipes balanced so that trap seals are not siphoned or blown out. Break either half and sewer gas comes into the building. Everything in this chapter — drain sizing, slope, traps, venting, cleanouts, interceptors — exists to keep waste moving out and sewer gas locked out.

3.1 Drain Sizing by Drainage Fixture Units (DFU)

Just as supply pipe is sized by WSFU, drainage pipe is sized by drainage fixture units (DFU) — a value representing the probable discharge load each fixture puts into the drainage system (CPC §702.0, Table 702.1). Each fixture carries a listed DFU value: a lavatory is 1 DFU, a typical shower is 2 DFU, and a water closet is a large load (commonly 3–4 DFU for a modern low-flow WC, historically higher), reflecting the surge of a flush.

The method mirrors the supply side:

1. Assign a DFU value to each fixture from the code table.
2. Total the DFU on the drain you're sizing.
3. Select the pipe size from the drainage sizing table for the chosen slope — because a given pipe can carry more DFU at a steeper slope. The building drain and building sewer are sized for the total connected DFU the same way (CPC §703.0, Tables 703.1 and 703.2).

Two hard rules govern drainage sizing:

• Never reduce a drain in the direction of flow. A drainage pipe must not decrease in size as it heads downstream toward the sewer. You can go bigger, never smaller. (The same "no reduction toward the terminal" logic applies to vents heading up toward their outlet.)
• Minimum sizes are set by fixture and by fixture count. A drain has limits on how many of a given fixture it may carry, independent of the raw DFU total: 3-inch pipe is capped at six water closets or five six-unit traps (2025 CPC Table 703.2, note 4). Size up when the count or the DFU total exceeds the table value.

Edition alert. This fixture-count note is one of the values that moved. Editions through the 2022 CPC split it — four water closets on a vertical stack, three on a horizontal branch. The 2025 CPC (adopting the 2024 UPC, effective January 1, 2026) states a single limit for the 3-inch row: "Not to exceed six water closets or five six-unit traps." If your jurisdiction is still permitting under the 2022 CPC, the older split still governs that permit — Health & Safety Code §18938.5 applies the standards in effect when the permit application was made.

Worked sizing questions are common. If a horizontal branch serves two water closets (say 3 DFU each = 6), two lavatories (1 each = 2), and two showers (2 each = 4), the branch carries 12 DFU — and because it serves water closets, the branch must be at least 3 inches (the minimum drain size for a water closet; two WCs is well inside the fixture-count cap on either edition).

Key Numbers & Facts — DFU Sizing - DFU = probable discharge load per fixture (CPC §702.0, Table 702.1). - Sample DFU: lavatory 1, shower 2, water closet ≈3–4 (flush surge). (Verify exact values on the current Table 702.1.) - Method: assign DFU → total → pick size from the drainage table for the slope (CPC §703.0). - Never reduce pipe size in the direction of flow. - 3-inch pipe → max 6 water closets (or 5 six-unit traps) (2025 CPC Table 703.2, note 4 — the current note sets one limit for the 3-inch row and no longer splits stack vs. branch). - Example capacity: 5-inch horizontal building drain/sewer at 1/4"/ft = 428 DFU (2025 CPC Table 703.2; note 5 — at 1/8"/ft multiply horizontal fixture units by 0.8, so ≈342).

3.2 Slope and Grade

Horizontal drainage pipe runs downhill so the flow carries solids along with it. The grade has to be a compromise: too little slope lets solids settle out and build a stoppage; too much slope can let the liquid outrun the solids, leaving them stranded. The code sets a minimum slope that depends on pipe diameter.

• Pipe 2-1/2 inches and smaller: minimum slope commonly 1/4 inch per foot (that's a 1:48 grade) toward the outfall.
• Pipe 3 inches and larger: may be permitted at 1/8 inch per foot where approved, because the larger bore still carries solids at the gentler grade (CPC §708.0).

