Washer standpipe rough-in — 18 to 42 inches above the trap weir with a 2-inch vented P-trap
Set the top of the washer standpipe 18–42″ above the trap weir (IRC P2706.1.2), with a 2″ vented P-trap (Table P3201.7). The discharge hose sits loose in the open top as an air gap. An S-trap is banned here — it self-siphons its seal on the washer’s fast discharge and lets sewer gas in.
What this diagram shows
A wall-section drawing of a clothes-washer drain rough-in. A vertical standpipe rises from a P-trap; a dimension line shows that the top of the standpipe must sit 18 to 42 inches above the trap weir, per IRC P2706.1.2. The standpipe and trap are labeled 2 inches minimum, per IRC Table P3201.7. The P-trap connects through a trap arm to a drain-and-vent stack that runs up to the vent and down to the drain, and the trap holds a water seal. The washer discharge hose sits loosely in the open top of the standpipe, forming an air gap so a backed-up drain cannot siphon the machine. A smaller inset on the right shows the wrong detail: an S-trap that drops straight into the floor with no vent, marked with a red X because it self-siphons its water seal on the washer discharge and lets sewer gas into the room.
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