R311.7.11.1

Section R311.7.11.1 is titled 'Treads of alternating tread devices' and covers the requirements for several design details of an alternating tread device. Tread depth, projected tread depth, tread width, riser height, method for measuring tread depth and riser height, angle of ascent, and initial tread location are established. The purpose of this section is to reduce trip and fall hazards.

The model shows an alternating tread device with minimum dimensions shown (unless otherwise noted), and the allowed range of angle of ascent. The intent of establishing the initial tread to begin at the same elevation as the platform, landing, or floor surface, is to assure that the first step up is no more than 9-1/2". The device does not actually have a tread at the same elevation as the floor - the floor itself is considered the first tread.