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How To Calculate The Unit Rise And Unit Run Of A Stair

Before stairs can be built, the unit rise and unit run need to be calculated. See figure 44A. To calculate the unit rise, measure the total vertical distance between finished floor to finished floor. Assume a total rise of  2100 mm and a desired rise of 180 mm. The desired rise is the height you want for each step. Divide the total rise by the desired rise to get the number of unit rises. 2100 mm divided by 180 mm =  11.67 unit rises. To make all unit rises equal, round off to the nearest whole number. 11.67 becomes 12. Divide the total rise by 12. 2100 divided by 12 =  175. 175 mm will be the height of each unit rise and riser. As a check, 175 mm times 12 =  2100 mm, the total rise. A unit rise of 175 is acceptable, according to Table 9.8.3.1., of Page 281 of the British Columbia Building Code 1998; because it lies between the upper and lower rise limits of 200 mm and 125 mm.

To calculate the unit run, subtract the sum of two unit rises from 550 and 700. The sum of one unit run plus two unit rises can not be less than 550 mm nor more than 700 mm. 175 + 175 = 350. 550 - 350 =  200. 700 - 350 =  350. 355 mm  and 210 mm are the maximum and minimum unit runs allowed according to Table 9.8.3.1. A run ranging from 210 mm to 350 mm can be chosen.

Figure 44A  The Parts Of A Stair

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1.Stairs: Calculate The Unit Rise & Unit Run Of Stairs.
2.Stairs: Calculate Stairwell Openings.
3.Stairs: Layout A Stringer When The Floor Frame Is The Top Step.
4.Stairs: Layout A Stringer When The Top Step Is Attached To The Floor Frame.
5.Stairs: Layout The Rise & Run Along The Stringer.

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Ronald Hunter

All images and text are copyright Ronald Hunter 2005 to 2011