Staircase design — minimum headroom clearance: What is the minimum head room that must be provided over a stair (measured vertically from the nosing line to the soffit above)?

Introduction / Context:
Head room over stairs ensures safe passage without the risk of head injury. Building practices and many codes prescribe a minimum vertical clearance measured from the line joining the nosings of treads to the underside of the soffit or any obstruction above.

Given Data / Assumptions:

• Conventional residential or institutional stair in RCC/masonry.
• Vertical headroom measured along the stair flight.
• Standard practice values expressed in centimeters.

Concept / Approach:
Common practice and many code-based guides adopt about 2.1 m as the minimum headroom for comfort and safety. This allows users to traverse the stair without having to stoop, accommodates taller individuals, and permits finishes/services below the slab or floor framing without infringing on clearance.

Step-by-Step Solution:
Identify the metric minimum: approximately 2.1 m = 210 cm.Check listed options and select 210 cm as the standard minimum.Note that larger values (e.g., 230 cm) are acceptable but not the minimum requirement.

Verification / Alternative check:
Architectural data handbooks and model codes internationally tend to cluster the minimum headroom around 2.0–2.1 m, with 2.1 m widely adopted for comfortable use and to reduce maintenance clashes with services.

Why Other Options Are Wrong:

• 200 cm / 205 cm: Often considered marginal or below many standard recommendations, potentially uncomfortable.
• 230 cm: Permissible but not the stated minimum.

Common Pitfalls:
Measuring from the tread rather than the nosing line; forgetting finishes/plaster thickness that can reduce actual clearance below the required minimum; ignoring soffit drops or duct runs.

Final Answer:
210 cm