How to Cut a Mitered Stringer

Lift the 2-by-12 and gaze down the narrow edge of the board. Flip the board over and do the look down the other narrow edge. Most long boards are bowed just slightly in one direction or another, based on the wood grain. This bow is called the board's crown. Draw an arrow on each board pointing toward the side of the board that is crowned upward. This will be the top edge of the stringer.

Measure the total height of the staircase, and the total distance the staircase will reach out from the building, deck or wall. These dimensions are called the rise and the run. For example, if the upper floor is 6 feet off the ground, and the stairs terminate 8 feet from the wall, the stringer is said to have a 6-by-8 rise over run.

Calculate the total length of the top edge of the stringer using the Pythagorean theorem, which says that for any right triangle, where A and B are the lengths of the legs of the triangle, and C is the length of the diagonal leg, or hypotenuse: A*A+B*B=C*C. In the example from Step 2, 36+64=C*C, or 100=C*C. Thus, C=10 feet. Set this dimension aside for later.

Lay the stringer on the ground or on sawhorses so you can work easily. Set the stringer so the crown is away from you. Take the builders square and place the 6-inch mark on the top corner of the board, on the same side of the board that has the crown. Place the 8-inch mark on the other leg of the framing square along the top edge of the board, so that the framing square is laying on the stringer. The elbow, or point at which the two legs of the square meet, should point toward you. Draw a line along the end of the stringer, marking the angle designated by the layout of the square. In the example used here, the angle marked on the board will be 60 degrees.

Cut off the top miter with the builders saw, and then measure from the top mitered corner the length figured for length C. In this example, measure down 10 feet from the mitered point of the stringer. Place a mark at this point, along the top (crowned) edge of the stringer. This point is the bottom end of the stringer.

Slide the framing square along the stringer, and make sure it stays in the same relative orientation to the board. Place the 8-inch mark on the lower leg of the square on the mark you put on the board. Place the 6-inch mark that is on the other leg of the square along the top edge of the stringer. Mark the line created by the lower edge of the square on the stringer. This line creates the complementary angle of the top miter, and the angle is at the right distance from the top of the board.

Cut off the end of the board along this angle with the builders saw.

Hold the stringer up into place and check the miters. Unless you are an experienced carpenter, the stringer may need to be trimmed. The angles on both ends of the board should be flush to the wall or floor, with an angle that meets both surfaces perfectly. Once the first stringer is cut properly, place it on top of the other stringer, trace the outline, and cut the second stringer to match.