Example: Total rise 118”/ 7. 5 = 15.73 — round up the total number of risers to 16 Total rise 118”/16 risers = 7-3/8” each rise
Knowing the number of risers tells you the number of tread — 15 (the sixteenth riser will be positioned approaching the upper floor with no tread on top). The run of the stair will then be 15 treads at 10 inches per tread, or 150 inches of total run. To determine the actu- al total length of the stair, you must add the nosing of the bottom step and the thickness of the top riser.
Example: Total tread run 150” + 1-1/4” nosing + 3/4” top riser = 152” total stair length
Next, calculate the length of the stairwell, or the width of the upper floor’s vertical shaft in which the stairs are located. This is a two-step calculation. First, account for the required headroom and the upper-floor construction, including floor-joist height, floor thickness and drywall thickness. For the purpose of our example, we will calculate the upper-floor con- struction to be a total thickness of 12-1/2 inches. This figure (12-1/2 in.) added to the desired head- room height of 80 inches will total 92-1/2 inches. Take this dimension and divide it by the riser height.
Example: 92-1/2” / 7-3/8” = 12.542
The answer ( 12.542) is the number of treads needed in the clear opening to make headroom. This would mean you now have 2. 45 treads that are located under the header. By multiplying 12.542 (number of treads in clear opening) by 10 inches (tread depth dimension) and adding 1-1/4 inch for the nosing and 3/4 inch for the top riser, you will achieve the stair- well length needed for the proper headroom. The result for our example is 127-7/16 inches for stairwell length. Most stairs that are located between two walls have a finished width of 36 inches. To accommodate this finished width, you will need to make your rough opening 37 inches.
The end result of our layout procedure is: 10” run, 7-3/8” rise, 127-7/16” stair well length, 37” stair well width, 36” stair finish width.
CUTTING THE STAIR Now that we have directions on paper as to how to build our stair, it is time to begin cutting. Carpenters always measure twice and cut once, a good rule to follow when cutting stairs. Stair building can be expensive; you only want to purchase the materials once.