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Chapter 5 — EXHAUST SYSTEMS

506.2 Construction — 2025 California Mechanical Code

California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-25 · 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) § 506.2 · Effective January 1, 2026 · Official source: Building Standards Commission

Ducts used for conveying products shall be airtight construction as approved by the Authority Having Jurisdiction, and shall not have openings other than those required for operation and maintenance of the system. Ducts constructed of steel shall comply with Table 506.2(1) or Table 506.2(2). Exceptions: (1) Class 1 product-conveying ducts that operate at less than 4 inches water column (0.9 kPa) negative pressure and con

vey noncorrosive, nonflammable and nonexplosive materials at temperatures not exceeding 250°F (121°C) shall be permitted to be constructed in accordance with SMACNA HVAC Duct Construction Standards–Metal and Flexible.

(2) Ducts used in central vacuuming 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-resistive walls or floor-ceiling or roof-ceiling assemblies shall be in accordance with the building code. Copper or ferrous pipes or conduit extending from within the separation between a garage and dwelling unit to the central vacuum unit shall be permitted to be used. The use of rectangular ducts conveying particulates shall be subject to approval of the Authority Having Jurisdiction. The design of rectangular ducts shall consider the adhesiveness and buildup of products being conveyed within the duct.

EXHAUST SYSTEMS

TABLE 506.2(2) MINIMUM SHEET METAL THICKNESS FOR RECTANGULAR DUCTS
NEGATIVE
PRESSURE
(inches
water
column)
REINF.
SPACING
(inches)		 LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT LONGEST SIDE OF DUCT
NEGATIVE
PRESSURE
(inches
water
column)
REINF.
SPACING
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 1
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)		 CLASS 2
(inches)
NEGATIVE
PRESSURE
(inches
water
column)
REINF.
SPACING
(inches)		 Up to 12 13 to 24 25 to 36 37 to 48 49 to 60 Up to 12 13 to 24 25 to 36 37 to 48 49 to 60
To 7 48 0.021 (24 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.068 (14 ga.) –– 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) ––
To 7 24 0.021 (24 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
To 7 12 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.021 (24 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
8 to 11 48 0.027 (22 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) –– 0.033 (20 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) ––
8 to 11 24 0.027 (22 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.033 (20 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
8 to 11 12 0.021 (24 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.027 (22 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
12 to 15 48 0.044 (18 ga.) 0.097 (12 ga.) –– –– –– 0.044 (18 ga.) 0.044 (18 ga.) –– –– ––
12 to 15 24 0.044 (18 ga.) 0.055 (16 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
12 to 15 12 0.027 (22 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.044 (18 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.055 (16 ga.) 0.055 (16 ga.) 0.055 (16 ga.)
16 to 20 48 0.068 (14 ga.) 0.112 (11 ga.) –– –– –– 0.068 (14 ga.) 0.112 (11 ga.) –– –– ––
16 to 20 24 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 12 0.033 (20 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.033 (20 ga.) 0.044 (18 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.)
CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 3
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)		 CLASS 4
(inches)
To 7 48 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) –– 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) ––
To 7 24 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
To 7 12 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
8 to 11 48 0.044 (18 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) –– 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) ––
8 to 11 24 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
8 to 11 12 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
12 to 15 48 0.044 (18 ga.) 0.097 (12 ga.) –– –– –– 0.055 (16 ga.) 0.097 (12 ga.) –– –– ––
12 to 15 24 0.044 (18 ga.) 0.055 (16 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
12 to 15 12 0.044 (18 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.097 (12 ga.) 0.097 (12 ga.) 0.097 (12 ga.)
16 to 20 48 0.068 (14 ga.) 0.112 (11 ga.) –– –– –– 0.055 (16 ga.) 0.112 (11 ga.) –– –– ––
16 to 20 24 0.068 (14 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.055 (16 ga.) 0.068 (14 ga.) 0.112 (11 ga.) 0.112 (11 ga.) 0.112 (11 ga.)
16 to 20 12 0.044 (18 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.068 (14 ga.) 0.055 (16 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 construction shall be permitted to be used in Class 1 duct systems. The thickness of aluminum ducts shall be not less than two Brown and Sharpe gauges thicker than the gauges required for steel ducts set forth in Table 506.2(1) and Table 506.2(2).

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California Mechanical Code (Title 24, Part 4)

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