// CODE SNIPPET

603.4 Metallic Ducts

Mechanical Code 2018 of Maryland > 6 Duct Systems > 603 Duct Construction and Installation > 603.4 Metallic Ducts

JUMP TO FULL CODE CHAPTER

Metallic ducts shall be constructed as specified in the SMACNA HVAC Duct Construction Standards—Metal and Flexible.

Exception: Ducts installed within single dwelling units shall have a minimum thickness as specified in Table 603.4.

TABLE 603.4
DUCT CONSTRUCTION MINIMUM SHEET METAL THICKNESS FOR SINGLE DWELLING UNITS^a
ROUND DUCT DIAMETER (inches)	STATIC PRESSURE
	¹/₂-inch water gauge		1-inch water gauge
	Thickness (inches)		Thickness (inches)
	Galvanized	Aluminum	Galvanized	Aluminum
< 12	0.013	0.018	0.013	0.018
12 to14	0.013	0.018	0.016	0.023
15 to 17	0.016	0.023	0.019	0.027
18	0.016	0.023	0.024	0.034
19 to 20	0.019	0.027	0.024	0.034
RECTANGULAR DUCT DIMENSION (inches)	STATIC PRESSURE
	¹/₂-inch water gauge		1-inch water gauge
	Thickness (inches)		Thickness (inches)
	Galvanized	Aluminum	Galvanized	Aluminum
≤ 8	0.013	0.018	0.013	0.018
9 to 10	0.013	0.018	0.016	0.023
11 to 12	0.016	0.023	0.019	0.027
13 to 16	0.019	0.027	0.019	0.027
17 to 18	0.019	0.027	0.024	0.034
19 to 20	0.024	0.034	0.024	0.034

For SI: 1 inch = 25.4 mm, 1-inch water gauge = 249 Pa.

Ductwork that exceeds 20 inches by dimension or exceeds a pressure of 1-inch water gauge shall be constructed in accordance with SMACNA HVAC Duct Construction Standards—Metal and Flexible.

603.4.1 Minimum Fasteners

Round metallic ducts shall be mechanically fastened by means of not less than three sheet metal screws or rivets spaced equally around the joint.

Exception: Where a duct connection is made that is partially inaccessible, three screws or rivets shall be equally spaced on the exposed portion so as to prevent a hinge effect.

603.4.2 Duct Lap

Crimp joints for round and oval metal ducts shall be lapped not less than 1 inch (25 mm) and the male end of the duct shall extend into the adjoining duct in the direction of airflow.

Related Code Sections

603.4 Duct Systems, Metallic Ducts

Metallic ducts shall be constructed as specified in the SMACNA HVAC Duct Construction Standards—Metal and Flexible . Exception: Ducts installed ...

Mechanical Code 2018 of Maryland > 6 Duct Systems > 603 Duct Construction and Installation > 603.4 Metallic Ducts