IBC 717.5.3
Shaft damper exceptions (IBC 717.5.3)
Duct penetrations of a shaft enclosure need a fire damper and a smoke damper, in practice usually a combination fire/smoke damper. The exceptions below remove one or both. Each one is shown with the result the tool gives when it applies.
Rules last reviewed October 12, 2026. How the rules are checked
Check your case in the tool (2024 IBC)Start the tool with the 2021 IBC · Start the tool with the 2018 IBC
The base rule
Fire damper: required · Smoke damper: requiredAll three editions
Fire damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no fire damper exception applies. IBC 717.5.3
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Combination fire/smoke damper required
The exceptions
Steel exhaust subducts
IBC 717.5.3 Exc. 1.1
Example: Exhaust subducts with a continuous roof fan, building not Group B or R
Fire damper: not required · Smoke damper: building official approval needed2021 and 2024 IBC
Fire damper: not required. The exhaust connection has a steel subduct at least 0.0187 inch thick extending 22 inches up the shaft, with a continuously powered exhaust fan at the top of the shaft. IBC 717.5.3 Exc. 1.1
Smoke damper: building official approval needed. The subducts remove the fire damper (Exception 1.1), but IBC 717.5.3 still requires a smoke damper because the Group B or R subduct exception (Exception 2) is not met. IBC 717.5.3.1 forbids a damper that would stop the required upward airflow in the shaft, so the rules conflict. Meet Exception 2, arrange the smoke damper so its closing cannot stop the shaft airflow, or get the building official to approve the arrangement. IBC 717.5.3, IBC 717.5.3.1
Full result in the tool: Code conflict: building official approval needed
Fire damper: not required · Smoke damper: required2018 IBC only
Fire damper: not required. The exhaust connection has a steel subduct extending 22 inches up the shaft with continuous upward airflow to the outside. IBC 717.5.3 Exc. 1.1
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Smoke damper required
Removes the fire damper only. The smoke damper is still required unless Exception 2 or another exception applies.
Penetration tested as part of the shaft wall
IBC 717.5.3 Exc. 1.2
Example: Duct into a shaft, penetration tested as part of the shaft wall design
Fire damper: not required · Smoke damper: requiredAll three editions
Fire damper: not required. The penetration was tested as part of the rated shaft assembly. IBC 717.5.3 Exc. 1.2
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Smoke damper required
Removes the fire damper only. The smoke damper is still required unless another exception applies.
Smoke control systems
IBC 717.5.3 Exc. 1.3 and 4
Example: Duct in an approved smoke control system where a damper would interfere
Fire damper: alternative protection · Smoke damper: not requiredAll three editions
Fire damper: alternative protection. The duct is part of an approved smoke control system and a fire damper would interfere with it, so approved alternative protection is used instead of the damper. IBC 717.5.3 Exc. 1.3, IBC 717.2.1
Smoke damper: not required. The duct serves an approved mechanical smoke control system, and a smoke damper would interfere with how that system works. IBC 717.5.3 Exc. 4
Full result in the tool: Approved alternative protection required
Parking garage shafts
IBC 717.5.3 Exc. 1.4 and 3
Example: Parking garage exhaust shaft separated from other shafts by 2-hour construction
Fire damper: not required · Smoke damper: not requiredAll three editions
Fire damper: not required. Parking garage exhaust or supply shafts separated from other shafts by 2-hour construction do not need fire dampers. IBC 717.5.3 Exc. 1.4
Smoke damper: not required. Parking garage exhaust or supply shafts separated from other shafts by 2-hour construction do not need smoke dampers. IBC 717.5.3 Exc. 3
Also: Where no damper is required, the duct penetration must still be firestopped so the wall keeps its rating (through-penetration or membrane-penetration protection). IBC 717.1.2, IBC 714.3 to 714.4.3
Full result in the tool: No damper required
Group B and R exhaust with subducts
IBC 717.5.3 Exc. 2
Example: Bathroom exhaust with steel subducts and a continuous roof fan, Group R, NFPA 13
Smoke damper: not requiredAll three editions
Smoke damper: not required. In a Group B or R building with NFPA 13 sprinklers, kitchen, clothes dryer, bathroom or toilet room exhaust with 26 gage steel subducts extending 22 inches up the shaft and a continuously powered exhaust fan at the top does not need smoke dampers. IBC 717.5.3 Exc. 2
Also: Where no damper is required, the duct penetration must still be firestopped so the wall keeps its rating (through-penetration or membrane-penetration protection). IBC 717.1.2, IBC 714.3 to 714.4.3
