A Typical Stair Tread

Widths Vary

• Tread widths will vary with the length of the stairway. A longer stair will use wider treads, a shorter one narrower treads. The angle of slope also influences tread width. Stairs on gentle slopes use wider treads and shorter risers, steep ones taller risers and shorter treads. Using the 17 1/2 rule, a stairway with 7-inch risers would have 10-inch treads.

Optimum Widths

• According to a National Institutes of Health publication, the optimum tread distance is 11 to 12 inches; the optimum rise was 7.2 inches, which pretty much confirms the 17 1/2 formula. The study covered male and female subjects of all ages. Most building codes require a minimum tread of 8 1/4 to 9 inches.

Calculating Width

• Basic tread width is calculated by dividing the total length of the stairway adjusted for the angle of the stairs. A stairway 10 feet long that needs 15 steps to cover the rise or angle would need treads of 8 inches. Adjusting the number of steps to 13 would get the tread width to 9 inches. Properly designed steps adjust these variables until treads and risers are in balance.

Curved Treads

• Curved stairways present special tread considerations. Treads on curved or "winder" stairs vary in width and the narrow end must still be wide enough to step on. Most building codes specify at least a 6-inch minimum depth at the narrow end, although specific formulas vary. Some are based on a minimum tread width 9 inches from the narrow end, for instance.

Landings

• Landings, points where stairs change direction or pause in the upward slope, can vary in width but need to be at least as long as the width of the steps. A 36-inch wide stairway, for instance, would need 36-inch wide landing treads. Landings are used on straight stairs most often when the slope is gradual; this allows for a taller and more comfortable riser height. Risers must be more than 4 inches high for comfort.