Treads on a stairway measure between 9 and 11 inches deep at different steps. The concern is:

Descending a stair is the more dangerous direction, and it is where inconsistent tread depth does its damage. Going down, the walker cannot see the tread they are stepping onto — the nosing above hides it — so foot placement is predicted from the previous step. A tread two inches shallower than the last leaves the heel over the edge or the foot placed short, and the fall that follows happens with the body's weight already moving forward and downward. That is why tread depth uniformity carries the same weight as riser uniformity in stair safety, and why the tolerance across a flight is small. A two-inch variation between steps in one flight is well outside any accepted range, and the inspector reports it as a fall hazard rather than as a construction irregularity.

Descending users place their feet by prediction because the nosing hides the tread below. Inconsistent depth breaks that prediction at the moment weight is committed forward and downward, which is a fall hazard.

Stairways require uniform tread depth and riser height within a small tolerance across a flight, because users navigate stairs by prediction rather than by looking at each step. Descent is the higher-risk direction because treads are visually obscured by the nosing above.

Down Is the Dangerous Direction. You cannot see the tread you are stepping onto — you are trusting the one before it to be the same.

Report the measured range rather than a single figure. 'Treads vary from 9 to 11 inches' communicates the hazard far better than an average.

• •Measuring one tread and one riser rather than comparing across the flight
• •Treating tread variation as a construction quality note
• •Assuming carpet or refinishing addresses the hazard