How to Replace a Stair Tread That Is Mortised Into Stringer

Replacing a mortised tread requires simple woodworking skills.

When a stair tread breaks or cracks in a new house, you call the builder to fix it. After all the warranties have expired, however, repair that tread before a member of the family or guest discovers the defect and turns an ankle -- or worse. Mortising -- or setting treads into grooves in the 2-inch thick stringers that line the stairs -- is a common way to secure treads without large numbers of screws or nails. Re-setting a mortised tread can be simple.

Things You'll Need

• Mallet
• Chisels
• Router and bit
• Saw
• Reciprocating saw
• Carpenter's square
• Hardwood tread
• Plumb line
• 2-penny finish nails
• Drill with bits
• Wood glue

Instructions

1. 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.

2. 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.

3. 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.

4. 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.

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

6. 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.

7. 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.

Tips & Warnings

• There is no one absolute method for stair-building. You'll need to study yours as you take them apart. If the treads are screwed into the riser from the rear, you'll need to destroy the tread to get it loose. Measure the position of baluster mortises before demolishing the step with a reciprocating saw.

• If the tread is attached to the stringer with nails and the stringer is closed in with wallboard, you may have to open a portion of the wall. Be prepared with a piece of wallboard, tape and plaster to replace the part of the wall you have to remove.

• Always wear eye protection when using power tools, especially when cutting metal screws or nails. Wear safety glasses when working above your head, too, as when drilling pilot holes for baluster nails.

References

• Fine Home Building: A Fast, Accurate Way to Make Mortised Stairs
• Sawdust on the Floor: Rebuilding the Stairway Part IV
• Fine Homebuilding: Building Finish Stairs