General Definition
- Stairway width requirements define the minimum clear width of a stairway to ensure safe and efficient evacuation during daily use and emergencies.
- These requirements vary based on factors such as occupant load, building use, fire safety, and accessibility standards.
- Both the International Building Code (IBC – USA) and the Building Standards Law (BSL – Japan) establish minimum stairway widths to ensure safe movement of occupants, with additional requirements for high-occupancy, high-rise, and public-use buildings.
- Stairway width is a crucial factor in ensuring safe egress, accessibility, and compliance with fire safety regulations.
- Both IBC (USA) and BSL (Japan) establish minimum stair width requirements, considering factors such as occupant load, fire protection, and building type.
IBC (USA) – Stairway Width Requirements
The International Building Code (IBC 2018, Section 1011.2) mandates minimum stairway widths based on occupant load and usage.
Key Requirements
- Minimum Stairway Width for Occupant Load > 50:
- 44 inches (1118 mm) minimum width for stairs serving an occupant load greater than 50 people.
- This ensures sufficient space for evacuation in case of an emergency.
- Reduced Stairway Width for Small Occupant Loads:
- 36 inches (915 mm) minimum width for stairways serving an occupant load of 50 people or fewer.
- Increased Stairway Width for High Occupant Loads:
- Additional width is required when the total required egress width exceeds the minimum 44-inch requirement.
- The formula 0.3 inches per occupant (sprinklered) or 0.2 inches per occupant (non-sprinklered) applies when determining total required width.
- Fire Safety Considerations:
- Stairways that serve high-rise buildings, assembly occupancies, or hazardous facilities may require wider stairs for safer egress.
- Smoke-proof enclosures may also impact stairway width requirements.
- Accessibility Compliance (ADA & ICC A117.1):
- Handrails must not reduce the required clear width by more than 4.5 inches on each side.
- Minimum headroom clearance = 80 inches (2032 mm) to prevent obstruction.
- Tactile and visual signage is required for wayfinding.
BSL (Japan) – Stairway Width Requirements
Japan’s Building Standards Law (BSL, 建築基準法, Article 119) establishes stairway width requirements based on building type and occupancy size.
Key Requirements
- Minimum Stairway Width for Large Office Buildings:
- 120 cm (47.2 inches) minimum width for stairs in large office buildings.
- This is wider than the IBC’s 44-inch requirement, emphasizing safer evacuation and ease of movement.
- Standard Minimum Stair Width for Most Buildings:
- 90 cm (35.4 inches) minimum width for general buildings and residential structures.
- Wider Stairs Required for High-Rise & Public Buildings:
- Stair width increases to 150 cm (59 inches) or more in high-occupancy office buildings, hospitals, and public-use facilities.
- Fire Safety Considerations:
- Stairs in high-rise buildings must be fire-resistant and smoke-proof to prevent fire spread during evacuation.
- Additional stairwells may be required if stair width alone cannot accommodate safe evacuation.
- Universal Design & Barrier-Free Compliance:
- Tactile guidance strips and handrails must be included to assist visually impaired individuals.
- Stairways must be designed for ease of use by individuals with disabilities, ensuring step consistency and non-slip surfaces.
Comparison Table: IBC vs. BSL – Stairway Width Requirements
| Feature | IBC (USA) – Required | BSL (Japan) – Required |
| General Minimum Stair Width | 44 inches (1118 mm) for occupant load > 50 | 120 cm (47.2 inches) for large office buildings |
| Smaller Occupant Loads (<50 people) | 36 inches (915 mm) | 90 cm (35.4 inches) |
| Wider Stairs for Large Facilities | Width increases based on occupancy load calculation | 150 cm (59 inches) or more for high-rise & public buildings |
| Fireproof & Smoke-Proof Requirements | Required for high-rise & hazardous buildings | Required for high-rise & public buildings |
| Barrier-Free Compliance | ADA / ICC A117.1 standards | Universal design & tactile guidance strips required |
Case Study
Stairway Width Design for an Office Building – Honolulu vs. Tokyo
- Project Overview:
- A company is constructing a 6-story office building with a total occupant load of 350 people per floor. The project is being developed in:
- Honolulu, USA (IBC jurisdiction)
- Toyosu, Japan (BSL jurisdiction)
- A company is constructing a 6-story office building with a total occupant load of 350 people per floor. The project is being developed in:
- The design team must ensure that stairway widths comply with both IBC (USA) and BSL (Japan) requirements.
