TinnerFlow™2nd YearHangers / SupportsRED · Stop / Verify
Sagging heavy duct means the support layout is not controlling the load. Add or revise supports per the approved hanger schedule before joints deform or the run gets covered.
Heavy gauge rectangular duct, smoke-control duct, lined duct, and large mains cannot be hung like light branch duct. If the joints dip between supports, the duct is telling you the spacing/support method is wrong.
The recovery is to verify the support schedule, add intermediate trapezes/rods, level the run, and recheck joints before sealing, insulation, inspection, or startup. Exact spacing and rod/strap size must follow the project/SMACNA/support detail, not a guess.
Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.