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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.
Note that this is parallel to the nose line of the finished stairs, but is not the same as the nose line. Make another line parallel to the first line, 11.25 inches apart. This denotes the width of a 2x12, the standard material used for stair stringers.