How to Cut Mitered Joints on Stairs

Start at the top of the stairs and work down. Measure the length of wood needed to cut from the last full piece of baseboard to the tip where the first stair begins. This is the first vertical section. Mark that distance on a piece of wood with a tape measure, pencil and carpenter square, so that you have a 45-degree angle going up and away from the mark you made for your first vertical section.

Place the piece of wood down on the saw’s tray and up against the fence with its face upwards. Turn the blade on the miter saw accordingly for cutting a 45-degree angle off the wood. Dry lay that piece in place.

Measure from the first stair tread’s top down the vertical face to the second stair tread. Mark a piece of wood accordingly. Hold the piece against the wall, and you will see that the upper part needs cutting off at an opposite 45-degree angle - matching the first cut with the two cuts joining, forming a 90-degree angle.

Use the carpenter square and draw that angle from the first pencil mark on the top of this piece. At the bottom of this vertical piece, make a same-direction 45-degree mark as well. Cut the ends off on the miter saw, and then dry lay the board in place so that its top section joins with the first cut around the stair tread forming a 90-degree angle. Notice that the bottom piece angles up and away from the tread and riser’s inside corner.

Measure from the first riser’s inside corner across the length of the second stair tread. Mark your piece of wood accordingly. Make the 45-degree marks so that the first mark joins with the riser’s 45-degree angle with the second mark going up and away from the tread’s outside corner where it meets the second riser. Make the cuts on the miter saw.

Repeat this process as you work down the stairs. Make two cuts for every inside corner and every outside corner with the 45-degree angles always matching up forming a 90-degree angle that wraps the stair treads and risers.