How to Build a Stair Run

Measure the distance, from bottom to top, that the stairs will span. Run the tape measure vertically, at a right angle to the surfaces, for an accurate measurement. The distance found is the total rise of the stairs. Record this figure.

Divide the total rise by 7, which is the ideal step rise, to find the number of steps required. For example, a total stair rise of 36.5 inches divided by 7 inches rise per step results is 5.21 steps. Since you cannot have 5.21 steps, you round the rises appropriately -- in this case, down to 5 steps.

Calculate the rise height for each step by dividing the total rise by the number of steps as previously determined. For instance, 36.5 inches total rise divided by 5 steps results in 7.3 inches each. Thus, every step is 7.3 inches above the one before it.

Write down the depth of the run you desire for each step. The ideal run is 11 inches, but you may prefer a deeper or shallower tread area.

Multiply the number of steps previously calculated by the step run desired to determine the total run of the stairs -- the length the stairs extend out from the top structure. For the example given of a 36.5 inch total rise with 5 steps, a run of 11 inches results in 5 multiplied by 11. The stairs will thus extend out 55 inches from the top structure.

Apply the Pythagorean Theorem to find the length of the stair stringer you need. The total rise and the total run form two sides of a right triangle since they meet at a 90-degree angle. The stringer is the hypotenuse. Square the total rise and square the total run previously determined. Add both together, then find the square root of this number to determine the stringer length.

For example, 36.5 inches total rise squared is 1,332.25, and 55 inches total run squared is 3,025. Added together, the result is 4,357.25. The square root of this is 66 when rounded down. Thus, the stringer length required is 66 inches.

Align a short, thin board or ruler with the step rise measurement on one end of a carpenter's square and the step run measurement on the other end. Clamp or tape the board securely in place. This transforms the square into a right triangle with a guide for outlining the steps.

Set the square on a 2-by-10 board cut to length. Allow the guide to rest flush with the board edge, causing the legs of the square to extend onto the board. Lay the square with the rise on the left to begin with a step up.

Cut the board, following the cut guide created. Use a handsaw at the inner portion of the cut to avoid cutting past the desired step run into the stringer. Square off the top or bottom ends as needed.

Hold the stringer in place and check for accuracy. Note any adjustments needed. If the stringer is accurate, lay it on top of another 2-by-10 and outline the steps. Mark and cut one stringer per every 12 to 16 inches of stair width for best results.

Attach the stringers in position. Nail wherever the stairs come in contact with the wood at the top or bottom of the stairs along with the sides. Alternatively, use mounting brackets and bolts as appropriate.

Cut 1-inch tread boards the width of the stairs. Nail across each step with 10d nails sunk into the stringers. Add boards to enclose the back of each step -- along the rise -- and attach similarly if desired.