If you've got damaged slats on any door, you probably already know that replacement won't be easy -- and in fact almost impossible on some types of doors. This is due to the fact that slats are typically mortised permanently into the sides of the door. They are difficult to remove or replace without notching and cutting into the door from the back. Replacement is usually not an option due to excessive labor and the possibility that your repair job won't turn out as planned. Fixing the slat in place is the solution.
Drill two small holes 1/8 inch from both sides of the door on the interior side, where the slat enters the side of the door on both sides. Use a drill/driver and 1/16-inch bit. The holes should be evenly spaced so that they penetrate through the face of the door and through the end of the slat. Jiggle the slat up and down, back and forth. If you can move the slat enough to see that the slat has a tenon -- a rectangular block on the end -- drill through the tenon.
Inject wood glue into the joints or separations where the slat enters the sides of the door. If you see uneven proportions or spaces between gaps, tap wooden wedges between adjoining slats to even the gaps.
Tap 1 1/4-inch brads into the holes using a hammer. Set the nails below the surface using a nailset and hammer. Fill the nail holes with putty crayon. Allow the glue to dry overnight before removing the wedges.
Cracks and Splits
Place strips of masking tape along both sides of any cracks or splits. Insert the tip of a craft knife into the crack and pry it open slightly.
Inject wood glue into the crack using the tip of a glue bottle. Remove the knife.
Wrap rubber-coated wire around the slat where it's been glued. Insert a wooden stick into the wire and twist it like a tourniquet to tighten the wire around the slat. If the crack is over 3 inches long, use additional tourniquets to tighten the crack.
Allow the wire to remain on the slat overnight. Remove the wire and tape. Use a stain marker to color any remaining glue lines.