How to Cut the Angle on a Stair Stringer

Place two clamps onto a framing square, with one clamp placed at the length of your proposed stair tread on one arm of the square (the stair run), and the other clamp placed at the length of your proposed stair risers (the stair rise).

Estimate how long each stair stringer will by measuring the diagonal distance on the framing square between the point of the rise and the point of the run, using the outside edge of the square as the points. Divide the height you wish to reach by the rise that you’re using to estimate the number of steps required. Multiple the number of steps, with one added as a safety margin, by the diagonal measurement, and you’ll have the estimated stringer length.

Set a 2-by-12-inch plank at least as long as the estimated stringer size across two sawhorses.

Place the framing square over the plank so that the two clamps are against the edge of the board. Move the framing square on the board until the outside edge of the square is flush with the corner on the side edge of the plank. The framing square should form an angle on the plank that resembles the rise and run of a step.

Hold the square in place and then trace along the outside edge of the square over the board with a pencil. The line running along the short width of the board will be the riser of your stair, while the line along the length of the board will be your tread.

Move the framing square down the length of the stringer until the edge is aligned with the marked thread of the first angle. Trace another angled step. Continue the process until you reach the end of the stringer.

Return to the first tread line where it meets the first riser line. Place a straightedge over the riser line and then continue the line across the entire width of the board. This line will be your cut line for the first riser.

Go to the last riser at the bottom of the stringer and make a mark down the height of the riser that’s the length of your proposed rise minus the height of the stair treads you intend to place onto the stringers.

Place a straightedge crossing the riser at this marked height at a 180-degree angle and then draw a line from the front of the riser to the rear of the panel to serve as a cutline for the base of the stringer against the ground.

Adjust the clamps on the framing square, raising each by the measured height of the stair tread. Place the framing square back over each marked step on the stringer, only lowering it along the riser lines until the tread edge of the square sits exactly the tread height distance lower in the plank than the previously marked tread lines.

Mark new tread lines at the new tread level to make an adjustment for the tread of the steps on the stringers. The new tread lines should extend from the rear riser to the front riser of each step.

Cut the stringer with a circular saw along the new tread lines, and then cut along the riser lines to create each step position. Make a last cut along the marked line for the base of the stringer against the floor. Do not overcut the drawn lines, as this would weaken the stringer. Instead, use a handsaw to cut the joining corners between the tread and stringer lines.

Use the first stringer as a template for marking the angles onto the other stringers needed for the staircase. Cut the other stringers using the marked lines with the circular saw, finishing the corners of the cuts with the handsaw as well.