What is the minimum size for a stair tread including nosing?

What is the minimum size for a stair tread including nosing?

• A. 210 mm to 235 mm with nosing
• B. 180 mm to 210 mm with nosing
• C. 235 mm to 260 mm with nosing
• D. 200 mm to 230 mm with nosing

Answer: (A)

The important idea is that tread depth must be sufficient for a safe step, and when a nosing is present you measure the depth including that nosing. The minimum depth that accommodates the nosing and still provides a safe stepping surface is about 210 mm, with typical code allowances extending the range up to around 235 mm depending on how the nosing is shaped. So the smallest acceptable depth including the nosing is 210 mm, which is why the range 210 to 235 mm is the correct choice. Ranges starting below 210 mm would be too shallow to step safely, while higher ranges meet the requirement but aren’t the minimum.

The important idea is that tread depth must be sufficient for a safe step, and when a nosing is present you measure the depth including that nosing. The minimum depth that accommodates the nosing and still provides a safe stepping surface is about 210 mm, with typical code allowances extending the range up to around 235 mm depending on how the nosing is shaped. So the smallest acceptable depth including the nosing is 210 mm, which is why the range 210 to 235 mm is the correct choice. Ranges starting below 210 mm would be too shallow to step safely, while higher ranges meet the requirement but aren’t the minimum.