Duct Hanger Spacing Chart — Support Rules for Metal & Flex
Who requires what, support-wise: the residential strap rule, the commercial SMACNA framework the IMC defers to, and the flex-duct spacing and sag limits that keep the capacity charts honest. The famous 10-foot rule is residential-only under current codes — the commercial answer lives in the SMACNA schedules, selected by duct size and spacing together.
Support requirements by duct system
| System | Support rule | Cite |
|---|---|---|
| Residential metal ducts (IRC) | Supported by 1/2"-wide 18-gauge metal straps or 12-gauge galvanized wire at intervals not over 10 feet, or other approved means | IRC M1601.4.4 |
| Commercial metal ducts (IMC) | Supported per the SMACNA HVAC Duct Construction Standards — hanger size and spacing come from its schedules by duct half-perimeter (spacing options run 4, 5, 8, and 10 feet) | IMC 603.10 |
| Flexible & factory-made ducts | Supported per the manufacturer's installation instructions and the UL 181 listing | IMC 603.10 / UL 181 |
Flex duct support rules
| Rule | ADC standard | SMACNA variant |
|---|---|---|
| Max support spacing | 4 ft | 5 ft |
| Max sag between supports | 1/2" per ft of spacing | 1/2" per ft of spacing |
| Support contact width | 1-1/2" min | 1" min |
| Vertical runs | Strapped at 6 ft max | — |
Placement beats spacing
Place hangers within 2 feet of every elbow and within 4 feet of every branch intersection, and count a connection to rigid duct or equipment as a support point. On commercial rectangular duct, hanger strap or rod size is selected from SMACNA's hanger schedule (its Table 4-1 lineage) by the duct's half-perimeter and the chosen spacing — this site cites that schedule rather than reproducing it, since gauge, reinforcement, and hanger selections interlock. Duct weight for the loading math comes from the sheet metal gauge chart (area × lb/ft², plus insulation), and wall thickness from the duct gauge chart. Sagging flex is a capacity problem before it's a support problem — the measured penalty is on the flex duct CFM chart.
Common questions
How often does ductwork need to be supported?
Residential metal duct under the IRC: every 10 feet, on 1/2"-wide 18-gauge straps or 12-gauge galvanized wire. Commercial duct under the IMC follows the SMACNA schedules, where hanger size and spacing (4, 5, 8, or 10 feet) are chosen together by duct half-perimeter. Flex duct is much tighter: every 4 feet per the ADC standard.
What is the maximum spacing for flex duct supports?
The ADC Flexible Duct Performance & Installation Standards (5th edition) say supports no more than 4 feet apart, with at most 1/2 inch of sag per foot of support spacing, on saddles at least 1-1/2 inches wide. SMACNA's duct construction standard printed 5 feet with the same sag limit — if your spec cites SMACNA, that variant applies, but 4 feet is the common conservative rule.
Is the 10-foot duct support rule still code?
Only for residential work. The pre-2015 IMC did print a blanket 10-foot hanger rule, but the current commercial code (IMC 603.10) defers entirely to the SMACNA standards instead. The 10-foot interval survives as the IRC's residential metal-duct strap rule — quoting it on a commercial job is citing a code that no longer says that.
Where do hangers go besides the spacing interval?
Within 2 feet of every elbow and within 4 feet of every branch intersection — turns and taps concentrate load and torque. A connection to rigid equipment or duct counts as a support point, which is why a short flex whip between two rigid connections may need no intermediate hanger at all.
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