Sanitary Drainage, Vents, and Storm Drainage
What this domain tests
Drainage sizing and slope, cleanouts, backwater protection, venting methods, and storm drainage kept separate from sanitary flow.
Core rules
- Slope: ¼ inch per foot for 2½-inch and smaller pipe, ⅛ inch per foot for 3- to 6-inch, 1/16 inch per foot for 8-inch and larger.
- Drainage fixture units size branches, stacks, and the building drain; the building drain and sewer must not be reduced in the direction of flow and must be at least as large as the largest branch connected.
- Cleanouts at the base of each stack, at changes of 45 degrees or more, at the building sewer, and at intervals not exceeding 100 feet; cleanouts must stay accessible.
- Fixtures with flood-level rims below the next upstream manhole cover need backwater valves or approved equivalent protection.
- Vents: each trap protected by an individual, common, wet, or circuit vent sized from the vent table. Wet venting requires the water closet to be the last fixture downstream. Vent terminals extend at least 6 inches above the roof and stay clear of operable openings, doors, and air intakes at the adopted distances.
- Air admittance valves only where the code allows, and always accessible.
- Storm water goes to a separate system: roof drains, leaders, area drains, and footing drains must not connect to the sanitary sewer.
Worked example
A 30-foot run of 2-inch pipe at ¼ inch per foot needs 7.5 inches of fall. If the plan shows only 4 inches of fall from the fixture to the stack, 2-inch pipe will not work, and dropping to 1½ inch does not help because slope and capacity both tighten. The honest answers are more fall, a soffit, or a pump.
Traps that cost marks
Using ⅛ inch per foot on 2-inch pipe; loading a wet vent with the water closet upstream; burying a cleanout behind a finished wall with no access panel; tying a roof drain into the sanitary sewer.