Step 4 · Calculate Tread Depth and Nosing

The tread depth is the distance from the front edge of one tread to the front edge of the next. For alternating tread stairs, this is often referred to as the 'run' per step. The 'nosing' is the part of the tread that overhangs the riser below. The combination of riser height and tread depth must comply with building codes to ensure safe footing. A common formula relating riser height (R) and tread depth (T) is 2R + T = 24-25 inches. However, for alternating tread stairs, the tread itself may not have a uniform depth, with one side being deeper than the other.