/ Field knowledge · IMC 504.10–504.11
A damper in the common dryer shaft is not 504.11.
A common dryer shaft is a life-safety riser, not a 4-inch flex leftover. IMC 504.10 commercial dryers: follow the manufacturer; fan motors stay outside the airstream; for multiple dryers the fan runs continuously or is interlocked so it runs whenever any dryer runs; ducts at least 6 inches from combustibles; the dryer-to-wall transition is a single listed UL 2158A duct, maximum 8 feet, and not concealed. IMC 504.11 shared multistory shafts: fire-resistance-rated per the International Building Code (IBC); no dampers in the common exhaust duct; rigid metal in the shaft, minimum 26-gauge sheet steel, straight — no offsets; fan continuous, on standby power, motor outside the airstream, status monitored with an audible or visual signal on failure; makeup air; a 12 inch by 12 inch cleanout at the base; no screens at the termination; dryers only — independent of toilets, kitchens, and other exhaust.
Educational 2024 International Mechanical Code (IMC) 504.10–504.11 from Daily Code Talk. Confirm the adopted edition. Distinct from 504.1–504.4 dwelling dryer terminations. This is not a shaft design and not a lint-velocity calculator. Packet: 504.11 PDF · ES.
- 504.10 commercial. Manufacturer first. Exhaust-fan motors outside the airstream. Multiple machines: continuous fan, or interlocked with every dryer. 6 inches to combustibles. Transition duct: one piece, UL 2158A, ≤ 8 ft, fully exposed — not daisy-chained flex behind a row of machines.
- 504.11 common multistory. Rated shaft. No fire or smoke damper in the common dryer duct (penetrations follow IMC 607.5.5 Exception 2). 26-gauge rigid metal, SMACNA, no offsets. Fan: 503.2 construction, outside the airstream, continuous, standby power, monitored. Makeup air. 12×12 cleanout at the base. No screens. Serves clothes dryers only.
- Do not share the shaft with toilet, kitchen, or house exhaust. Do not “add a damper later for the rating.” Do not bury the 8-foot connector in the soffit.
On the set: tag the shaft “COMMON MULTISTORY DRYER SHAFT PER IMC 504.11.” Show the fan out of the airstream, the standby-power source, the monitor, the 12×12 at the base, and the 6-inch combustible clearance. The transition at each machine is a listed UL 2158A single piece, not a note that says “flex.”
A damper in the common dryer shaft “because the wall is rated”
Lint plus a closed damper is a chimney. Offsets in the shaft fill with lint the same way. If the fan is not on standby power, the first outage turns every stacked laundry into an unvented appliance.
Source: Daily Code Talk — IMC 504.10–504.11 · tool: Commercial / shaft screen · dwelling: 504.1–504.4 · fans: 503.
What to send
How many dryers share the riser (or that they are commercial machines), whether any damper is shown in the shaft, a section of the shaft (straight vs offsets), and whether the fan is on standby power.
If the common dryer shaft has a damper — or an offset
Send the shaft section and the fan note. Masterbuild will say whether 504.11 still has a damper in the common duct — or 504.11 still has no 12×12 at the base and no standby power.
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. Fan motors outside the airstream, UL 2158A ≤ 8 ft exposed, 6 in to combustibles, no dampers in the common duct, 26-gauge straight metal, continuous fan on standby power with a failure signal, makeup air, 12×12 cleanout, and dryers-only service are 2024 IMC 504.10–504.11 education from Daily Code Talk — not a shaft design. This page is not legal advice.