When should a third stairway stringer be used?

A third stairway stringer is appropriate when the stair width exceeds 30 inches. This requirement ensures proper support and stability for the tread and riser system, particularly in wider stairs where increased weight and usage can affect the safety and structural integrity. The addition of a third stringer helps distribute the load more evenly across the stairway, reducing the risk of sagging and providing a safer means of egress.

Wider stairs not only support more significant weight but also accommodate more foot traffic. Without appropriate support, wider stairs can lead to dangerous conditions, such as structural failure or discomfort for users. Therefore, establishing these guidelines is critical for both safety and compliance with building codes.

The other options suggest using a third stringer for higher widths, which may not align with standard construction practices and safety regulations. For stair widths exceeding 36 or 48 inches, while additional considerations for support may become necessary, the established threshold of 30 inches is widely accepted in building guidelines for requiring a third stringer.