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.
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.
Common questions
What does the code say about high loop for Dishwasher Drain Protection?
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)
What does the code say about air gap fitting for Dishwasher Drain Protection?
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)
What does the code say about connection point for Dishwasher Drain Protection?
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)
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.