Honolulu (IBC) Compliance
- Challenge:
- The original stair width design was 44 inches, which meets the minimum IBC requirement but may not accommodate 350 occupants efficiently.
- Solution:
- Increased stair width to 52.5 inches (1334 mm) based on the occupant load calculation.
- Ensured handrails do not reduce the required clear width by more than 4.5 inches per side.
- Installed tactile warning strips and non-slip surfaces for ADA compliance.
- Incorporated fire-resistant materials for high-rise building compliance.
Tokyo (BSL) Compliance
- Challenge:
- The original stair width design of 44 inches did not meet BSL’s 120 cm minimum for large office buildings.
- Solution:
- Increased stair width to 120 cm (47.2 inches) to meet BSL requirements.
- Ensured tactile guidance strips and handrails comply with barrier-free regulations.
- Designed stairwells with fire-resistant partitions to meet fire safety standards.
- Allowed for a wider 150 cm (59 inches) stair width in public-use areas.
Final Universal Design Approach for Both Locations
To ensure compliance with both IBC (Honolulu) and BSL (Tokyo), the project adopted a universal design strategy:
- Designed stairways at 120 cm (47.2 inches) wide, exceeding IBC’s 44-inch minimum and meeting BSL’s requirements.
- Incorporated fire-resistant stair enclosures to comply with both IBC and BSL fire safety standards.
- Installed tactile warning strips, non-slip surfaces, and universal handrails for accessibility.
- Maintained a consistent riser height and tread depth to improve safety and usability for all occupants.
Key Takeaways
- BSL requires wider stairways (120 cm / 47.2 inches) in large office buildings, while IBC requires a minimum of 44 inches (1118 mm) for occupant loads > 50.
- IBC uses an occupancy-based width calculation, whereas BSL sets fixed width minimums for different building types.
- BSL enforces stricter fireproof stairwell designs, ensuring safer egress in high-rise office buildings.
- A universal stair design at 120 cm (47.2 inches) ensures compliance with both codes while improving safety, accessibility, and egress efficiency.
Global Approach
To establish cohesive and universally applicable stairway width standards in Group B occupancy buildings, integrating the International Building Code (IBC, USA) and the Building Standard Law (BSL, Japan), the following global approach is recommended:
- General Minimum Stairway Width
- Recommended Global Minimum: 44 inches (1120 mm)
- Ensures broad compliance and comfortable, safe passage for occupant loads over 50, aligning closely with IBC and slightly exceeding BSL requirements.
- Smaller Occupant Loads (less than 50 persons)
- Recommended Global Minimum: 36 inches (915 mm)
- Matches IBC guidelines and accommodates practical circulation, aligning closely with BSL smaller occupancy standards.
- Wider Stairs for Large or High-Rise Facilities
- Width should incrementally increase based on calculated occupant load, with a minimum width of:
- 59 inches (1500 mm) for high-rise buildings, large public facilities, or significant occupant loads.
- Ensures safe and efficient evacuation, meeting or exceeding stringent requirements from both IBC and BSL.
- Width should incrementally increase based on calculated occupant load, with a minimum width of:
- Fireproof and Smoke-Proof Requirements
- Stairways in high-rise buildings and facilities with increased fire risk must be fire-rated, smoke-proof, and pressurized to prevent smoke infiltration and maintain clear egress paths.
- Complies with rigorous standards from both jurisdictions, enhancing global occupant safety.
- Barrier-Free and Universal Design Compliance
- Incorporate ADA/ICC A117.1 (USA) standards alongside universal design principles including tactile guidance strips and accessible handrails as required by Japanese guidelines.
- Ensures stairways accommodate users with diverse needs, promoting inclusive access and international compliance.
Implementing these harmonized global guidelines provides a safe, inclusive, and regulatory-compliant stairway design suitable for international building projects.