/ Field knowledge · IMC 603.8
Bury it after the leak test — not before.
Underground ducts are not “bury it and hope.” IMC 603.8: ducts must be approved for underground installation. Metallic ducts without an approved coating must be encased in at least 2 inches of concrete. Slope 1/8 in per foot to drainage with access. Seal, secure, and test before burial or encasement — including International Energy Conservation Code (IECC) leak testing. Plastic underground ducts: PVC or HDPE per the material rules, 150°F maximum design temperature. 603.14: ducts cannot sit in or within 4 inches of earth unless they comply with 603.8.
Educational 2024 International Mechanical Code (IMC) 603.8–603.8.3 and 603.14 from Daily Code Talk. Confirm the adopted edition. Distinct from grease-duct test-before-concealment and flood / weather (603.13–603.16). This is not an underground-duct design.
- 603.8: approved for underground. Metallic without approved coating: encase in at least 2 in of concrete.
- 603.8.1–603.8.3: 1/8 in per ft slope to drainage with access. Sealed, secured, and tested before burial or encasement, including IECC leak testing. Plastic: PVC/HDPE rules, 150°F max design temperature.
- 603.14: not in or within 4 in of earth unless 603.8 is met. A duct 3 in off grade with no coating is still an underground-duct problem.
- 603.7 (related): rigid-duct penetrations of walls, floors, ceilings, roofs, and air-transfer openings protected per 607. Dwelling-to-private-garage: ducts continuous, minimum 26-gage sheet steel, no openings into the garage. Dampers not required there unless International Building Code (IBC) Chapter 7 requires them.
Bare spiral in the slab, no slope, no leak test
Or a duct 2 in off the dirt. Those fail inspection after the pour — the expensive time to find out.
Source: Daily Code Talk — IMC 603.7–603.10 · tool: Underground-duct screen · flood: 603.13 · hangers/seams: SMACNA 006 / UL 181A–B in the same DCT post.
What to send
Where ducts go below grade or within 4 in of earth, the coating or 2 in concrete path, and whether a pre-burial leak test is specified.
If the duct is in the slab with no test-before-burial note
Send the route. Masterbuild will say whether 603.8 still has no 2 in encasement — or 603.14 still has a duct within 4 in of earth.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. 2 in concrete, 1/8 in per ft, 150°F plastic, 4 in of earth, and 26-gage garage ducts are 2024 IMC 603 education from Daily Code Talk — not an underground-duct design. This page is not legal advice.