/ Field knowledge · IMC 502.1.2
Gasoline vapor does not wait at the canopy.
Gasoline vapor is heavier than air. It sits on the slab. IMC 502.1.2: in fuel-dispensing areas, the bottom of exhaust or inlet openings must be ≤ 18 inches above the floor to capture heavy flammable vapors. A high ceiling grille in a canopy or a louver at 8 ft is the miss 502.1.2 is written to catch. Do not treat this as ordinary environmental exhaust — high concentrations of flammable vapor are a 509 hazardous-exhaust question.
Educational 2024 International Mechanical Code (IMC) 502.1–502.1.4 from Daily Code Talk. Confirm the adopted edition. Distinct from 304.5 hydrogen (lighter than air, ceiling) and 404 garage. This is not a fuel-station design.
- 502.1: exhaust anywhere equipment or processes throw off dust, fumes, spray, gas, or smoke in amounts that can irritate or affect health/safety — even if the room name looks “normal.”
- 502.1.1: put inlets where contaminants are heaviest, not where the duct route is convenient.
- 502.1.2 fuel-dispensing: bottom of exhaust or inlet openings ≤ 18 in above the floor.
- 502.1.3: equipment/service rooms built and exhausted so contaminants do not spread to occupied spaces.
- 502.1.4: high concentrations of dust or hazardous vapors → IMC 509 hazardous exhaust, not a toilet fan.
- 502.2 aircraft fueling: compartments with piping, pumps, separators, hose reels ventilated at floor level or within the floor.
On the set: M-101 tag “LOW EXH INLET ≤ 18 IN AFF” at fuel-dispensing. M-601: exhaust type (environmental / hazardous / fuel-vapor) and code reference. Hydrogen and CNG repair invert the pickup (low inlets, high outlets) — different fuel, different density, different 502 row.
A canopy exhaust at the roof deck over the dispensers
Heavy vapor never reaches it. 18 in AFF is the educational check. Pair with 509 if concentrations are hazardous, and with 304.5 if the fuel is hydrogen.
Source: Daily Code Talk — IMC 502.1–502.2 · tool: 18-inch AFF screen · hydrogen: 304.5 · 509: hazardous exhaust · garage: 404.
What to send
Whether this is gasoline / diesel / CNG / hydrogen, a section showing inlet height above finished floor, and whether the exhaust is dedicated to outdoors.
If the only inlet over the dispensers is a high louver
Send the section. Masterbuild will say whether 502.1.2 still has no opening within 18 in of the floor — or 502.1.4 still treats fuel vapor as environmental exhaust.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. 18 in above the floor for fuel-dispensing openings is 2024 IMC 502.1.2 education from Daily Code Talk — not a fuel-station design. Hydrogen is a different 304.5 path. This page is not legal advice.