Full result in the tool: No damper required
Dryer and kitchen exhaust
IBC 717.5.3 Exc. 5
Example: Clothes dryer exhaust into an ordinary (not common multistory) exhaust shaft with subducts and a roof fan, Group R, NFPA 13
Fire damper: prohibited · Smoke damper: not requiredAll three editions
Fire damper: prohibited. The mechanical code prohibits fire and combination fire/smoke dampers in this exhaust duct, and the building code waives them here. IBC 717.5.3 Exc. 5, IMC 504.2
Smoke damper: not required. No smoke damper is needed (IBC 717.5.3 Exc. 2: Group B or R building with NFPA 13 sprinklers, 26 gage steel subduct extending 22 inches up the shaft, continuously powered exhaust fan at the top), and IMC 504.2 prohibits devices that would obstruct the exhaust flow. IBC 717.5.3 Exc. 2, IMC 504.2
Full result in the tool: Dampers prohibited in this duct
Shafts that need continuous upward airflow
IBC 717.5.3.1 (2021 and 2024 IBC)
Example: Exhaust subducts with a continuous roof fan, building not Group B or R
Smoke damper: building official approval needed2021 and 2024 IBC
Smoke damper: building official approval needed. The subducts remove the fire damper (Exception 1.1), but IBC 717.5.3 still requires a smoke damper because the Group B or R subduct exception (Exception 2) is not met. IBC 717.5.3.1 forbids a damper that would stop the required upward airflow in the shaft, so the rules conflict. Meet Exception 2, arrange the smoke damper so its closing cannot stop the shaft airflow, or get the building official to approve the arrangement. IBC 717.5.3, IBC 717.5.3.1
Full result in the tool: Code conflict: building official approval needed
Smoke damper: required2018 IBC only
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Smoke damper required
Where shaft ductwork enters a rooftop penthouse
IBC 713.12.1 (2021 and 2024 IBC)
Example: Where the shaft ductwork enters a rooftop mechanical penthouse and connects directly to the equipment (that penetration only)
Fire damper: not required · Smoke damper: not required2021 and 2024 IBC
Fire damper: not required. No fire/smoke damper is needed where the shaft ductwork enters the rooftop structure: the shaft rises through the roof into a rooftop mechanical penthouse and its ductwork connects directly to the HVAC equipment. Other duct penetrations of the shaft walls still follow IBC 717.5.3. IBC 713.12.1
Smoke damper: not required. No smoke damper is needed at that penetration either (IBC 713.12.1 covers fire/smoke dampers). IBC 713.12.1
Also: The rooftop structure must comply with IBC 1511. Duct penetrations elsewhere in the shaft walls still follow the shaft damper rules. IBC 713.12.1, IBC 1511
Also: Where no damper is required, the duct penetration must still be firestopped so the wall keeps its rating (through-penetration or membrane-penetration protection). IBC 717.1.2, IBC 714.3 to 714.4.3
Full result in the tool: No damper at the rooftop structure penetration
Fire damper: required · Smoke damper: required2018 IBC only
Fire damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no fire damper exception applies. IBC 717.5.3
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Combination fire/smoke damper required
Dryer and kitchen exhaust shafts also follow mechanical code rules. See dryer, grease and hazardous exhaust ducts.
The subduct checklist
The tool asks these questions for an exhaust duct connecting to an exhaust shaft. The checklist changed in 2021.
Is the duct an exhaust connection into an exhaust shaft?
For example, bathroom or kitchen exhaust from each floor connecting into a vertical exhaust shaft.
Which of these are true for the exhaust shaft?
2018 IBC only
A subduct is a short steel duct that turns up inside the shaft, so smoke must travel up 22 inches before it can spread to other floors.
In the 2018 IBC these items serve two exceptions. Exception 1.1 drops the fire damper where a steel subduct extends at least 22 inches and air flows continuously upward to the outside (the tool accepts a continuously running fan at the top as providing that airflow). Exception 2 drops the smoke damper in a Group B or R building with NFPA 13 sprinklers, where the subduct is also at least 0.0187 inch thick and the fan at the top runs continuously:
- The exhaust connection has a steel subduct
- The subduct wall is at least 0.0187 inch thick (No. 26 gage)
- The subduct extends at least 22 inches vertically inside the shaft
- Air in the shaft flows continuously upward to the outside
- An exhaust fan at the top of the shaft runs continuously, powered as required by IBC 909.11
IBC 717.5.3 Exc. 1.1 and 2
2021 and 2024 IBC
A subduct is a short steel duct that turns up inside the shaft, so smoke must travel up 22 inches before it can spread to other floors.
