Over time stair treads can break or become damaged from repeated use. The area of the tread that receives the most traffic wears out first and breaks away from the rest of the tread. When this happens the area needs to be cut out and replaced with new wood. If the damage is severe, replacement of the entire tread may be needed instead.
Outline the damaged area by drawing a horizontal line that is parallel to the stair edge and short vertical lines on each side of the damaged area with a straightedge.
Set the blade of a circular saw to the thickness of the stair tread. Make a guide for the saw's base plate to follow by nailing a strip of wood to the tread just outside of the horizontal line.
Cut on the horizontal line by using the strip of wood as the saw's base plate's guide. Stop the blade and remove the circular saw when the longest line meets with the vertical side lines. Do not cut on the vertical side lines.
Cut the vertical side lines by making 45-degree angle cuts with a tenon saw. Stop when the cuts reach the horizontal line cut. Do not cut beyond the horizontal line. Remove the damaged section.
Remove the strip of wood from the tread by removing the nails with the back of a hammer.
Remove waste from the corners of the cut out section with a chisel, while making sure not to damage the riser.
With a table or circular saw plane the underside of a new piece of nosing so that the riser fits into it. Cut 45-degree angles out of the new nosing piece so that it will fit snugly into the cut out section.
Apply wood glue to the cut edges of the new nosing piece and stick the section into place. Clamp the section in place by layering a piece of polyurethane, a strip of hardboard, and a strip of wood that is a few inches longer than the nosing section on top of the section. Screw the ends of the strip of wood into the stair's tread with a screwdriver.
Reinforce the butt joint by drilling ¼ inch holes with a drill and ¼ inch drill bit into the nosing section and the riser. Hammer wooden dowels into the drilled holes.
Allow the wood glue to dry. Unscrew the screws in the strip of wood with a screwdriver to remove the wood strip, hardboard strip and piece of polyurethane.
Sand the repaired section to be flush with the rest of the stair tread with sandpaper.
Call a professional to repair whole stair treads or for areas that are severely damaged. Repaired area will be noticeable. Use caution when using saws.