Stringers--the sloping sides of stairs that support the treads and risers--are the key to a structurally sound staircase. Laying them out and cutting them involves some simple geometry that provides an understanding of how stairs are designed and built. Local and national building codes contain requirements relating to stair construction, so consult these before getting started.
Measure the vertical distance between the surface of the first floor and the surface of the second floor. This measurement will be used to determine the number of steps needed for the stairs. Divide the measurement by a number that gives you a step height, or "riser," between 7 and 8 inches each. The standard residential step is about 7 1/2 inches.
Multiply the number by the tread depth to determine the "run" of the stairway. Stair treads are typically 11 1/2 to 13 inches wide, including an overhang or "nosing". Determine the approximate length of board needed for each stringer by multiplying the number of treads by 15 inches.
Place one 1 1/2-inch by 11 1/2-inch board on sawhorses for layout. This board will be the first stringer and the template for marking the remaining two stringers.
Define one end of the board as the base and place the framing square on the edge from which the tread and riser notches will be cut. Adjust the framing square so that the shorter arm aligns the riser measurement determined in Step 2 with the edge of the board. Adjust the longer arm to align the tread depth determined in Step 3 with the board's edge.
Hold the framing square securely in place and mark a pencil line along the edge of the square at both angles. Note: All marking must be done from the same edge (inside or outside) of the framing square for consistency.
Mark a 90-degree angle from the first tread line, where it meets the outside edge of the stringer. This is the cut line of the first riser.
Measure and mark the first riser line at the riser height from Step 2 minus the thickness of your tread material. (This cut will sit on the floor at the beginning of the stair.)
Repeat Step 6, placing the tread measurement at the edge end of the previous riser mark. This step is repeated until the correct number of treads and risers established in Step 2 have been marked on the stringer board.
Determine the top end cut by marking a 90-degree angle to the tread line, measured from the front edge of the step line to the depth of the tread, minus the nose overhang and thickness of the riser material. For a 1-inch tread nose overhang and 3/4-inch riser boards, the cut would be the tread width minus 1 3/4 inches.
Cut the stringer along the lines with a circular saw. Stop cutting when you reach the adjacent line, then use a handsaw to finish the saw cuts at the inside corners.
Verify the fit of the stringer by holding it in place at the stair location. Use the completed stringer as a template to mark the remaining two stringer boards for cutting. Stringers are installed with one on each side of the stair well opening and one positioned at the centre.
Small metal guides are available that attach to the framing square to define the tread and riser measurements and make repetitious marking easier. These inexpensive guides are sold at hardware and building supply stores.
When the first stringer has been marked with all the steps and risers, it is a good idea to make your top end and bottom base cuts and hold it in place before cutting tread and riser cuts. This allows you to confirm that your angles and cut layout are correct before making all your cuts and perhaps wasting material.