The bottom of a doorway consists of a bottom sill and threshold. The threshold sits directly between the door when it is closed to prevent drafts, moisture or pests from entering a structure. Adjusting the door bottom when installing new flooring or replacing an old door can cause a gap between the threshold and door. To fix this gap, insert a filler piece between the threshold and sill. The filler piece should be a solid piece of lumber, such as pine, or another type of wood that matches the current sill, threshold or flooring material in that particular area.
Open the door to expose the threshold that is attached to the door sill. Cut through caulking placed around the outer edges of the threshold using a utility knife. Remove leftover bits of caulking to expose the sides of the threshold.
Trace around the threshold while it is still secured to the door sill before removing it. Remove the fasteners, such as slotted or Phillips screws, that secure the threshold to the door sill. Pull the threshold up to separate it from the door sill.
Clean off debris that may be left over from all sides of the threshold. Lay the threshold on the piece of lumber that you're using to create the filler. Trace along the sides of the threshold using a sharp pencil. Confirm that the traced lines match with the threshold.
Align the cutting wheel of the oscillating or cutting tool with the traced lines on the filler piece and cut the lumber. Attach the lumber to a solid work surface using C-clamps if it slips while you make the cuts. Change to a sanding accessory to lightly smooth the cut edges.
Equip the drill or rotary tool with a drill bit that is slightly smaller in diameter than the screw fasteners that you're using to secure the filler piece and threshold to the door sill. Place the threshold on the filler piece and check that all edges are aligned properly. Drill pilot holes through the filler piece using the threshold’s predrilled holes.
Reseal or paint the filler piece to match the surrounding sill or threshold.
Lay the filler piece and threshold on the sill and align them with the traced lines that indicate where the threshold was previously installed. Slip a screw into each pilot hole and tighten them by hand or with a drill or rotary tool.