Home / Code Guides / Dishwasher Drain Protection

Dishwasher Drain Requirements — Air Gap vs. High Loop

The entire point of the dishwasher drain rule is keeping dirty sink water from siphoning back into a machine full of clean dishes. There are two accepted ways to do it, and which one you can use depends on where you are building.

Requirement	Model Code Standard	Section
High loop	The dishwasher waste line must rise to the underside of the counter and be securely fastened there before dropping to its connection — the loop is the backflow break	IRC P2717.2, P2717.3
Air gap fitting	A listed air gap device through the countertop or sink deck is the alternative: the drain line enters the fitting, breaks to atmosphere, and a second hose carries the water down to the disposer or tailpiece	IRC P2717.2; local amendment
Connection point	The waste line ties into a wye branch on the sink tailpiece or into the dishwasher inlet on a food waste disposer — always upstream of the trap, never into the trap arm past the trap	IRC P2717.2, P2717.3
Shared trap	A sink and dishwasher, with or without a food grinder, are permitted to discharge through a single 1-1/2 in. trap	IRC P2717.2, P2717.3
Disposer knockout	Where the line lands on a new disposer, the factory knockout plug in the dishwasher inlet has to be driven out — leaving it in is the single most common reason a new dishwasher will not drain	Manufacturer instructions per IRC R102.4
Water supply protection	The dishwasher’s water connection is separately protected against backflow into the potable supply, normally by the appliance’s integral air gap or a listed device	IRC P2717.1

Worth knowing

Whether a high loop alone is acceptable is a local question. California requires a countertop air gap by state amendment, and a number of other jurisdictions and individual inspectors do the same — confirm before the counters are set, because a retrofit air gap means a new hole in stone.
A high loop only works if it is actually fastened at the top. A hose zip-tied loosely and sagging to the cabinet floor provides no protection and will not pass.
Air gaps that sputter onto the counter are telling you the outlet hose to the disposer is kinked or the disposer inlet is partly blocked — it is a clog signal, not a defective fitting.
Some dishwasher manufacturers require an air gap in their installation instructions regardless of local code, and those instructions are enforceable under the IRC.
Never route the dishwasher drain into a standpipe or into the trap arm downstream of the sink trap. It may drain beautifully and still be a violation.

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.

Deck Footings →Deck Ledger →GFCI Protection →Egress Windows →Stairs →Handrails & Guards →