/ Field knowledge · IMC 506.3
Wrap it after the light test — not before.
Grease duct is not “whatever steel is on the truck.” IMC 506.3: 16 gage steel or 18 gage stainless (or listed UL 1978 factory-built), continuous liquid-tight welds, 500 fpm minimum, 18 in to combustibles when no enclosure is required, and a leakage test — light test and water spray — before shaft, wrap, or coatings hide the joints. Decide the path early: welded 16 ga, UL 1978, or duct plus listed wrap evaluated to ASTM E2336.
Educational 2024 International Mechanical Code (IMC) 506.1–506.3.6 from Daily Code Talk. Confirm the adopted edition. Distinct from 506.3.13 termination, 507.2 hood construction, and no damper in grease shafts. This is not a grease-duct design.
- 506.1: grease ducts and exhaust equipment match the hood and appliances served. 506.2: outdoors or corrosive environments need approved corrosion protection.
- 506.3.1: steel minimum 0.0575 in (16 ga) or stainless 0.0450 in (18 ga). Exception: listed factory-built grease duct (UL 1978) installed per IMC 304.1.
- 506.3.2: continuous liquid-tight weld or braze (limited listed-device / listed-duct exceptions). Joints: butt, welded flange, or telescoping/bell overlap — no ledges that trap grease. Hood-to-duct liquid-tight and smooth. Duct-to-fan: flanged, gasketed, bolted; gaskets/sealants rated for continuous 1500°F. Vibration isolation only at the fan connection, approved/listed noncombustible method.
- 506.3.2.5: test before concealment. Prior to shaft, wrap, or coatings that block full inspection, pass a leakage test (light test and water spray). Put “TEST PRIOR TO CONCEALMENT” on the drawings so the inspection is not missed.
- 506.3.3: noncombustible supports for gravity plus seismic. No fasteners penetrating duct walls. 506.3.4: minimum 500 fpm (short transitions up to 3 ft can be exempt if shaped to avoid grease traps).
- 506.3.5: separate grease duct per Type I hood unless same story, same or adjoining rooms, no rated penetrations, and no solid-fuel appliances.
- 506.3.6 when no enclosure is required: 18 in to combustible construction; 3 in to noncombustible construction and gypsum on noncombustible framing. Exceptions for UL 1978, listed equipment, or listed wrap evaluated to ASTM E2336.
Wrapped before the light test
Or 18 ga carbon steel. Or a slip-joint that traps grease. Or 18 in clearance drawn with no enclosure strategy. Those are first-cycle comments — and failed concealment inspections.
Source: Daily Code Talk — IMC 506.3 · tool: Grease-duct path · slope: 506.3.7 · enclosure: 506.3.11 · termination: 506.3.13 · Type I: 507.2.
What to send
The grease-duct material/gage (or UL 1978 listing), whether wrap or a shaft is proposed, and whether a test-before-concealment note is on the set.
If the duct is wrapped and nobody called a light test
Send the route and the spec. Masterbuild will say whether 506.3.1 still has the wrong gage — or 506.3.2.5 still has no test-before-concealment path.
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. 16 ga / 18 ga, 500 fpm, 18 in / 3 in, 1500°F gaskets, UL 1978, ASTM E2336, and test-before-concealment are 2024 IMC 506.3 education from Daily Code Talk — not a grease-duct design. This page is not legal advice.