Sanitation System (C-42) · Lesson 5 of 6

Building Sewer Laterals: Fall, Size, Trench, and Cleanouts

"Building Sewer Laterals: Fall, Size, Trench, and Cleanouts" is lesson 5 of 6 in the Sanitation System (C-42) Learning Hub. It opens with "Fall: The Slope Rules for a Building Sewer" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Fall: The Slope Rules for a Building Sewer
  2. Size the Sewer by Fixture Units
  3. Trench Width and Bedding for Plastic Sewer Pipe
  4. Cleanouts on the Lateral
  5. Distances, Testing, and Abandoned Lines

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Trench width" through "Cleanouts".

What you'll be able to do

  • Explain the main idea of "Building Sewer Laterals: Fall, Size, Trench, and Cleanouts" in plain language you can use on the exam.
  • Apply this step with confidence: "Trench Width and Bedding for Plastic Sewer Pipe".
  • Apply this step with confidence: "Cleanouts on the Lateral".
  • Apply this step with confidence: "Distances, Testing, and Abandoned Lines".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Building Sewer Lateral: 4-Inch, 130 FeetPROFILE: 130 ft lateral (fall exaggerated)HOUSEC/O 1C/O 2TANKfall = 32.5 in over 130 ft100 ft between cleanouts (max 100)30 ftSIZE AND FALL (175 fixture units)Fall: 130 ft x 1/4 in = 32.5 in (2 ft 8-1/2 in)4 in at 1/4 in/ft: 216 FU, over 175 - OK4 in at 1/8 in/ft: 216 x 0.8 = 172.8, under 175 - FAIL6 in at 1/8 in/ft: 720 x 0.8 = 576 FU - OKCleanouts: at the house and every 100 ft maxTest: water fill (no air test on plastic DWV)TRENCH (4.5 in OD pipe)≥ 20.5 inbed ≥ 4 in6 in liftsto 12 inover pipe,85% proctorwidth = OD + 16 in4-in sewer at 1/4 in/ft, 32.5 in of fall, trench 20.5 in wide with 4-in bedCleanouts at the building and at 100 ft; at 1/8 in/ft the 175 FU would need 6-in pipe2025 CPC Sections 718.1, 719.1, 314.4.1 and Table 703.2 (footnote 5)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Fall: The Slope Rules for a Building Sewer

2025 California Plumbing Code (CPC) Section 718.1 requires building sewers to run in practical alignment at a uniform slope of not less than 1/4 inch per foot toward the point of disposal. The exception, only where the AHJ approves and it is impractical because of street sewer depth or the building layout, allows 4-inch through 6-inch pipe at not less than 1/8 inch per foot and 8-inch and larger pipe at not less than 1/16 inch per foot. A 100-foot run at 1/4 inch per foot drops 25 inches, so the invert at the tie-in must be checked before you cut the trench.

Step 2: Size the Sewer by Fixture Units

Section 717.1 says the sewer size is based on the total fixture units drained, and no building sewer may be smaller than the building drain. For 6-inch and smaller pipe, Table 717.1 sends you to Table 703.2: horizontal drainage maximums are 35 units for 3-inch, 216 for 4-inch, 428 for 5-inch, and 720 for 6-inch, based on 1/4 inch per foot. Table 703.2 footnote 5 says that at 1/8 inch per foot you multiply the horizontal fixture units by 0.8. For 8-inch and larger, Table 717.1 gives a maximum and a minimum load for each slope.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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