How to Fix a Broken Stringer on the Stairs

Inspect the entire staircase. Look for screws or nails popping up and loose treads -- the flat portion of each step, where your foot rests -- and risers, if present -- the upright piece at the back of the step, running between each step. Plan to repair or replace any structural weakness while working on the stringers.

Look closely at the stringers. Notice if the crack or split is relatively small and localized or if it runs for several inches or through the stringer's thickness.

Remove trim, tread or risers to look closer or access the area underneath the stairs to examine the bottom edge of the stringer. Generally made into a closet, the room below the staircase may feature exposed stringers. If covered with ceiling sheathing instead, pull or cut the ceiling covering away carefully. You may need to remove 2-by-4-inch boards along the edges, framing the ceiling, to reveal the stringers completely.

Slide a length of 2-by-4-inch lumber underneath the bottom end of the stair stringers and wedge it tightly in place to slightly lift the cracked or split stringer, if possible. A hydraulic floor jack or similar jack, inserted under the stringer slightly forward of the bottom area, may work better. Place a block on top of the jack plate and exert upward pressure while watching the crack or split. If the void grows smaller, it is an excellent candidate for repair. If the crack or split remains unchanged consider replacing the stringer instead.

Hold a section of angle iron, approximately 18 to 24 inches long, depending on the length of the crack or split and if you are able to get to both sides of the stringer involved, up against the stair stringer. Center it over the split or crack. Use a metal mending plate, alternatively, for smaller cracks that do not completely penetrate the stringer's thickness.

Drill four to six pilot holes, slightly smaller in diameter than the mending plate or angle iron's bolts or screws, in position as indicated. Situate the anchors equally on both sides of the crack or split and do not attach closer to the void than 3 inches, even if you must skip holes. Drilling through the stringer close to the split or crack will weaken the stringer even more.

Bolt or screw the angle iron or mending plate in place, driving 2-inch bolts, 1/2 inch in diameter, through the plate into the stringer. Pre-drill holes on the other side of the stringer if the void runs through the stringer and the opposite face is accessible. Attach similarly.

Cut a section of 2-by-4-inch lumber to a minimum of 4 feet in length. Spread a generous bead of construction adhesive along the length, squiggling the glue to ensure it spreads across the board. Hold the board up to the stringer, from underneath, and clamp in place until dry to further reinforce the mended stringer. Drive screws through the splint and into the stringer every 8 to 10 inches, avoiding the area directly above and to the side of the split or crack by 3 or 4 inches.

Remove the 2-by-4 wedged beneath the bottom step or let down the jack, slowly, to lower the stairs. Watch the mend as the weight of the stairs returns to the stringer. If the crack or split remains gaped or other signs of distress occur consider replacing the stair stringer.