The dishwasher air gap is one of the more confusing pieces of plumbing code in US kitchens because the requirement varies dramatically by jurisdiction and the alternative (the high-loop method) is functionally equivalent in most real-world conditions. Buyers and DIY installers who do not check local code often install the high-loop everywhere, which is fine in 38 states and a code violation in the 12 that mandate the air gap.
This guide walks through what the air gap does, which jurisdictions require it as of 2026, what the high-loop alternative is, and how to verify your local code before starting an install.
What the air gap actually does
The dishwasher drain hose connects somewhere downstream of the dishwasher to either the garbage disposal inlet or a dishwasher tailpiece on the sink drain. Both of these connection points are at or below sink-drain level, which means if the sink drain backs up (clog, slow drain, septic backup), wastewater can flow back through the drain hose into the dishwasher.
This is a hygiene concern. Drain water carries food particles, grease, soap residue, and bacteria. Having it backflow into the dishwasher where it sits in contact with clean dishes is undesirable.
Two mechanisms prevent this backflow:
The air gap, a physical break in the drain line where water cannot siphon backward through a column of air. The water has to flow uphill through one inlet, into a cylinder open to the atmosphere, and back down through a second outlet. Atmospheric pressure prevents the uphill flow.
The high-loop, where the drain hose is routed up to a high point (typically within 1 inch of the underside of the countertop) before descending. The high point acts as a partial air gap. If the drain backs up below the loop, water has to climb the loop to enter the dishwasher, which it generally cannot do under normal backflow conditions.
The air gap is the more reliable backflow prevention. The high-loop is functionally adequate for most household conditions but fails in edge cases (severe drain backup combined with rapid plumbing pressure changes).
Why some states require the air gap and others do not
The split traces back to the Uniform Plumbing Code (UPC) vs. the International Plumbing Code (IPC).
The UPC, used in California, Washington, Hawaii, Minnesota, Oregon, and several other western and northern states, requires an air gap for dishwasher drainage.
The IPC, used in most southern, midwestern, and eastern states, allows either an air gap or a high-loop.
Some states adopt one code base and modify it locally. Texas is officially IPC but Austin and several other municipalities have adopted air gap requirements. New York follows IPC but New York City has its own plumbing code that requires an air gap in commercial settings and recommends it in residential.
The result is a patchwork where you cannot assume the state code applies in your municipality without checking.
States that require an air gap as of 2026
The following states have a statewide or near-statewide air gap mandate as of 2026:
- California (California Plumbing Code, updated 2022)
- Washington (Uniform Plumbing Code adoption)
- Oregon (most municipalities, statewide UPC adoption)
- Minnesota (Minnesota Plumbing Code)
- Hawaii (Uniform Plumbing Code adoption)
The following states have municipalities with explicit air gap requirements without a state-level mandate:
- Texas (Austin, Houston in some districts, several other cities)
- Colorado (Denver, Boulder, several Front Range municipalities)
- Utah (Salt Lake City and surrounding areas)
- Arizona (Tucson, Flagstaff, several mountain communities)
- Nevada (Las Vegas, Reno in some districts)
- Wisconsin (Milwaukee, Madison)
The following states explicitly allow either method statewide:
- Florida
- Georgia
- North Carolina
- South Carolina
- Tennessee
- Virginia
- Pennsylvania (with municipal variation in Philadelphia)
- Most of the Northeast and Midwest
This is a snapshot in time. Plumbing codes are updated on 3 to 6 year cycles and individual jurisdictions adopt updates on their own timetables. Always verify with the local building department before relying on this list.
How to verify local code before installing
The fastest verification path is:
Call the local building department in your municipality and ask “Does my city require a dishwasher air gap for a kitchen dishwasher install?” Most building departments answer this question in a sentence over the phone.
Look up the local plumbing code online. Most municipalities publish current code adoption on the building department website. Search for “[city name] plumbing code” or “[city name] dishwasher air gap requirement.”
Ask the plumber or appliance installer who is doing the work. Local pros know which municipalities require what and will install to local code by default.
Check your homeowners insurance or HOA documents. Some HOAs require code compliance for plumbing modifications and will inspect on resale.
If you cannot verify and you are doing the install yourself, the conservative choice is to install the air gap. It does not violate any code that allows the high-loop alternative, and it satisfies any code that mandates it.
