Things You'll Need
3 inch L brackets
2 inch treated deck screws
Stair stringers are the side boards that support the risers (the front of the step) and treads (the top of the step) of your staircase. If your stringer fails, the staircase cannot be safely used. Home maintenance should include a yearly inspection of safety related elements, such as stair stringers, to ensure that any damage is found early so that effective repairs can be made. Repairing stair stringers is a fairly simple process that can be done with common tools and materials that can be purchased from your local hardware store or lumber yard.
Assessing the Damage
Examine the stringer to determine what repairs need to be made. Check the top and bottom connections and tighten any loose joints with a hammer. Reinforce loose joints with a 3 inch L bracket and 3/4 wood screws.
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Repair loose steps with an L bracket and wood screws between the tread and the riser. Drive a shim between the end of the tread and the top of the stringer to tighten it before installing the L bracket.
Check the stringer board itself for damage. Slight splits or cracks can be repaired with mending plates and splints. Any split that remains open when the the load is removed from the stair is an indication that the stringer should be replaced. Conduct this repair only if you are an experienced carpenter; get professional advice if you are unsure.
Repairing the Stringer
Lift the stairs with a 2-by-4 brace on top of a hydraulic bottle jack to take the weight off of cracked or split stringers to make the repair possible.
Apply a heavy duty mending plate, at least four inches long, to the bottom edge of the stair stringer spanning the damaged area. Use 2 inch drywall screws to attach the plate.
Install a hammer-down mending plate to the outside of the stringer, spanning the damaged area. Hammer the plate flat to the stringer.
Cut a 48 inch piece of 2-by-4 lumber to use as a splint. Apply construction adhesive to the face of the 2-by-4. Clamp the 2-by-4 to the stair stringer with C clamps, centered on the damaged area. Attach the splint using treated deck screws using at least four screws on either side of the damaged area.
Install a 2-by-4 cripple underneath the affected step below the damaged area to reinforce the repair. Cut the 2-by-4 to the height of the bottom of the step when measured from the floor. Attach the cripple with screws to the stringer at the top and with an L bracket to the floor. Let the jack down slowly and check to be sure that the mended stringer stays in position with a load on top.