Plain English:
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 degrees 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 framing
Exceptions exist for UL 1978 ducts, listed equipment, or listed wrap systems evaluated to ASTM E2336.
On Plans: Why it matters
Common redlines: wrong gauge, non-liquid-tight joints, missing test language before concealment, and clearances shown with no enclosure strategy.
Code Path: Where to show it
M-001: Type I grease duct basis note (material/ga, liquid-tight joints, 1500 degrees F gaskets at fan connection, 500 fpm, test prior to concealment, clearance/enclosure method).
M-101: Route duct with min. clearances.
Check: 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.
Review Risk: Don't
Don't penetrate grease duct walls with screws/bolts.
Don't combine Type I hoods across stories or rated assemblies.
Field Tip:
Add a "IMC 506 CHECK" for kitchen designs: DUCT TYPE, GAUGE, LIQUID-TIGHT JOINTS, 1500 degrees F FAN GASKET, 500 FPM, TEST PRIOR TO CONCEALMENT, CLEARANCE/ENCLOSURE.
Masterbuild QA Lens
Grease duct design starts with the hood classification (Type I or Type II) and the cooking appliance classification. The wrong combination generates a redline that affects the duct material, gauge, clearances, and the fire suppression system. Confirm the appliance-to-hood match is shown on M-501, not just described in the spec.
Drawing / Submittal Check
For grease duct basics (506.1-506.3.6): confirm the duct material and wall thickness are called out on the schedule and match the plan. Show all access panel locations on the plan - IMC 506 requires access at every change in direction and at maximum 20-foot intervals. Grease duct routing through concealed spaces without access panels shown will fail inspection and require demolition to provide access.
Common Review Risk
The most common 506.1-506.3.6
When To Escalate
Escalate when the grease duct gauge or welding specification cannot be verified during shop fabrication - when the contractor proposes to fabricate the duct on-site without a third-party inspection. Continuous-weld liquid-tight construction that is not verified during fabrication cannot be confirmed after installation. A third-party inspection of the fabricated duct before installation is the only reliable check.
Rated Assembly Coordination
Grease duct systems must be enclosed in a rated shaft or have an equivalent rated enclosure for the full duct length. The shaft rating, construction type, and access panel locations must all be shown on the drawings before framing begins.
Load Assumption Check
Grease duct sizing begins with the exhaust CFM from the listed hood. Confirm the design CFM is set by the hood manufacturer's listing or the code minimum - not rounded down for fan cost savings - before sizing the duct and exhaust fan.