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Stair Stringers (7) Stair stringers form the backbone of a set of stairs. By code, stairs need to be at least 36 wide. Although it's usually possible to get away with just using two stringers, we recommend you add a third one down the middle. This extra stringer will ensure your stairs are exceptionally rigid.
This compensates for the tread thickness you install. The top is also squared, with the back of the stringer cut to the length of the run. With the stringers thus laid out and readied, all that remains is to install each and run treads across the steps. Stairs4U: Stringer Layout - Learn How to Design a Stair Stringer.