What the air gap install actually looks like
An air gap is a small chrome cylinder (about 1.5 inches diameter, 1.5 to 2 inches tall above the counter) with a flange that sits on the countertop. Below the counter, it has two hose inlets:
- Inlet 1, smaller diameter, accepts the dishwasher drain hose from below
- Outlet 2, larger diameter, sends drain water to the disposal or sink tailpiece
The drain water flows up the dishwasher hose, into the air gap chamber where it falls freely through 1/2 to 1 inch of air, and out through the disposal hose under gravity. The physical air gap inside the cylinder is the backflow prevention.
The countertop hole for the air gap is typically 1-3/8 inches. Many countertops already have this hole knockout drilled (intended for an air gap or for a soap dispenser). If your countertop does not have a hole and is laminate, plastic, or butcher block, you can drill it yourself with a hole saw. Granite, quartz, and solid surface countertops require a diamond hole saw and are best handled by the fabricator.
When the air gap matters in real life
The air gap protects against drain backflow contaminating the dishwasher. The scenarios where this actually happens:
A garbage disposal clog backs up the sink and drain water rises in the disposal inlet. Without an air gap or high-loop, this water can flow back through the dishwasher drain hose. With an air gap, it cannot.
A septic system backs up during heavy rain or pump failure, pressurizing the entire drain system. Same scenario.
A sewer main backup pushes wastewater up through household drains. Rare but happens, particularly in older municipalities with combined storm-sewer systems.
In all three scenarios, a high-loop prevents most backflow under normal pressure but can fail under significant pressure or rapid water surge. The air gap is mechanically immune to backflow regardless of pressure.
For a single-family home on municipal sewer in a dry climate, the practical risk of dishwasher contamination from drain backflow is very low and the high-loop is functionally adequate. For a home on septic, in a flood-prone area, or in a basement-level kitchen where drainage is gravity-marginal, the air gap is genuinely safer.
Bottom line
Verify your local code before installing. In 12-plus states the air gap is required. In the rest the high-loop is acceptable. The cost difference is $15 for an air gap fitting plus 30 minutes of install time, which is not a significant burden for the additional safety.
If you are replacing a dishwasher in a jurisdiction that requires the air gap and the previous install used a high-loop (this happens in older homes), the replacement is the moment to bring it up to current code.
See our dishwasher installation costs guide, the dishwasher installation when replacing an old one walkthrough, and the methodology page for our full appliance framework.
Frequently asked questions
What is a dishwasher air gap and why does it exist?+
An air gap is a small chrome cylinder mounted on the countertop next to the faucet, with two hose connections underneath. The dishwasher drain hose connects to one inlet, and a second hose runs from the air gap outlet to the disposal or sink drain. The physical gap of air inside the cylinder prevents drain water from siphoning backward into the dishwasher, which would contaminate the clean dishes. It is a backflow prevention device required by some plumbing codes.
What is the high-loop alternative and when is it acceptable?+
The high-loop method routes the drain hose up to within 1 inch of the underside of the countertop before descending to the disposal or drain. The high point creates a similar siphon-break to an air gap. The high-loop is accepted in most US states and is the default install method outside states with explicit air gap mandates. Most modern dishwashers include hose clips to secure the loop in place.
Which states explicitly require an air gap in 2026?+
California (per the California Plumbing Code, updated 2022), Washington, Oregon (in most municipalities), Minnesota, Hawaii, and parts of Texas (Austin and several other municipalities have explicit local code). Several other states have municipalities that require it without a state-level mandate. Always verify with the local building department before relying on a generic state-level answer.
Will I fail an inspection if I use a high-loop in an air gap state?+
Yes, in most cases. A code inspector in California or Washington will look for the air gap on the countertop as part of a kitchen inspection. If it is missing, the install will be flagged for correction. For new construction and permitted remodels this is unavoidable. For an unpermitted DIY replacement of an existing dishwasher, no one will see it unless you sell the house and a home inspector flags it.
Can I install an air gap myself or do I need a plumber?+
You can install it yourself if the countertop already has the hole drilled for it (most countertops have a 1-3/8 inch knockout for an air gap or for an unused soap dispenser hole). Drilling a new hole in granite or quartz requires a diamond hole saw and is best left to a fabricator. Plastic, laminate, and butcher block can be drilled with a standard hole saw.
