How to Replace a Stair Tread That Is Mortised Into Stringer

Pull visible nails from the top of balusters on the step to loosen them from the underside of the railing. Mark around the top of the balusters with a pencil on the underside of the railing. Pull the balusters gently away from the banister, lifting them out and away from the tread.

Rotate the step up and away from the riser to remove the old tread from the staircase. You may need to tap the step upward with a mallet under the front center if it's glued to the riser beneath it, to loosen the step.

Create a new tread, using wood that's similar to the old tread. Cut it the same size and use a table or hand router to fashion a rounded bullnose on the front edge of the tread that matches the original tread.

Dry-fit the tread into the mortise on the stringer. If it doesn't fit snugly, trim enough wood off the bottom along the edge of the tread to slide the new tread into the mortise. Tap the tread in snugly and mark the points to drill for the baluster tenons, either by measurements from the original or by dropping a plumb line from the centers of the outlines on the underside of the banister.

Drill the mortises on the new tread to fit the tenons at the bases of the balusters.

Lay a bead of wood glue along the stringer and the top of the riser at the front of the tread. Rotate the new tread in, back edge first, then place the side into the mortise. Tap the tread in and wipe any glue drips with a damp, clean cloth.

Replace the balusters, tipping them into their base mortises. Check them with a fluid or digital level to ensure that they stand perpendicular. Fasten the balusters to the banister with 2-penny nails.