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Home / Code Guides / Laundry Standpipe & Washer Box

Washing Machine Standpipe Requirements — Height, Size & Trap

A washing machine pumps out fast and hard, and a standpipe that is too short, too small, or badly trapped will put that water on the floor. The IRC pins down the height range, the trap, and the size.

RequirementModel Code StandardSection
Standpipe heightThe standpipe extends not less than 18 in. and not more than 42 in. above the trap weir — tall enough that a discharging washer does not overflow, short enough that the trap still refillsIRC P2706.1.2
Trap heightThe standpipe trap is installed not less than 6 in. and not more than 18 in. above the floorIRC P2706.1.2
SizeA clothes washer standpipe is a 2 in. trap and drain — 1-1/2 in. cannot keep up with a modern high-speed pump-outIRC P2706.1.2, Table P3201.7
VentingThe standpipe trap is vented like any other trap, within the trap-arm limit for a 2 in. arm: 8 ft of developed length from the trap weir to the vent fittingIRC P3101.2, Table P3105.1
Air gap at the hoseThe washer discharge hose drops loosely into the standpipe and is never sealed, taped, or glued into it — that gap is what keeps the machine from being siphoned or backed up intoIRC P2706.1
AccessThe standpipe and its trap have to stay accessible, which is what the recessed washer box is for — supply valves and drain in one serviceable openingIRC P2706.1.2, P3201.6
Supply connectionHot and cold supplies terminate at the box with shutoff valves, and the hose connections are protected from backflow by the air gap at the machineIRC P2903.9.3

Worth knowing

  • A standpipe running above 42 in. is the most common real-world violation, usually created after the fact by adding a pedestal or raising the box to clear a countertop.
  • The height limits are measured from the trap weir, not from the floor. A trap mounted high in the wall plus a tall standpipe fails twice over.
  • A washer on any floor above grade belongs in a drain pan piped to an indirect waste. The IRC does not require it for laundry, but many jurisdictions and most insurers do.
  • When the standpipe backs up, the culprit is usually lint buildup or an undersized 1-1/2 in. legacy drain — not the machine’s pump.
  • A laundry branch tied in downstream of a kitchen disposal branch will smell. Keep the laundry on its own path to the stack where the framing allows it.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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