Height over Stairs

These clear heights are intended to provide sufficient headroom clearance to prevent a reasonably tall adult from making accidental head contact with the ceiling or other objects at that height (e.g., light fixtures, sprinklers heads, or piping). The lower minimum clear height for stairs in dwelling units is rationalized on the basis of greater occupant familiarity. 

The clear height requirements for stairs are illustrated in Figure 9.8.-1.

Quoted from Illustrated User's Guide - NBC2015