Drainage, Waste & VentQuestion 102 of 360
Using the half-diameter rule, the minimum vent size for a 4 inch drain is:
a.1-1/2 inch
b.2 inch
c.3 inch
d.4 inch
Explanation
Half of a 4 inch drain is 2 inches, so the minimum vent is 2 inches (well above the 1-1/4 inch floor). A 1-1/2 inch vent is less than half the drain diameter and undersized. A 3 or 4 inch vent exceeds the minimum required, though developed length could increase the required size.
Law Reference: 2022 California Plumbing Code (CPC) §904.0Practice all 360 questions free — no signup required.
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