Broken slats or balusters on staircases not only look bad, but are potentially a safety concern. The balusters support the handrail between the top of the stairs and the bottom banister. It is important to replace broken balusters as soon as you notice the problem. In addition to supporting the handrail, they also act as protection to prevent small children and animals from falling off the side of the stairs, similar to a guardrail that runs the length of the stairs.
Saw the broken baluster in half with a hand saw. This will allow you to remove the two halves between the bottom of the railing and the stairs. The bottom of the baluster has a round tenon or peg that slides into a mortise or hole in the stairs. This is typically glued in place.
Grip the bottom half of the baluster with a pipe wrench or pliers and twist it to break the glued mortise-and-tenon joint. Pull the bottom half out. Remove the top half by working it back and forth with your hands until it is loose enough to remove. Set the broken parts aside.
Sand the mortise in the stair tread of the removed baluster with medium-grit sandpaper to remove any remaining glue.
Measure the distance between the bottom of the handrail and the top of the stair tread in the position of the removed baluster. Place a bevel gauge against the bottom of the handrail and adjust the gauge to the angle of the handrail.
Mark the measurement and angle on the top end of the replacement baluster with a pencil. Adjust the mitre saw to the correct angle and cut the replacement baluster to size.
Apply wood glue to the bottom tenon of the replacement baluster and to the mortise hole in the stair tread. Slide the bottom end into the mortise in the stair tread and pivot the top end of the baluster into place.
Drive a finish nail at an angle through the baluster into the hand rail with a hammer. The nail should be on the back side of the baluster where the angle is highest. Set the nail 1/16 inch below the surface of the wood using a nail set.
Apply wood finish or paint to the baluster with a paintbrush. Allow the finish to dry at for least four hours before touching the baluster or adding a second coat.
Apply glue to loose baluster slats to prevent further damage and the need for replacement. Make nailing the baluster easier by drilling a pilot hole at the top of the baluster before installation.
Wear eye protection glasses when making wood repairs.