How to Install Baseboards at Stairs

Measure the wall from the nearest corner to the base of the staircase. This is the length of trim needed.

Locate your studs and mark them with a pencil. You may use a stud finder, or locate the stud by knocking along the drywall until your knock comes back solid. Studs are generally placed about every 16 inches, so your tape measure can be used to find the rest.

Check the floor for levelness, using a 4-foot level. If it isn't level, find the lowest point and loosely attach a piece of scrap trim.

Mark off the trim height using the top of the scrap piece of trim and your level. Do this not just at the staircase, but all the way around the room.

Snap a chalk line over your marks to create a line. You will line up the top edge of your baseboard trim with this line during installation.

Set your sliding T-bevel at a random angle and lock it into place. This angle is a jumping-off point.

Place the bevel along the skirting where the baseboard will begin its ascent or descent.

Mark the angle with a light line, using your carpenter's pencil.

Flip the T-bevel over so it rests against the skirting on the other side of the angle, making certain that the point of origin is the same at the base. Make another light line to mark that angle. You should have an upside down triangle.

Choose the center between your two lines. This is called splitting the angle.

Adjust the T-bevel to align with your dot. Draw a line.

Flip the T-bevel over and draw a line where the line falls again. If the two lines do not match up completely, repeat, finding the center and readjusting the bevel until the lines are on top of one another. This is your miter angle.

Transfer the angle to a piece of scrap trim for safe-keeping so you can continue using your T-bevel without losing your angle.

Transfer the angle again to your trim when you are ready to cut. Make the cut using a skill saw. Cut on the outside of your lines so that you can adjust the boards for a tight fit by sanding down the edges.

Tack the first piece of trim to the wall starting at a corner. Use a compass to measure the depth between the chalk line and the top corner of the trim. The top edge of the trim will be resting higher than the chalk line if your floor is not level. Maintain this angle by not adjusting the legs of the compass, and pull it along the floor, drawing a scribe line on the base of the board. Cut along this line at a 2- to 5-degree bevel and then block-plane the trimmed bevel right to the pencil line. When done correctly, your baseboard trim should now fit perfectly between the floor and the chalk line.

Nail on the baseboard at your stud marks, butting the mitered corners together where the trim meets the staircase. Use two 8D finishing nails at each stud, angling them slightly toward the floor.