IMC 506 is where a "Type I hood" becomes a life safety duct system. If the grease duct story is vague, plan review and inspections get ugly fast.
Plain English: Plain-English Highlights
Type I grease ducts (506.1-506.3.6)
Corrosion protection where exposed.
Materials: 16 ga steel or 18 ga stainless, or listed factory built grease duct (UL 1978) per IMC 304.1.
Joints and penetrations: continuous liquid tight weld or braze (limited listed exceptions). No screws through duct wall.
Fan connections: flanged, gasketed, bolted. Use 1500°F rated gaskets and sealants where required.
Test prior to concealment (shaft, wrap, coatings): light test or water spray test.
Minimum velocity: 500 fpm.
Clearances when not enclosed: 18 in to combustibles, 3 in to noncombustible construction.
Drain + clean (506.3.7-506.3.9)
Slope to prevent pooling: 2% minimum. If horizontals exceed 75 ft: 8.3% slope.
Cleanouts: every 20 ft max, within 10 ft of turns over 45°, and within 3 ft on both sides of in line fans.
Doors: liquid tight, same thickness as duct, 1500°F gaskets.
Underground + enclosures (506.3.10-506.3.12)
Underground: test before burial, fully encase in 4 in min concrete, slope to reservoirs, label cleanout access.
Enclose Type I grease ducts through ceilings, walls, floors, or concealed spaces. Minimum 1 hour and not less than the penetrated assembly.
Enclosure options: rated shaft, listed wrap (ASTM E2336), or factory built enclosure (UL 2221).
Cleanouts in rated enclosures need rated access panels labeled "ACCESS PANEL. DO NOT OBSTRUCT."
Terminations (506.3.13)
Above roof: 40 in minimum above roof surface.
Separation distances to buildings, property lines, openings, and outside air intakes apply. Dimension it on plans.
Type II hoods (506.4)
Rigid metallic duct per Chapter 6. Seal where positive pressure or moisture or heat laden.
Terminations: follow hood manufacturer plus minimum clearance and height rules. Do not discharge onto walkways.
Exhaust equipment and PCUs (506.5)
Type I fan housing built like grease duct unless UL 705 listed. Motor outside airstream.
Upblast fans: hinged, flex cable, restraint, grease duct 18 in above roof, provide grease drainage.
PCUs: UL 8782 listed with required alarms, access, clearances, and wash down drains routed to grease interceptor.
Code Path: Where to show it
M-001: IMC 506 basis note (duct type, test, velocity, cleanouts, enclosure method, terminations, Type II, equipment).
M-101 and roof plan: slope arrows, cleanout tags, enclosure extents, and termination dimensions.
Field Tip: Field tip
Use a 3 step "KITCHEN EXHAUST QA" sweep:
1. Containment: gauge or listing, liquid tight, test note.
2. Maintenance: slope + cleanouts (20-10-3).
3. Discharge: terminations dimensioned.
Comment "IMC506" if you want a paste ready M-001 master note + QA checklist.
Masterbuild QA Lens
Exhaust systems need a source-to-discharge story. Identify what is being captured, how it is captured, how it is routed, where it terminates, and what interlocks or separations protect the building.
Drawing / Submittal Check
Verify source classification, hood or pickup point, duct material, route, cleanouts or access, fan selection, discharge location, make-up air, controls, and required coordination with fire protection or alarms.
Common Review Risk
The expensive miss is treating all exhaust the same. Grease, dryer, dust, hazardous, smoke control, battery, and specialty exhaust systems carry different proof requirements.
When To Escalate
Escalate when exhaust involves grease, hazardous materials, combustible dust, battery charging, smoke control, rated shafts, energy recovery, or any discharge that can re-enter the building.
Rated Assembly Coordination
When rated construction is involved, the drawings should identify the assembly, damper type, access location, actuator/control basis, fire alarm interface if applicable, and who coordinates the opening.
Kitchen Exhaust Coordination
For kitchen exhaust, tie the hood schedule, appliance lineup, grease duct route, cleanouts, fan discharge, fire suppression interface, and make-up air strategy into one reviewable story.