Two more grade principles matter on the job and on the exam:

• Uniform, continuous grade. The fall should be steady with no low spots — a sag or "belly" in the line traps waste and breeds stoppages.
• Self-scouring velocity. The slope is chosen to keep the flow moving fast enough to scour the pipe (carry solids) without leaving the liquid behind. This is the physical reason behind both the minimum and the practical maximum.

Remember the direction of the ratio: 1/4" per foot is steeper than 1/8" per foot. The smaller pipe needs the steeper minimum.

Key Numbers & Facts — Slope - Pipe ≤ 2-1/2": min slope 1/4 in per foot (1:48). - Pipe ≥ 3": 1/8 in per foot permitted where approved (CPC §708.0). - 1/4"/ft is steeper than 1/8"/ft — smaller pipe, steeper minimum. - Grade must be uniform/continuous (no bellies) and keep a self-scouring velocity.

3.3 Traps and Trap Seals

A trap is the U-shaped fitting under every fixture that holds a water seal — a plug of standing water that blocks sewer gas from coming up through the fixture into the building. The trap is the front-line barrier of the whole DWV system, and the code protects it carefully.

• Trap seal depth. Each fixture trap must hold a liquid seal generally between 2 and 4 inches deep (CPC §1008.0), unless a deeper seal is specifically required (as for some traps subject to evaporation). Too shallow and it siphons or evaporates away; too deep and it can restrict flow.
• One trap per fixture — no double trapping. Each fixture is separately trapped, with a trap placed as close as practical to the fixture outlet. A fixture must not be double-trapped — a second trap in series locks the flow by trapping air between the two seals. Some assemblies of like fixtures (a three-compartment sink, say) are allowed to share under specific code provisions, but the default is one trap, one fixture (CPC §1002.1).
• Prohibited traps. S-traps, bell traps, and traps with moving (mechanical) parts are prohibited. An S-trap self-siphons its own seal; a bell trap clogs and loses its seal; moving-part traps fail. Use a P-trap, which is designed to be vented and hold its seal.
• The trap arm. The trap arm is the length of drain between the trap's weir and the vent. Its length and slope are limited to protect the seal — which leads directly to the next section.

Key Numbers & Facts — Traps - Trap seal depth: 2 to 4 inches (CPC §1008.0). - One trap per fixture; no double-trapping (locks flow); trap set close to the fixture outlet (CPC §1002.1). - Prohibited: S-traps, bell traps, moving-part traps. Use a vented P-trap. - Trap arm = drain from the trap weir to the vent; length and slope limited.

3.4 Trap-to-Vent Distance and Venting

Every trap must be vented so that when the fixture (or others on the line) discharges, the moving water does not siphon the seal out of the trap. The maximum developed length of the trap arm — from the trap to the vent — is limited by the size of the trap arm, set out in a code table (CPC §1002.2). The bigger the trap arm, the farther the vent may be:

Trap arm size	Minimum distance, trap to vent	Max developed length, trap to vent
1-1/4 in	2-1/2 in	2 ft 6 in (30 in)
1-1/2 in	3 in	3 ft 6 in (42 in)
2 in	4 in	5 ft (60 in)
3 in	6 in	6 ft (72 in)
4 in	8 in	10 ft (120 in)
Over 4 in	2 × diameter	10 ft (120 in)

Verified against the 2025 California Plumbing Code, Table 1002.2, "Horizontal Lengths of Trap Arms" (values unchanged from the 2022 CPC). Three things the table itself says that candidates miss: the maximums assume the arm is held to 1/4 inch per foot (note 1); the minimum is never less than two pipe diameters (§1002.2); and the table excepts water closets and similar fixtures — for those, the developed length measured from the face of the closet flange to the inner edge of the vent may not exceed 6 feet (note 2).

