Sheet Metal & Air Conditioning Contractors’ National Association


Are you up-to-date on the latest International Mechanical Code changes?

Not only do residential contractors have to work with the International Residential Code but they also must be aware of the International Mechanical Code.

The International Mechanical Code (IMC) has changed over the last ten years in the code cycle process. Residential contractors can see how this process has evolved in Table 603.4 Duct Construction Minimum Sheet Metal Thickness for Single Dwelling Units of the IMC.

Below is the progression of the changes in Table 603.4 from 2006 to the current 2015 IMC.

2006 IMC

Table 603-4 1

In 2006, Table 603.4:

  • Allowed 30-gage sheet metal.
  • Duct had no maxium length: Duct over 14 inches could be constructed from 28 gage for round and 26 gage for rectangular.
  • Did not contain any language refering to SMACNA standards.

2009-2012 IMC

Table 603-4 2


In 2009, Table 603.4:

  • Deleted 30-gage sheet metal.
  • The Table showed Duct had no maxium length: Duct over 20 inches could be constructed from 24 gage for round.
  • Added Note A that refered to SMACNA standards for duct size over 14 inches for rectangular.

2015 IMC

Table 603-4 3

In 2015, Table 603.4:

  • Has sheet metal thickness for both ½ inch and 1 inch water gage and limits the sizes to 20 inches.
  • Updated Note A to state: “Ductwork that exceeds 20 inches by dimension or exceeds a pressure of 1-inch water gage shall be constructed in accordance with SMACNA's HVAC Duct Construction Standards—Metal and Flexible.
  • Added back 30 gage or 0.013 sheet metal thickness (minimal) to the Table depending on duct dimension.

If you have any questions or concerns please contact SMACNA’s Technical Resources Department through the Technical Resources Inquiry webpage.