Stair stringers refer to stairs constructed from structural lumber that are supported by 2 or 3 diagonal members called stringers against which the ends of the risers (the vertical face of the step) and treads (the horizontal surface of the step) are terminated. Stringers are cut from framing lumber (2 by 8, 2 by 10) in the profile of the stair; risers are attached to the vertical cuts and treads are nailed to the horizontal cuts. Stringers cut to the proper dimensions are essential for building a solid set of stairs.
Design the Stairs
The first step is determining the size of the stair components by obtaining the overall dimensions. The total number of risers (the total rise) is equal to the overall dimension between the finished floor levels. The number and size of the risers are found by dividing this distance into risers of equal height, providing the riser height does not exceed local building codes. An overall vertical height between floors of 9 feet 4 inches would give 16 risers each at 7 inches. The overall horizontal length of the stairs (the total run) is the total number of treads. Treads 11 inches or slightly greater in width will give a comfortably sized step and are commonly used for residential stairs. The number of risers will determine the number of treads; the number of treads is always 1 less than the number of risers. 15 treads at 11 inches each will give an overall horizontal length, or a total run of 13 feet 9 inches.
Select the Stringer Boards
Choose straight lumber for the stringers, meaning boards that are neither bowed nor twisted. The width of the stringer boards is selected so that a 4-inch distance is maintained between the inner corner of the riser/tread cut perpendicular to the lower edge of the stringer.
Determine the Tread Width
The length of the tread cut on the stringer will not equal the width of the finished tread. The part of a tread that projects beyond the outer edge of the riser is referred to as a nosing. Allow for nosings when designing the stairs and cutting the stringer. Nosings typically extend ½ inch beyond the riser. The finished tread will be ½ inch plus the thickness of the riser greater than the tread cut into the stringer.
Layout and Cut the Stringers
The stringers must be cut to fit the stair opening in length and in width, with the riser cuts plumb and the tread cuts level when the stairs are in place. Lay a framing square flat on the board so that the horizontal blade aligns with the top edge of the board at the length of the tread and the vertical blade is aligned at the length of the riser. Trace out the step and continue to slide the square down the board at the same angle so that the total run is stepped off horizontally in equal increments. Scribe a plumb cut at the top end and a level cut at the bottom end and cut the stringer pattern with a circular saw. Using the pattern as a template, trace out and cut the other stringer or stringers.