Residential stair design: What is the recommended minimum clear width of a stair in typical residential buildings?

Introduction / Context:
Stair width affects usability, safety, and evacuation. Residential requirements are less onerous than public buildings, but minimum widths should still allow comfortable and safe passage for occupants carrying items such as laundry or furniture.

Given Data / Assumptions:

• Typical single-family or small residential building.
• Local byelaws vary, but common practice provides a baseline.
• Clear width is measured between finished faces of handrails/walls.

Concept / Approach:
Many residential standards adopt a practical minimum near 0.85–0.90 m for main stairs, balancing circulation comfort and plan economy. Wider stairs are encouraged for duplexes or frequent furniture movement, whereas public buildings typically require ≥1.0 m to 1.2 m or more depending on occupancy. For tight plans, 0.85 m is a sensible lower limit to avoid awkward passing and to accommodate hand clearance.

Step-by-Step Solution:
Identify the building type: residential (not assembly or institutional).Apply common practice minimum width for residential stairs.Among choices, 85 cm matches typical lower bound; narrower widths (55–70 cm) are impractical and unsafe.Select 85 cm as the recommended minimum.

Verification / Alternative check:
Several handbooks recommend 0.85–0.90 m minimum for residential stairs, with 1.0 m or more preferred for comfort.

Why Other Options Are Wrong:
55 cm, 70 cm: Too narrow for safe two-way maneuvering and moving objects.100 cm, 120 cm: Acceptable or preferred, but not the minimum in small residences.

Common Pitfalls:
Ignoring handrail intrusions into clear width; locating switches, risers, or nosings that reduce usable width at turns or winders.

Final Answer:
85 cm.