How to Cut Stair Stringers

Calculating Your Measurements

Measure out how much total rise your stairs need. Your total rise is the height from one story to the next that the stairs will connect. The total rise should be set in most cases, so you simply need to measure the height. This distance should be from the top of the finished floor below and the finished floor at the top. If the floor is unfinished when you are calculating your stringers, you need to take that into account.

Determine how tall you want each step to be. The height of each step can vary a bit but there are general standards. It is standard to have your riser be about 7 inches (18 cm) tall, so use this measurement unless you have specific parameters that prevent it, such as limited headroom. This measurement is sometimes called the individual rise of the stairs.

Divide the overall rise by the individual rise to get the number of stairs. For this calculation you are simply figuring out how many stairs it will take to get the height you need. Use a calculator or do the arithmetic by hand, as it is generally pretty simple. For example, if you want 7 inches (18 cm) tall stairs and your overall rise needs to be 56 inches (140 cm), then 56/7=8. You will need 8 stairs. The number of treads you have (individual runs) will automatically be one less than the number of risers.

Determine how much run each stair will need. Your total run is the horizontal distance between the top and the bottom of the stairs. How far the stairs stick out can vary depending on your needs, but keep in mind that stairs should usually be at an angle of about 40 degrees. In general, the run of each stair should be about 10 inches (25 cm) in order for people’s feet to fit comfortably as they walk up them. There are many stair calculators that you can use online to figure out stair measurements. You simply need to input your rise and the angle you want for your stairs and they will calculate the other measurements you need, including your run.

Calculate how long the stringer needs to be. Once you have the overall rise and run of the stairs, you can calculate how long the stringer needs to be. You can either use an online stair calculator, a hypotenuse calculator, or you can do the math yourself. To calculate the length yourself, you will need to use the Pythagorean theorem, which is a + b = c. So, for example, if the rise needs to be 60 inches (150 cm) and the run needs to be 84 inches (210 cm), then you would calculate 60 + 84 = c, with “c” being the length of 99 inches (250 cm).

Double check your measurements before you start marking your cuts. Ensure your math and calculations are correct so that you don’t waste your time and materials cutting risers that don’t fit. It’s better to take a few minutes to double check yourself than to waste a whole lot of time redoing your work.

Marking Your Cuts

Start laying out the first stair. Place a framing square near the end of a 2 x 12 (38 x 286 mm) board, leaving a few inches at the end before the end of the square. Use the individual rise and run figures marked on the outside scales of the square that matches your desired measurements. These figures should touch the upper edge of your board. The short end of the square (tongue) should be on the rise measurement. The long end of the square (body) should be on the run measurement. The board should be at least 12 inches (30.48cm) longer than the planned length of the stringer, just to give yourself some play.

Mark the outline along the square's outer edges. Move the square down to extend the run line to the lower board edge, if necessary. This is the outline of your top stair.

Lay out the rest of the stairs. Slide the framing square along the board so your scale run figure touches the end of your first marked run line. Double check that your rise and run figures are lines up with the top edge of the board and then mark your second stair. Continue down the board, making sure to line up the scale's rise and run figure at the top edge. Mark the new outline and repeat until you have marked 1 extra pair of runs and rises.

Mark the bottom of the stringer. In order to make the first step the same height as the others, you need to subtract the depth of the thread from the rise, so the finished stair is still 7 inches (18 cm). Simply make another mark to the right of the run line that is parallel and equal to the thickness of the thread. This marks the stringer bottom.

Making Your Cuts

Prepare to use a circular saw safely. Put on your personal protective equipment, including safety glasses. It's also important to use general safety precautions when using a circular saw. These include keeping your body parts away from the blade while the tool is running and being sure that the cord is out of the path of the tool as it's running. You should also clamp down your stringer board, so that it doesn’t move while you cut it.

Cut down the lines you marked on the stringer with a circular saw. Start the saw running before it makes contact with the wood. Then slowly work the saw in from the outer edge of the mark towards where it ends, where the rise and run meet. Leave about .5 inches (1.3 cm) of the line uncut, which you will finish with a hand saw.

Finish your cuts with a handsaw. Cutting beyond where the rise and run lines meet can severely weaken your structure. Instead of accidentally going too far with the circular saw, choose to finish your cuts with a handsaw so that you can be exact.

Trim the bottom and top of the stringer. Cut the top of the stringer along the first rise line. Then trim the bottom run line, which was marked so that it is shorter than the others by an amount equal to the thread thickness of your stairs.

Use the first finished stringer as a template. Use this one as a stringer template for all of your other stringers so they all match exactly. In most cases, you will need only 1 or 2 more stringers, although a very wide stairway could require more.