Two companion rules ride along with the distance table:

• The trap arm's total fall must not exceed one pipe diameter over its developed length. If the trap arm drops more than one diameter before reaching the vent, the fixture outlet effectively falls below the vent opening and the trap can self-siphon.
• Vents may not be reduced in size as they run up toward the terminal (the mirror image of the drainage rule).
• Vents are graded to drain. Horizontal vent runs are sloped back toward the drainage pipe they serve, so condensation and any rainwater entering the roof terminal drain away by gravity instead of pooling and blocking the vent.

You should recognize the common venting methods:

• Individual vent — serves a single fixture trap; connects to the vent system or runs to open air.
• Common vent — a single vent serving two fixture traps connected at the same level (for example, two fixtures back-to-back on a shared vertical drain), permitted within code limits (CPC §905.6).
• Wet vent — a pipe that serves as a drain for one fixture and a vent for another at the same time, allowed only under specific code conditions.
• Combination waste and vent — used where a conventional vent is impractical, such as floor sinks and island sinks (CPC §910.0).

Finally, vent termination: vents must extend through the roof and terminate a set distance above the roof and away from openings (doors, windows, air intakes) so sewer gas can't re-enter the building (CPC §906.0). In cold climates subject to frost closure, a vent through the roof must be enlarged (commonly to at least 3 inches), and that increase in size must be made at least 12 inches inside the building before the vent penetrates the roof, so the enlargement doesn't frost shut at the surface.

Key Numbers & Facts — Venting - Max trap-arm developed length by size: 1-1/4"→30 in, 1-1/2"→42 in, 2"→5 ft (60 in), 3"→6 ft (72 in), 4"→10 ft (120 in) (2025 CPC §1002.2 / Table 1002.2). - Minimum trap-to-vent distance = 2 × the trap-arm diameter (§1002.2); the table's maximums assume 1/4 in per foot on the arm (Table 1002.2, note 1). - Water closets are excepted from that table — a WC's developed length, closet-flange face to the inner edge of the vent, may not exceed 6 ft (Table 1002.2, note 2). - Trap-arm total fall ≤ one pipe diameter. - Individual vent = 1 trap; common vent = 2 traps at the same level (CPC §905.6); wet vent = drains one, vents another. - Combination waste & vent for floor sinks / island sinks (CPC §910.0). - Vents extend through roof, terminate a set distance above roof & away from openings (CPC §906.0). - Cold-climate roof vent enlarged (≈3 in), size increase made ≥12 in inside before the roof. - Vents never reduce in size toward the terminal.

3.5 Cleanouts and Interceptors

Two DWV features remain: the access points that let you clear a stoppage, and the devices that keep the wrong things out of the sewer.

Cleanouts are fittings that give access to clear a stoppage — a capped opening through which a drain can be rodded (snaked). The code requires cleanouts installed and located so the drainage piping can be reached and cleared (CPC §707.0), including:

• At changes of direction — a cleanout is generally required at a significant change of direction (such as a 90° turn on the building drain) so a blockage at the elbow can be reached.
• Along long horizontal runs — cleanouts are spaced at code intervals so the whole line can be reached from an access point.
• At the base of stacks and at the building drain's connection to the sewer (a two-way cleanout is common where the building drain leaves the building).

Interceptors (and separators) remove harmful or clogging substances before they reach the sewer:

• Grease interceptor — required where kitchens and food-service operations discharge grease, to keep grease out of the sewer where it would congeal and block the line (CPC §1014.0). Food-handling equipment such as a commercial food-prep sink or an ice machine must discharge to the drainage system by an indirect waste with an air gap to a floor sink or receptor (so a sewer backup can't contaminate food equipment).
• Sand and oil interceptors — used at garages and industrial drains to catch heavy solids (sand) and petroleum (oil) before they enter the drainage system.

Key Numbers & Facts — Cleanouts & Interceptors - Cleanouts give rodding access; required at changes of direction and along long runs (CPC §707.0). - Grease interceptor for kitchen/food-service grease (CPC §1014.0); food equipment drains via indirect waste + air gap to a floor sink. - Sand/oil interceptors at garages/industrial drains. - Never reduce drain size toward the sewer (recurring rule).