R311.7.5.2.1

Section R311.7.5.2.1 is titled 'Winder treads' and covers the requirements for winder tread dimensions. A minimum depth at the walkline and any point within the clear width, method of making depth measurement at the walkline, and depth consistency at the walkline within a flight are established. The section also clarifies that winder treads depth at the walkline does not need to be consistent with rectangular treads in the same flight. The purpose of this section is to reduce trip and fall hazards.

The model shows a partial stairway which includes winders. The measurement of the winders' tread depth are highlighted between the intersections of the vertical plane projection of the nosings and the walkline. Also shown is a vertical plane from the outside edge of the inside handrail to the treads, which is where the minimum winder tread depth measurement must be taken, and the minimum tread depth measurement highlighted.

Not shown in the model is the requirement for consistent winder tread depth within a flight. An inspector needs to measure every winder tread in every flight to assure that in each flight, the largest winder tread depth doesn't exceed the smallest by more than 3/8".