Sheet Metal (C-43) · Lesson 4 of 8

Grease Duct: Gauge, Slope, Access, and Clearance (CMC 510 and 507.4)

"Grease Duct: Gauge, Slope, Access, and Clearance (CMC 510 and 507.4)" is lesson 4 of 8 in the Sheet Metal (C-43) Learning Hub. It opens with "A Grease Duct Stands Alone" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. A Grease Duct Stands Alone
  2. Metal and Welds
  3. Slope, Drains, and No Traps
  4. Access Openings and Supports
  5. Clearance to Combustibles and Enclosures

The worked example in the course applies this lesson to a realistic job in 5 steps, starting with "Clearance to the wood joist".

What you'll be able to do

  • Explain the main idea of "Grease Duct: Gauge, Slope, Access, and Clearance (CMC 510 and 507.4)" in plain language you can use on the exam.
  • Apply this step with confidence: "Slope, Drains, and No Traps".
  • Apply this step with confidence: "Access Openings and Supports".
  • Apply this step with confidence: "Clearance to Combustibles and Enclosures".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Grease Duct: Slope, Metal, and ClearanceFall needed on a horizontal run60 ft runfall = 15 in1/4 in per ft (75 ft or less)90 ft runfall = 90 in (7 ft 6 in)1 in per ft (over 75 ft)not to scaleGrease duct checklistMetal (Sec. 510.5.1):carbon steel 0.060 in (No. 16 MSG)stainless steel 0.048 in (No. 18 MSG)Slope (Sec. 510.1.3):75 ft or less: 1/4 in per footover 75 ft: 1 in per footClearance to combustibles (Sec. 507.4):18 in for bare duct9 in: 28 ga sheet on 1 in spacers3 in: 22 ga sheet on 1 in mineral woolDuct may touch noncombustible surfacesfor up to 50% of its area per foot (507.4.7)60 ft run: 15 in of fall. 90 ft run: 90 in (7 ft 6 in) of fall.Access: at least one 20 x 20 in opening on a horizontal duct (Sec. 510.3.3).2025 CMC Sec. 510.1.3, 510.3.3, 510.5.1, 507.4, 507.4.2, 507.4.2.1, 507.4.7.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: A Grease Duct Stands Alone

The 2025 California Mechanical Code (CMC) Section 510.1 says ducts shall not pass through fire walls, Section 510.1.1 says all ducts lead directly to the exterior of the building, and Sections 510.1.2 and 510.1.8 say the Type I exhaust system shall not be interconnected with any other building ventilation or exhaust system and must be independent. Listed grease ducts are installed to their listing and the manufacturer instructions (Section 510.4). The rest of this lesson is about the other grease ducts, the field-fabricated sheet metal ducts of Section 510.5.

Step 2: Metal and Welds

Section 510.5.1 requires carbon steel not less than 0.060 inch (No. 16 MSG) or stainless steel not less than 0.048 inch (No. 18 MSG), and the duct must be supported by that same metal. That is thicker than the hood, which only needs 0.048 inch steel (Section 508.3). Section 510.5.3 requires all seams, joints, penetrations, and duct-to-hood collar connections to have a liquid-tight continuous external weld. Butt welded connections are not permitted (Section 510.5.4). For telescoping and bell-type joints the inside section is always uphill of the outside section with an overlap of not more than 2 inches (Section 510.5.5). Bolts, screws, and rivets must not penetrate duct walls (Section 510.1.6), and a leakage test (light test, water pressure test, or approved equivalent per ASHRAE 154) is required before the duct is used or concealed (Section 510.5.6).

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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