What air gap means
An air gap is an open vertical space between a water outlet, or a waste pipe, and the flood rim of the fixture or drain it empties into. Nothing can be pulled back across open air.
Where it matters on a plumbing job
Dishwasher drains, water softener discharge and RO units all need an air gap or another approved method. Inspectors check them on remodels, so build them into the install.
What the model code says
- The code defines a drainage air gap as the open vertical distance between a waste pipe's outlet and the flood-level rim of the fixture it discharges into. (IRC 2021, Chapter 2)
Check which code edition your building department enforces for air gap before you quote.
What it costs
The dishwasher installation cost guide has the typical job at $990 and the usual range at $390 to $1,500. The guide shows its sources and what moves the price.
Work it out
- Cost guide: Dishwasher installation cost
- Licensing: Plumbing license rules by state
- Tools, templates and guides: Free plumbing resources
Sources: 2021 International Residential Code, Chapter 2, Definitions (ICC). Checked October 9, 2026.