Exception 1.1 drops the fire damper when all three items are met. Exception 2 drops the smoke damper when they are met for kitchen, clothes dryer, bathroom or toilet room exhaust in a Group B or R building with NFPA 13 sprinklers:
- The exhaust connection has a steel subduct at least 0.0187 inch thick (No. 26 gage)
- The subduct extends at least 22 inches vertically inside the shaft
- An exhaust fan at the top of the shaft runs continuously, powered as required by IBC 909.11, keeping airflow upward to the outdoors
IBC 717.5.3 Exc. 1.1 and 2
Is this exhaust from a kitchen, clothes dryer, bathroom or toilet room?
Are the spaces served by this exhaust shaft in Group B (business) or Group R (residential) occupancies?
Common situations by edition
| Situation | 2024 IBC | 2021 IBC | 2018 IBC |
|---|---|---|---|
| Supply or return duct into a 2-hour duct shaft | Combination fire/smoke damper required | Combination fire/smoke damper required | Combination fire/smoke damper required |
| Bathroom exhaust with steel subducts and a continuous roof fan, Group R, NFPA 13 | No damper required | No damper required | No damper required |
| Exhaust subducts with a continuous roof fan, building not Group B or RDiffers by edition | Code conflict: building official approval needed | Code conflict: building official approval needed | Smoke damper required |
| Where the shaft ductwork enters a rooftop mechanical penthouse and connects directly to the equipment (that penetration only)Differs by edition | No damper at the rooftop structure penetration | No damper at the rooftop structure penetration | Combination fire/smoke damper required |
| Parking garage exhaust shaft separated from other shafts by 2-hour construction | No damper required | No damper required | No damper required |
| Clothes dryer exhaust into an ordinary (not common multistory) exhaust shaft with subducts and a roof fan, Group R, NFPA 13 | Dampers prohibited in this duct | Dampers prohibited in this duct | Dampers prohibited in this duct |
Select a result to open it in the tool with every answer filled in, including the conditions not listed here.
Questions
Which shaft exceptions remove the smoke damper as well as the fire damper?
Parking garage shafts and, since 2021, rooftop penthouse connections remove both. In a smoke control system the smoke damper is dropped and approved alternative protection replaces the fire damper. Subducts remove the smoke damper only in Group B and R buildings with NFPA 13 sprinklers.
Fire damper: not required · Smoke damper: not requiredAll three editions
Fire damper: not required. Parking garage exhaust or supply shafts separated from other shafts by 2-hour construction do not need fire dampers. IBC 717.5.3 Exc. 1.4
Smoke damper: not required. Parking garage exhaust or supply shafts separated from other shafts by 2-hour construction do not need smoke dampers. IBC 717.5.3 Exc. 3
Also: Where no damper is required, the duct penetration must still be firestopped so the wall keeps its rating (through-penetration or membrane-penetration protection). IBC 717.1.2, IBC 714.3 to 714.4.3
Full result in the tool: No damper required
Does an air transfer opening into a shaft get the subduct exceptions?
No. The subduct exceptions are for exhaust ducts connecting to an exhaust shaft.
What changed for subducts in the 2021 IBC?
The subduct needs a minimum steel thickness and the shaft needs a continuously powered exhaust fan at the top. Dampers that would stop the required upward airflow are not allowed.
Fire damper: not required · Smoke damper: building official approval needed2021 and 2024 IBC
Fire damper: not required. The exhaust connection has a steel subduct at least 0.0187 inch thick extending 22 inches up the shaft, with a continuously powered exhaust fan at the top of the shaft. IBC 717.5.3 Exc. 1.1
Smoke damper: building official approval needed. The subducts remove the fire damper (Exception 1.1), but IBC 717.5.3 still requires a smoke damper because the Group B or R subduct exception (Exception 2) is not met. IBC 717.5.3.1 forbids a damper that would stop the required upward airflow in the shaft, so the rules conflict. Meet Exception 2, arrange the smoke damper so its closing cannot stop the shaft airflow, or get the building official to approve the arrangement. IBC 717.5.3, IBC 717.5.3.1
Full result in the tool: Code conflict: building official approval needed
Fire damper: not required · Smoke damper: required2018 IBC only
Fire damper: not required. The exhaust connection has a steel subduct extending 22 inches up the shaft with continuous upward airflow to the outside. IBC 717.5.3 Exc. 1.1
Smoke damper: required. Shaft enclosure penetrations need fire and smoke dampers, and no smoke damper exception applies. IBC 717.5.3
Full result in the tool: Smoke damper required