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Chapter 5 — EXHAUST SYSTEMS

506.1 Materials — 2025 California Mechanical Code

California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-10-19 · California

CBSC: official editions, supplements and errata

Italicized text is a California amendment to the model code, as printed in the official publication.

Cite as: 2025 California Mechanical Code (Title 24, Part 4) § 506.1 · Effective January 1, 2026 · Official source: Building Standards Commission

Materials used in product-conveying duct systems shall be suitable for the intended use and shall be of rigid sheet metal.

Exceptions:

(1) Asbestos-cement, concrete, clay, or ceramic materials shall be permitted to be used where it is shown that these materials will be equivalent to metal ducts installed in accordance with this chapter.

(2) Ducts serving a Class 5 system shall be permitted to be constructed of approved nonmetallic material where the corrosive characteristics of the material being conveyed make a metal system unsuitable and where the mixture being conveyed is nonflammable.

Approved nonmetallic material shall be either a listed product having a flame-spread index not exceeding 25 and a smoke-developed rating of 50 or less on both inside and outside surfaces without evidence of continued progressive combustion, or shall have a flame-spread index not exceeding 25 and shall be installed with an automatic fire-sprinkler protection system inside the duct.

(3) Ducts used in central vacuum cleaning systems within a dwelling unit shall be constructed of materials in accordance with ASTM F2158 or the applicable standards referenced in Chapter 18. Penetrations of fire walls or floor-ceiling or roof-ceiling assemblies shall be in accordance with the building code.

Copper or ferrous pipes or conduits extending from within the separation between a garage and dwelling unit to the central vacuuming unit shall be permitted to be used.

TABLE 506.2(1) MINIMUM SHEET METAL THICKNESS FOR ROUND DUCTS
NEGATIVE
PRESSURE
(inches
water
column)
REINF.
SPACING
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)
NEGATIVE
PRESSURE
(inches
water
column)
REINF.
SPACING
(inches)		 Up to 7 8 to 11 12 to 15 16 to 19 20 to 23 24 to 35 36 to 47 48 to 59 60
To 7 0 0.021 (24 ga.) 0.021 (24 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.127 (10 ga.) –– ––
To 7 96 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
To 7 48 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.)
To 7 24 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.)
8 to 11 0 0.021 (24 ga.) 0.027 (22 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) –– –– ––
8 to 11 96 0.021 (24 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
8 to 11 48 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
8 to 11 24 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
12 to 15 0 0.021 (24 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) –– –– ––
12 to 15 96 0.021 (24 ga.) 0.027 (22 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.097 (12 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
12 to 15 48 0.021 (24 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.)
12 to 15 24 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
16 to 20 0 0.021 (24 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.112 (11 ga.) –– –– –– ––
16 to 20 96 0.021 (24 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 48 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.112 (11 ga.)
16 to 20 24 0.021 (24 ga.) 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.)
CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)
To 7 0 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.127 (10 ga.) –– ––
To 7 96 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
To 7 48 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
To 7 24 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
8 to 11 0 0.027 (22 ga.) 0.027 (22 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) –– –– ––
8 to 11 96 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
8 to 11 48 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
8 to 11 24 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
12 to 15 0 0.027 (22 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) –– –– ––
12 to 15 96 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.097 (12 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
12 to 15 48 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.)
12 to 15 24 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
16 to 20 0 0.027 (22 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.112 (11 ga.) –– –– –– ––
16 to 20 96 0.027 (22 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 48 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.)
16 to 20 24 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)
To 7 0 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.127 (10 ga.) –– ––
To 7 96 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
TABLE 506.2(1) MINIMUM SHEET METAL THICKNESS FOR ROUND DUCTS
To 7 48 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
To 7 24 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
8 to 11 0 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) –– –– ––
8 to 11 96 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
8 to 11 48 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
8 to 11 24 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
TABLE 506.2(1) (continued) MINIMUM SHEET METAL THICKNESS FOR ROUND DUCTS
NEGATIVE
PRESSURE
(inches
water
column)		 REINF.
SPACING
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)
NEGATIVE
PRESSURE
(inches
water
column)		 REINF.
SPACING
(inches)		 Up to 7 8 to 11 12 to 15 16 to 19 20 to 23 24 to 35 36 to 47 48 to 59 60
12 to 15 0 0.133 (20 ga.) 0.133 (20 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) –– –– ––
12 to 15 96 0.133 (20 ga.) 0.133 (20 ga.) 0.144 (18 ga.) 0.144 (18 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.197 (12 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
12 to 15 48 0.133 (20 ga.) 0.133 (20 ga.) 0.144 (18 ga.) 0.144 (18 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.168 (14 ga.)
12 to 15 24 0.133 (20 ga.) 0.133 (20 ga.) 0.144 (18 ga.) 0.144 (18 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.168 (14 ga.)
16 to 20 0 0.133 (20 ga.) 0.144 (18 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.112 (11 ga.) –– –– –– ––
16 to 20 96 0.133 (20 ga.) 0.133 (20 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 48 0.133 (20 ga.) 0.133 (20 ga.) 0.144 (18 ga.) 0.144 (18 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.)
16 to 20 24 0.133 (20 ga.) 0.133 (20 ga.) 0.144 (18 ga.) 0.144 (18 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.168 (14 ga.)
CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)
To 7 0 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) –– ––
To 7 96 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
To 7 48 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
To 7 24 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
8 to 11 0 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.197 (12 ga.) –– –– ––
8 to 11 96 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
8 to 11 48 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
8 to 11 24 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.197 (12 ga.)
12 to 15 0 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) –– –– ––
12 to 15 96 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
12 to 15 48 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.155 (16 ga.) 0.168 (14 ga.) 0.168 (14 ga.) 0.197 (12 ga.) 0.197 (12 ga.) 0.097 (12 ga.)
12 to 15 24 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
16 to 20 0 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.112 (11 ga.) –– –– –– ––
16 to 20 96 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 48 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
16 to 20 24 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)

For SI units: 1 inch = 25.4 mm, 1 inch water column = 0.249 kPa

Aluminum ducts shall not be used in systems conveying flammable vapors, fumes, or explosive dusts, nor in Class 2, 3, or 4 systems. Galvanized steel and aluminum ducts shall not be used where the temperature of the material being conveyed exceeds 400°F (204°C). Metal ducts used in Class 5 systems that are not resistant to the corrosiveness of the product shall be protected with an approved corrosion-resistant material.

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▸Contents — California Mechanical Code (Title 24, Part 4)
California Mechanical Code (Title 24, Part 4)

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