Closed skirtboards that slip into a pocket
Skirtboards between two walls are called closed skirtboards, a skirtboard that must be mitered into the risers is called an open skirtboard and is more complicated because each tread and riser must be notched into the skirtboard.
If the framers leave a wide enough pocket, finish carpenters can slip the skirtboard between the stringer and the drywall and avoid having to notch any of the treads and risers. This is the simplest variation, and it is what is covered in this video.
Step 1: define the top of the skirtboard and find the angle
Spencer uses the width of his level as the offset for the top of the skirtboard above the stair nosing.
To find the angle, use a digital level to dial it in for you. lay the long digital level along the top of the treads, hit the angle finder button on the digital level and there's the angle: 37 degrees.
Or at least one of them.
Thirty-seven degrees is the plumb cut angle. The horizontal angle will be 90 degrees minus 37 degrees, or 53 degrees.
Snap a chalk line along the top of the skirt for the entire length of the stairway. This will help make sure that when you splice two boards together that you keep that top line flat.
Step 2: Define the transition points
Find the points where the baseboard and skirtboard will meet. That transition point is where the skirtboard gets plumb cut.
Spencer tacks a finish nail at the transition point so that he can hook his tape measure for an overall skirt board length.
He writes the number on the wall—behind where the skirtboard will; cover it—and draws a quick diagram of the skirtboard to act as a map at the saw station.
He fills in the known dimensions: 96-3/8 inches overall, 5-7/8 inches baseboard height.
Install the skirtboard
With the skirt board cut, he can slip it into the pocket next to the stairs. He tacks a 10d nail into the top so that he has something to grab while adjusting the placement.
With the top of the skirt on the chalk line and the bottom of the skirt at the transition point, he nails the skirt into the studs.
Two biscuits and pinch dogs help to pull the joint between the skirt board and baseboard together. Connecting these joints with biscuits and glue will prevent later cracks.
Mitering the skirt/baseboard around a corner
Run the skirt long, so that you can scribe the backside. Scribing is better than measuring because sometimes walls are not perfectly plumb, floors are not perfectly level, and corners are not perfectly 90-degrees.
If the corner occurs before the transition point, you'll need to make a different type of transition. An inch or two before the corner, step the skirtboard down to the baseboard height with a plumb cut.
Other transitions are possible and acceptable alternatives.
Spencer wants to take this time to recommend a Stabila digital level "It has paid for itself many times for me."
—Spencer Lewis is a finish carpenter in Fort Wayne, Indiana. His YouTube channel has a playlist of stair trimout videos, which is a good place to start for more info.