Trade Exam · Module 3
DWV Rough-In
Rough in drain, waste and vent piping the way the 2025 California Plumbing Code actually reads: minimum trap and trap arm sizes and fixture unit values from Table 702.1, pipe sizing from Table 703.2, trap arm length limits from Table 1002.2, the fittings allowed for each change of direction, support spacing from Table 313.3, and the nail plate, sleeve and fire penetration rules.
Key Takeaways
- Table 702.1 minimum trap and trap arm sizes: lavatory 1-1/4 in, bathtub 1-1/2 in, domestic kitchen sink 1-1/2 in (with a 2 in minimum drain), shower 2 in, clothes washer standpipe 2 in, water closet 3 in.
- Private dwelling DFUs: lavatory 1, bathtub 2, shower 2, kitchen sink 2, clothes washer 3, 1.6 gpf water closet 3.
- Table 703.2 horizontal limits: 1-1/2 in = 1 DFU, 2 in = 8, 3 in = 35, 4 in = 216. No water closet on 2 in pipe.
- Trap arm maximums (Table 1002.2): 1-1/4 in 30 in, 1-1/2 in 42 in, 2 in 60 in, 3 in 72 in, 4 in 120 in. Water closet to vent: 6 ft max.
- Horizontal to vertical may use a sanitary tee; horizontal to horizontal and vertical to horizontal need a wye, a combination wye and 1/8 bend, or equivalent sweep (706.2 to 706.4).
- Schedule 40 ABS and PVC DWV: support every 4 ft horizontally and at each horizontal branch connection; vertically at the base and each floor with mid-story guides (Table 313.3).
- Plastic or copper pipe within 1 in of the face of framing needs an 18 gauge steel nail plate extending 1-1/2 in past the pipe (312.9).
Why This Matters
Rough-in is the stage where the drain, waste and vent (DWV) piping goes in before walls and floors are closed. It sits inside Underground and Rough Systems, which the CSLB C-36 study guide weights at 22 percent of the trade exam and describes as piping layout and installation plus system testing.
The questions are code questions, so they test specific numbers. The good news is that almost every number in this lesson comes from four places in the 2025 California Plumbing Code (CPC): Table 702.1, Table 703.2, Table 1002.2 and Table 313.3. Learn where those numbers live and you will recognize them on test day.
Traps and Trap Sizes
Every fixture must be separately trapped with an approved liquid seal trap, unless it has an integral trap, and only one trap is allowed on a trap arm (1001.2). No fixture may be double trapped, and S traps, bell traps and crown vented traps are prohibited (1004.1). The trap seal must be at least 2 inches and not more than 4 inches deep unless the AHJ requires a deeper seal (1005.1).
The trap is sized from Table 702.1, and 1003.3 says the trap may not be smaller than the table size or more than one pipe size larger, and it must be the same size as the trap arm it connects to. Minimum trap and trap arm sizes from Table 702.1 that come up often:
- Lavatory: 1-1/4 inch
- Bathtub or combination bath and shower: 1-1/2 inch
- Domestic kitchen sink (with or without disposer or dishwasher): 1-1/2 inch trap, and a note to the table requires a 2 inch minimum drain
- Laundry sink: 1-1/2 inch, also with the 2 inch minimum drain note
- Shower, single head: 2 inch (a footnote allows 1-1/2 inch for a bathtub to shower retrofit where the shower is no larger than 36 by 60 inches)
- Clothes washer standpipe: 2 inch
- Floor drain: 2 inch
- Water closet: 3 inch
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