2013 California Standards for Accessible Design Pocket Guide

11B-504.3 Open risers.

Open risers are not permitted.

Exceptions:

1.      On exterior stairways, an opening of not more than ½ inch (12.7 mm) may be permitted between the base of the riser and the tread.

2.      On exterior stairways, risers constructed of grating containing openings of not more than ½ inch (12.7 mm) may be permitted.