What is the minimum tread depth required for spiral stairs measured from the narrower edge?

The minimum tread depth for spiral stairs, as specified in building codes, is typically 7 1/2 inches when measured from the narrower edge. This measurement is important for safety and usability, ensuring that users have enough space to place their feet securely while navigating the stairs. A depth of 7 1/2 inches strikes a balance between providing adequate support and maintaining the compact design that spiral stairs are known for.

While other options suggest greater depths, they do not align with the established minimum requirement that aims to provide a safe and comfortable experience for users. These greater depths may exceed what is necessary for proper function in a spiral stair design, which is meant to save space while still allowing for safe passage.