If grease can pool, it will. IMC 506 Part 2 is the "drain it and clean it" section for Type I hood grease ducts.
Plain English:
506.3.7 slope to prevent pooling
Route/install so grease cannot collect. Provide continuous slope toward the hood or a grease reservoir.
Minimum slope: 1/4 in per 12 in (2%).
If horizontal duct exceeds 75 ft: slope increases to 1 in per 12 in (8.3%).
Exception: Factory-built grease duct follows its listing/manufacturer instructions.
506.3.7.1 grease reservoirs (when used) must:
Bottom of horizontal duct or riser.
Full duct width, min 12 in long, min 1 in deep.
Bottom sloped to a drain.
Include a cleanout with direct access (typically on side/top).
Follow manufacturer instructions if it's a manufactured device.
506.3.8 cleanouts and openings:
No openings unless needed for operation/maintenance.
If sections can't be cleaned from hood or discharge: cleanouts max 20 ft apart and within 10 ft of turns >45 degrees.
Doors must be liquid-tight, steel (same thickness as duct), no fasteners penetrating duct wall, and gaskets/sealants rated 1500 degrees F.
506.3.8.1 personnel entry option:
If duct is large enough, provide at least one approved/listed 22 in x 20 in opening in horizontals and at top of risers.
With personnel entry, the standard cleanout spacing in 506.3.8 is not required (but supports must handle the extra load).
506.3.8.2 in-line fan cleanouts: Provide cleanouts on inlet and outlet sides within 3 ft of fan duct connections.
506.3.9 horizontal cleanout specifics:
Max 20 ft spacing and within 10 ft of turns >45 degrees.
Bottom cleanouts only if no other location works and must include internal damming to prevent pooling.
Minimum opening 12 in x 12 in (or 12 in on one side where constrained).
Place cleanouts at reservoirs and within 3 ft of horizontal discharge fans.
On Plans: Why it matters
Most grease duct failures are predictable: flat runs, missing cleanouts, or cleanouts placed where nobody can service them.
Code Path: Where to show it
M-101: Slope arrows + slope rate on every horizontal grease duct run.
M-601: Tag cleanouts (CO-1, CO-2) and note spacing intent.
Details: Cleanout door detail and access coordination when ducts are wrapped or in enclosures.
Check: Do
Draw the slope direction and rate.
Lay out cleanouts like a maintenance system, not an afterthought.
Review Risk: Don't
Don't leave long "flat" grease ducts.
Don't hide cleanouts behind hard ceilings/equipment with no access.
Field Tip:
Use a quick "20-10-3" check during layout: cleanouts every 20 ft, within 10 ft of any turn >45 degrees, and within 3 ft on both sides of any in-line fan. Tag them CO-# so the access plan is obvious.
Masterbuild QA Lens
Grease duct clearances to combustibles are 18 in minimum for listed systems - unless a reduced-clearance listed duct or listed duct wrap is used. Confirm the clearance dimension is shown on the duct section detail and that the listing basis for any reduced clearance is stated on M-001. Assumed clearances without dimensions will fail field inspection.
Drawing / Submittal Check
For 506.3.7-506.3.9 (clearances, enclosures, protection): the drawings must show the clearance to combustibles on a detail section, the enclosure construction (1-hour or 2-hour depending on building type), and the shaft liner if the duct penetrates rated assemblies. Confirm the duct slope to the grease reservoir is shown on the riser. A grease duct section through a rated assembly without an enclosure detail is an automatic comment.
Common Review Risk
Clearance to combustibles (506.3.7) is the inspection failure that creates the most demolition work. When a grease duct is installed too close to wood framing, ceiling joists, or roof structure, the correction requires pulling the ceiling and installing a fire-rated enclosure. Confirm the duct path and clearances on the architectural drawings before the mechanical set is issued.
When To Escalate
Escalate when the required clearance to combustibles cannot be achieved with any routing through an existing building, and for any grease duct enclosure that must penetrate a rated assembly in a way that is not covered by an existing UL design number. These situations require either a code modification, an AHJ-approved alternative, or a redesign of the kitchen layout.
Rated Assembly Coordination
Sections 506.3.7-506.3.9 govern grease duct enclosure construction, clearances to combustibles, and enclosure access. Show shaft wall assembly type, UL listing, and each access panel location on the shaft detail drawing.
Load Assumption Check
Sections 506.3.7-506.3.9 affect grease duct routing, which changes duct length and static pressure. Confirm the fan selection accounts for the actual equivalent length of the grease duct system, including all elbows and transitions.