Sanitary Drainage, Storm Drainage, and Sewer Systems
What the exam tests
This domain covers building drains, building sewers, sanitary drainage, storm drainage, sumps, cleanouts, and backwater protection. The exam tests slope, sizing, and separation of storm water from sanitary flow.
Core rules
- Gravity drainage must slope continuously. Common adopted slopes are 1/4 in/ft for small pipe and 1/8 in/ft for larger pipe, with 1/16 in/ft for very large pipe; verify the table in the adopted code.
- Cleanouts are required at the sewer, at changes of direction, and at maximum intervals; keep them accessible.
- Backwater valves are required where fixture flood levels are below the next upstream manhole, as the code specifies.
- Storm drainage must be separate from sanitary drainage unless the code allows combined flow. Roof drains, leaders, and area drains need traps or approved connections.
- Test the sewer before backfilling and bed the pipe properly.
Worked example
A 4-in building sewer runs 80 ft at 1/8 in/ft. Fall = 80 x 0.125 = 10 in. If the upstream invert is 30 in below grade, the downstream invert is 20 in below grade. Confirm that the downstream connection is deep enough and that the slope is within the adopted table; a 1/4-in/ft slope would drop 20 in, which may be too deep for the connection.
Traps that cost marks
Using 1/4 in/ft on large pipe without checking the table; connecting a roof drain to the sanitary sewer; burying a cleanout; omitting a backwater valve; failing to test before backfill.