2010 ADA Standards for Accessible Design Pocket Guide

608.7 Thresholds.

Thresholds in roll-in type shower compartments shall be ½ inch (13 mm) high maximum in accordance with 303. In transfer type shower compartments, thresholds ½ inch (13 mm) high maximum shall be beveled, rounded, or vertical.

EXCEPTION: A threshold 2 inches (51 mm) high maximum shall be permitted in transfer type shower compartments in existing facilities where provision of a ½ inch (13 mm) high threshold would disturb the structural reinforcement of the floor slab.