Alternating Tread Flight
By Huu Khanh Nguyen · Updated October 28, 2026 · stair, flight
A steep stair flight of staggered half-width treads, each step on the left or the right of a central stringer in turn.
Parameters
| Name | Default | Unit |
|---|---|---|
| Number of risers (the flight has risers - 1 treads, alternately left and right) | 8 | |
| Riser height, vertical distance between two successive treads | 173.7 | mm |
| Going, horizontal distance between the noses of two successive treads (y) | 117.2 | mm |
| Clear width of the flight between the handrails (x) | 508 | mm |
| Tread depth, front to rear (y) | 266.7 | mm |
| Tread thickness passed to the tread (z) | 38 | mm |
| Overlap of a tread past the centre line, negative leaves a gap (x) | -25.4 | mm |
| Spine width across the flight (x) | 50.8 | mm |
| Spine depth normal to the slope | 44.45 | mm |
| Finish of the flight's own parts | white | |
| Slope of the flight from the horizontal, degrees (OSHA 50 to 70) | Math.atan(rise / going) * 180 / Math.PI | ° |
| Bracket length along the tread, shortened where the floor would cut the seat | Math.min(88.9, (rise - thickness - 6.35) / (rise / going)) | mm |