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:
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 degrees 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 degrees, and within 3 ft on both sides of in line fans.
Doors: liquid tight, same thickness as duct, 1500 degrees 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.
Check: Grease Exhaust Summary Completeness
M-001 or a dedicated exhaust sheet must list each kitchen hood: Type (I or II), appliances served, exhaust rate, makeup air rate, duct material, fan type, and fire suppression system reference.
Confirm duct clearances to combustibles are shown on details, access panels are located and dimensioned, and shaft penetration protection is coordinated with structural and architectural drawings.
Cross-reference the fire suppression shop drawing to confirm nozzle coverage matches the final appliance layout before commissioning.
Review Risk:
Grease exhaust inspection failures after the ceiling is closed are the most expensive commercial kitchen corrections - clearance violations and missing access panels require demolition to correct.
Hood-to-appliance coordination failures after construction - hood sized for original appliances when appliances changed - require field modification of the fire suppression system.
Missing access panel locations on the reflected ceiling plan is a pre-occupancy hold that cannot be resolved without field-cutting the ceiling.
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.
Masterbuild QA Lens
For the Section 506 grease exhaust summary, four items must be in the permit set before the kitchen review proceeds: (1) duct material, gauge, and clearances are on M-501; (2) access panel locations are dimensioned on the isometric; (3) the fire suppression system coordination note references the specific suppression listing for the hood; and (4) the grease collection point location and drainage path are shown. Missing any one of these four is a first-cycle comment.
Drawing / Submittal Check
The grease exhaust summary should be on M-001 or a dedicated exhaust sheet: list each kitchen hood, its Type (I or II), the appliances served, the exhaust rate, the makeup air rate, the duct material, the fan type, and the fire suppression system. The fire suppression system must also be cross-referenced on the fire protection drawings.
Common Review Risk
Grease exhaust inspection failures discovered after the ceiling is closed are the most expensive corrections in commercial kitchen construction. Clearance violations, missing access panels, and unprotected shaft penetrations all require demolition to correct. The only way to prevent them is a grease exhaust-specific field review before any ceiling is closed. Confirm this review is a milestone in the construction schedule.
When To Escalate
Escalate when the commercial kitchen project is in an existing building with an existing grease exhaust system that cannot be upgraded to current code without major structural work. Existing-building grease exhaust upgrades are a common source of hidden scope - the AHJ may require full compliance when any alteration is made to the existing system.
Rated Assembly Coordination
Commercial kitchen grease duct coordination involves rated shaft construction, clearance-to-combustibles, access panels at every floor, and suppression system tie-in. All of these need to be resolved on the drawings - not in the field.
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.