Daily Code Talk #58: IMC 506 Part 1 (Grease Duct Basics 506.1–506.3.6)

🧠 Plain-English Highlights• 506.1 general: Grease ducts and exhaust equipment must match the hood and appliances served.• 506.2 corrosion: Outdoors or corrosive environments require approved corrosion protection.• 506.3.1 materials:– Steel grease duct: min 0.0575 in (16 ga)– Stainless: min 0.0450 in (18 ga)Exception: Listed factory-built grease duct (UL 1978) installed per IMC 304.1.• 506.3.2 joints and penetrations: Continuous liquid-tight weld or braze (with limited listed-device / listed-duct exceptions). Goal: no grease leakage.• 506.3.2.1 joint types: Butt, welded flange, or telescoping/bell overlap. Overlaps must avoid ledges that trap grease and must not create big step changes.• 506.3.2.2 hood-to-duct: Liquid-tight and smooth, no grease traps. Limited alternative flange + high-temp gasket option, or listed collar connection.• 506.3.2.3 duct-to-fan: Flanged, gasketed, bolted. Gaskets/sealants rated for continuous 1500°F.• 506.3.2.4 vibration isolation: Only at the fan connection and must be an approved/ listed noncombustible method.• 506.3.2.5 test before concealment: Prior to shaft/wrap/coatings that block full inspection, pass a leakage test (light test & water spray test).• 506.3.3 supports: Noncombustible supports for gravity + seismic loads. No fasteners penetrating duct walls.• 506.3.4 velocity: Minimum 500 fpm in grease ducts (short transitions up to 3 ft can be exempt if shaped to avoid grease traps).• 506.3.5 separation: Separate grease duct per Type I hood unless same story, same/adjoining rooms, no rated penetrations, and no solid-fuel appliances.• 506.3.6 clearances (when no enclosure is required):– 18 in to combustible construction– 3 in to noncombustible construction and gypsum on noncombustible framingExceptions exist for UL 1978 ducts, listed equipment, or listed wrap systems evaluated to ASTM E2336.🏗️ Why it mattersCommon redlines: wrong gauge, non-liquid-tight joints, missing test language before concealment, and clearances shown with no enclosure strategy.🗺️ Where to show it• M-001: Type I grease duct basis note (material/ga, liquid-tight joints, 1500°F gaskets at fan connection, 500 fpm, test prior to concealment, clearance/enclosure method).• M-101: Route duct with min. clearances.✅ Do• Decide early: welded 16 ga, UL 1978 listed duct, or duct + listed wrap (ASTM E2336).• Add a “TEST PRIOR TO CONCEALMENT” callout so inspections don’t get missed.⛔ Don’t• Don’t penetrate grease duct walls with screws/bolts.• Don’t combine Type I hoods across stories or rated assemblies.🔧 Field tipAdd a “IMC 506 CHECK” for kitchen designs: DUCT TYPE, GAUGE, LIQUID-TIGHT JOINTS, 1500°F FAN GASKET, 500 FPM, TEST PRIOR TO CONCEALMENT, CLEARANCE/ENCLOSURE.Comment “IMC506” if you want a paste-ready M-001 note.