IBC 717.5.3
Fire and smoke dampers at shaft enclosures (IBC 717.5.3)
Duct penetrations of a shaft enclosure need both a fire damper and a smoke damper, usually a combination fire/smoke damper, unless an exception removes one or both.
What counts as a shaft enclosure: The rated walls around a vertical duct or exhaust shaft that connects floors.
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
When each damper is required
Every result the tool can give for a duct carrying heating, cooling or ventilation air, or an air transfer opening, through this assembly. Labels show where a result applies only to ducts, only to air transfer openings, or only in some editions.
Fire damper
- Required
Shaft enclosure penetrations need fire and smoke dampers, and no fire damper exception applies.
IBC 717.5.3
- Alternative protectionDucts only
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
- Not requiredDucts only, 2021 and 2024 IBC
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
- 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
- Not required
The penetration was tested as part of the rated shaft assembly.
IBC 717.5.3 Exc. 1.2
- Not requiredDucts only, 2021 and 2024 IBC
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
- Not requiredDucts only, 2018 IBC only
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
- Building official approval neededDucts only, 2021 and 2024 IBC
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
- Not requiredDucts only, 2021 and 2024 IBC
No smoke damper is needed at that penetration either (IBC 713.12.1 covers fire/smoke dampers).
IBC 713.12.1
- 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
- Not requiredDucts only
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
- Not requiredDucts only
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
When a damper is not the answer
- Not permitted
A duct that does not serve the shaft’s own purpose (for example a supply or return duct crossing an elevator hoistway or a chute) may not penetrate the shaft wall, except as a membrane penetration on the outside of the shaft enclosure (IBC 713.8.1).
Also required
- The rooftop structure must comply with IBC 1511. Duct penetrations elsewhere in the shaft walls still follow the shaft damper rules. (ducts only, 2021 and 2024 IBC) IBC 713.12.1, IBC 1511
- Flexible ducts and flexible air connectors may not pass through any fire-resistance-rated assembly, and flexible air connectors may not go through any wall, floor or ceiling at all. (ducts only) IBC 717.7
- 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
- Ductwork for a stair’s own pressurization is allowed by IBC 1023.5. IBC 1023.5
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 |
| Supply duct crossing an elevator hoistway wall | Not permitted | Not permitted | Not permitted |
Select a result to open it in the tool with every answer filled in, including the conditions not listed here.
Exceptions and conditions the tool checks
These are the questions the tool asks, in order, with the explanation it gives. Section numbers are shown for each edition where they differ.
Is this a shaft that exists to carry ducts or exhaust?
A duct that does not serve the shaft’s own purpose (for example a supply or return duct crossing an elevator hoistway or a chute) may not penetrate the shaft wall, except as a membrane penetration on the outside of the shaft enclosure (IBC 713.8.1).
IBC 713.7.1, IBC 713.8.1
Is this where the shaft rises through the roof into a rooftop mechanical penthouse, with the shaft ductwork connected directly to the HVAC equipment?
2021 and 2024 IBC
The rooftop structure must comply with IBC 1511.
IBC 713.12.1
Is this a parking garage exhaust or supply shaft separated from other building shafts by at least 2-hour construction?
IBC 717.5.3 Exc. 1.4 and 3
Is the duct part of an approved smoke control system where a damper would interfere with how the system works?
Engineered smoke control systems (IBC 909) sometimes need ducts or openings to stay open during a fire. The smoke control design must show this.
IBC 909, IBC 717.2.1
Was this duct tested as part of the rated wall assembly itself (ASTM E119 or UL 263)?
Answer yes only if the fire-resistance design for this wall (for example a UL design) includes this penetration as tested. A listed firestop system tested to ASTM E814 or UL 1479 does not count.
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?
Is the building fully protected by an automatic sprinkler system?
- Yes, NFPA 13 throughout. IBC 903.3.1.1
- Yes, NFPA 13R throughout. IBC 903.3.1.2, used in some residential buildings
- No, partial or NFPA 13D only.
What is the required fire-resistance rating of the assembly?
Shafts need 2 hours where they connect four stories or more, otherwise 1 hour, and never less than the floors they pass through (IBC 713.4).
Code changes that affect this assembly
- Exhaust subducts in shafts: Since 2021 the subduct needs a minimum steel thickness and the shaft needs a continuously powered exhaust fan.
- Shafts with continuous upward airflow: New in 2021: dampers are prohibited where closing them would stop a required upward airflow.
- Shafts into rooftop mechanical penthouses: New in 2021: no fire/smoke damper where a shaft rises into a rooftop penthouse and connects directly to the equipment.
Questions
Do shaft penetrations need a combination fire/smoke damper?
Yes, unless an exception applies. Where both dampers are required, a combination fire/smoke damper or a separate fire damper and smoke damper may be used.
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
Do steel subducts remove the shaft dampers for bathroom exhaust?
In Group B and R buildings with NFPA 13 sprinklers, both dampers can be dropped.
Fire damper: not required · Smoke damper: not required2021 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: 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
Fire damper: not required · Smoke damper: not 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: 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
What happens with subducts in other occupancies?
The subducts still remove the fire damper, but not the smoke damper, and since 2021 a damper may not stop the required upward airflow.
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
Can a duct pass through an elevator or stair shaft wall?
No.
Not permittedAll three editions
A duct that does not serve the shaft’s own purpose (for example a supply or return duct crossing an elevator hoistway or a chute) may not penetrate the shaft wall, except as a membrane penetration on the outside of the shaft enclosure (IBC 713.8.1).
Also: Ductwork for a stair’s own pressurization is allowed by IBC 1023.5. IBC 1023.5