Trade Exam · Module 2
Plumbing Math
The C-36 exam gives you a calculator, but you still have to know which formula to use. This lesson covers the math plumbers use every week: drain slope and total fall under 2025 CPC 708.1, pressure and elevation (0.433 psi per foot, 2.31 feet per psi), offset travel, adding drainage fixture units from Table 702.1 and checking them against Table 703.2, roof area for storm drainage, and pipe volume. Every worked example has been recomputed.
Key Takeaways
- Horizontal drainage piping slopes at least 1/4 inch per foot (2 percent). Only a building drain 4 inches or larger may go to 1/8 inch per foot, and only where 1/4 is impractical and the AHJ approves it first (CPC 708.1).
- Total fall = run in feet x slope in inches per foot. 20 feet at 1/4 inch per foot = 5 inches.
- 1 foot of water = 0.433 psi, and 1 psi = 2.31 feet of water. Pressure drops 0.433 psi for every foot you rise.
- 45 degree offset: travel = offset x 1.414. For 60 degree fittings the multiplier is 1.155; for 22-1/2 degree fittings it is 2.613.
- DFU values (Table 702.1): lavatory 1, kitchen sink 2, shower 2, bathtub 2 (private), clothes washer 3, 1.6 gpf water closet 3 private or 4 public.
- Horizontal drain capacity (Table 703.2, at 1/4 inch per foot): 2 inch 8 DFU, 3 inch 35 DFU, 4 inch 216 DFU, 6 inch 720 DFU. No water closet on 2 inch drainage.
- At 1/8 inch per foot, multiply the horizontal DFU capacity by 0.8 (Table 703.2 note 5).
Why This Matters
CSLB's C-36 study guide says some questions require mathematical computation and that a calculator will be provided. You cannot bring your own; CSLB's exam FAQ lists calculators among prohibited items. So arithmetic is not the hard part. The hard part is setting the problem up: knowing that slope is inches per foot, that pressure changes 0.433 psi per foot of height, and which code table a fixture unit comes from. These calculations show up in Planning and Estimating (about 22 percent of the exam) and in the rough-in sections too.
Drain Slope and Total Fall
The Code Rule
2025 CPC 708.1 requires building drains and other horizontal drainage piping to run at a uniform slope of not less than 1/4 inch per foot (2 percent) toward the point of disposal. Where the depth of the street sewer, structural features or the building layout make 1/4 inch impractical, a building drain 4 inches or larger may slope not less than 1/8 inch per foot (1 percent), but only when first approved by the Authority Having Jurisdiction.
So the default for every size is 1/4 inch per foot. The 1/8 inch option is a limited exception for large building drains, not a rule for all pipe 3 inches and up.
The Formula
Total fall (inches) = run (feet) x slope (inches per foot)
- 1/4 inch per foot = 0.25
- 1/8 inch per foot = 0.125
To turn inches per foot into percent: 0.25 / 12 = 0.0208, about 2 percent, which is why the code states 1/4 inch per foot "or 2 percent."
Why Slope Matters
Too little slope and the flow is too slow to carry solids. Too much slope and the liquid can run ahead of the solids and leave them behind. Uniform slope at the code minimum or a little more is the goal.
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