Stairs may be the hardest part of building a home. Only one or two out of every 10 framers are capable of configuring a stair opening without a detailed plan. Configuring a stair opening involves many variables. Making the correct calculations will enable you to create ergonomically correct stairs. Keep in mind that each set of stairs will be different and must be configured separately.
Determine the correct rise, run and pitch for your stairway by using the golden rule for stairs: rise x 2 + run = 24 to 25 inches. As an example: (7.25 x 2) + 10 = 24.5 inches. As with any pitch, rise and run dictate how steep or shallow the stairs are. The rise is the vertical distance from one tread to the next, and the run is equal to the tread (the horizontal part of each step). A 12/12 pitch—which is a 12-inch rise for every 12-inch run—will equal a 45-degree angle. This is much too steep for stairs. A 6/12 pitch is much closer to a stairway.
Check the height of your stairs. As an example, let's use a rise of 6-13/16 inches and a tread depth (or run) of 10-11/16 inches. These stairs will have a 6/10 pitch in this example. An 8-foot ceiling will have a distance of about 109 inches from one level to the next. At a rise of 6-13/16 inches, you will need 16 rises and 15 treads. Although in this example the numbers are exact, often there is usually a slight amount left over or under. The maximum variance allowed to prevent tripping is 1/4 inch, so make sure any variance remains within this amount. Take this variance out of the bottom step—on this step, there will be the least distance to fall if someone trips on it.
Check the length of the stairs by multiplying your tread depth by the number of treads. Example: 10-11/16-x-15-inch treads = 160-5/16 inches (total run). Now include space for an approach and landing by adding to your total run at least 39 inches of existing floor space to both ends for an appropriate approach and landing area. For example: 160-5/16 inch + 39 + 39 = 238-5/16 inch (total length of stairs). Last, your stairs should be about 39 inches wide.
Commit to a method. Now that you have an example, you can estimate if there is room for your stairs. Find a book or method and commit to it. This will avoid issues that may arise from combining methods. Use this article as a grading scale when choosing your book or method. Double-check your calculations before starting and have fun with your project.