Standpipe

A standpipe is a vertical drain receptor — typically a 2-inch pipe extending 18–42 inches above the trap — that provides an indirect (air-gap) connection between an appliance and the drain system. Washing machines are the most common application: the washer's flexible drain hose hooks over the standpipe opening and drains into it. This air gap prevents dirty drain water from being siphoned back into the machine.

Why standpipes matter

Without a standpipe, connecting a washing machine directly to a drain pipe creates a submerged connection that allows drain water (and sewer gas) to be pulled back into the machine by vacuum pressure when water drains rapidly. The standpipe's open top provides both an air gap (backflow prevention) and a vent for the fixture trap below.

Standpipe requirements (IPC)

• Minimum 2-inch diameter for washing machine standpipes
• Minimum height: 18 inches above the trap weir
• Maximum height: 42 inches above the trap weir (so the washer hose can reach without excessive length)
• The drain hose should slip loosely into the standpipe — not sealed — to maintain the air gap
• The standpipe must be vented either individually or via a wet vent

Other standpipe applications

In commercial buildings, fire standpipes are completely different systems — vertical water mains inside stairwells used by firefighters. In residential plumbing, standpipes also appear at utility sinks, mop sinks, and condensate drain lines from HVAC equipment. Condensate standpipes are typically 1¼–1½ inch and drain air handler condensate indirectly into a floor drain.

Common problems

Standpipe clogs are usually caused by lint from washing machine drains. Unlike a sink p-trap, the trap under a standpipe is typically under the floor and can be difficult to access. Hydrojetting is the most effective way to clear a standpipe and its downstream drain.