Sheet Metal (C-43) · Lesson 3 of 8

Type I Kitchen Hoods: Metal, Overhang, and Net Exhaust (CMC 508)

"Type I Kitchen Hoods: Metal, Overhang, and Net Exhaust (CMC 508)" is lesson 3 of 8 in the Sheet Metal (C-43) Learning Hub. It opens with "Where a Type I Hood Is Required" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Where a Type I Hood Is Required
  2. Build It: Metal, Welds, and Supports
  3. Size the Canopy: Overhang and Height
  4. Pick the Duty Level and Read the Table
  5. Net Exhaust, Outlets, and the Label

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Hood length" through "Metal and supports".

What you'll be able to do

  • Explain the main idea of "Type I Kitchen Hoods: Metal, Overhang, and Net Exhaust (CMC 508)" in plain language you can use on the exam.
  • Apply this step with confidence: "Size the Canopy: Overhang and Height".
  • Apply this step with confidence: "Pick the Duty Level and Read the Table".
  • Apply this step with confidence: "Net Exhaust, Outlets, and the Label".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Type I Hood: Net Exhaust and SizeWall-mounted canopy hood, side viewcooking lineType I hood14 ft longnet 5,600 cfm4 ft max6 inHood 14 ft = 13 ft cooking line + 6 in each open endNet exhaust, metal, supportsHeaviest duty under hood: HEAVY (broiler)Wall canopy, Table 508.5.1.3: 400 cfm per ftNet = 400 x 14 ft = 5,600 cfmInternal-discharge makeup air (MA): 1,000 cfmHood exhaust = 5,600 + 1,000 = 6,600 cfmOutlets: 14 ft / 12 ft each = 2 outletsSteel: 0.048 in (No. 18 MSG), Sec. 508.3Supports: 650 + 800 = 1,450 lb, Sec. 508.4Overhang 6 in min, lip 4 ft max (508.5.1)Duty level = heaviest appliance (508.5.1.1)Net 5,600 cfm; fan moves 6,600 cfm; 2 outlets; 0.048 in steelTable values are NET airflow: net = total hood exhaust minus internal makeup air.2025 CMC Sec. 508.3, 508.4, 508.5.1, Table 508.5.1.3, Equation 508.5.1.1, Sec. 508.7.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Where a Type I Hood Is Required

The 2025 California Mechanical Code (CMC) Section 508.1 requires a Type I hood at or above commercial deep-fat fryers, broilers, grills, hot-top ranges, ovens, barbecues, rotisseries, and similar equipment that emits comparable amounts of smoke or grease in a food-processing establishment, which does not include a dwelling unit. A few exceptions apply, such as appliances that meet UL 197 reduced-emissions limits and listed recirculating systems. Listed hood assemblies are installed to their listing and the manufacturer instructions (Section 508.2).

Step 2: Build It: Metal, Welds, and Supports

Section 508.3 says the part of a hood that collects cooking vapors and residue must be made of and supported by steel not less than 0.048 inch (No. 18 MSG) or stainless steel not less than 0.036 inch (No. 20 MSG), or an approved material of equal strength and fire and corrosion resistance. Listed hoods are excepted. All seams, joints, and penetrations that direct and capture grease-laden vapors need a liquid-tight continuous external weld to the hood lower outermost perimeter (Section 508.3.2), and a wall-mounted hood must fit tight against the back wall (Section 508.3.1). Hoods are secured by noncombustible supports able to carry the expected hood weight plus 800 pounds (Section 508.4).

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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