/ Field knowledge · IMC 506.3.7–506.3.9
If grease can pool, it will.
If grease can pool, it will. IMC 506.3.7–506.3.9 is drain-it-and-clean-it. Continuous slope toward the hood or a grease reservoir: minimum 1/4 in per 12 in (2%). Horizontal runs over 75 ft: 1 in per 12 in (8.3%). Cleanouts where sections cannot be cleaned from the hood or discharge — maximum 20 ft apart and within 10 ft of turns greater than 45 degrees. Most grease-duct failures are flat runs, missing cleanouts, or cleanouts nobody can reach.
Educational 2024 International Mechanical Code (IMC) 506.3.7–506.3.9 from Daily Code Talk. Confirm the adopted edition. Distinct from 506.3 materials / test-before-concealment and 506.3.11 enclosure. This is not a grease-duct design. Factory-built grease duct follows its listing.
- 506.3.7: route so grease cannot collect. Continuous slope toward the hood or a reservoir. 1/4 in per 12 in (2%) minimum; 1 in per 12 in (8.3%) if the horizontal duct exceeds 75 ft.
- 506.3.7.1 reservoirs (when used): bottom of horizontal duct or riser; full duct width; minimum 12 in long and 1 in deep; bottom sloped to a drain; cleanout with direct access (typically side or top).
- 506.3.8: no openings except for operation/maintenance. Cleanouts liquid-tight, steel same thickness as the duct, no fasteners penetrating the duct wall, gaskets/sealants rated 1500°F.
- 506.3.8.1 personnel entry: if the duct is large enough, at least one approved/listed 22 in × 20 in opening in horizontals and at the top of risers. With personnel entry, 506.3.8 spacing is not required (supports must handle the extra load).
- 506.3.8.2 in-line fans: cleanouts on inlet and outlet sides within 3 ft of fan duct connections.
- 506.3.9 horizontal: max 20 ft spacing and within 10 ft of turns >45°. Bottom cleanouts only if no other location works, with internal damming to prevent pooling. Minimum opening 12 in × 12 in (or 12 in on one side where constrained). Place cleanouts at reservoirs and within 3 ft of horizontal discharge fans.
On the set: slope arrows and slope rate on every horizontal grease-duct run. Cleanout locations dimensioned to turns and fans. Do not put a cleanout where the ceiling or a shaft makes it unreachable.
A 90-foot flat run with one cleanout at the hood
Grease pools. The inspector cannot clean past the first elbow. The wrap goes on anyway. That is a first-cycle comment and a fire-risk residue problem.
Source: Daily Code Talk — IMC 506.3.7–506.3.9 · tool: Slope / cleanout screen · construction: 506.3 · enclosure: 506.3.11.
What to send
The grease-duct plan with run lengths, turns greater than 45°, fan locations, and any cleanouts already shown.
If the grease duct is drawn flat
Send the plan. Masterbuild will say whether 506.3.7 still has no 2% slope — or 506.3.9 still has a 20 ft / 10 ft cleanout miss.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. 2% / 8.3% slope, 20 ft / 10 ft cleanouts, 12×12 and 22×20 openings, and 1500°F gaskets are 2024 IMC 506.3 education from Daily Code Talk — not a grease-duct design. This page is not legal advice.