How to Trim a Bifold Door

Hold your level over the top of the doorway, 1/4 inch up from the edge of the casing (the part of the opening that sits directly against the door). Mark a pencil line along the top.

Measure the line with a tape measure and add 1/2 inch to the measurement. Transfer the measurement to a piece of trim, making the marks on the bottom edge of the trim and leaving a few inches at either end.

Set the trim on your miter saw, on its back, with the marks on the bottom edge facing the blade. Set one of the marks directly under the blade. Turn the blade so it points away from the trim at 45 degrees. Make the cut.

Set the other mark under the blade and repeat the process, turning the blade to 45 degrees in the opposite direction (still pointing away from the trim) and cut it.

Hold the board on the line above the doorway, with the short bottom edge on the line and the longer top edge facing up. The bottom edge should come out over the ends of the opening by 1/4 inch on either side, and the mitered cuts should flare out and up. Secure the piece with trim nails and a hammer, putting nails along the top and bottom of the trim about every foot.

Measure, cut and install the two side pieces of trim in the same way, except miter just the top end of each board, square-cutting the bottom end to sit on the floor. The mitered top ends of the two side boards should butt against the miter ends of the top piece to make the corners.