How to Install Exterior Trim on a Patio Door

Measure from your patio or step up to 3/16 inch above the bottom, of the upper door frame board, and make a pencil mark. Repeat this step on the opposite side of the door. This is the mark where you will place the lowest point of your side trim boards.

Draw at least 8 pencil marks, 10 to 12 inches apart on your door jambs 3/16 inch from the inside edge of the door jambs, toward the outside edge of your door jambs. Your door jambs are the wooden frame that holds your patio doors. You'll place your side trim boards against these pencil marks.

Hold one of your 8-foot-long trim boards against your left door jamb. Make sure the thinnest edge of your trim board is resting against your pencil marks, and the top of your trim board covers the outside corner of the top jamb. Make a pencil mark on the inside edge of your trim board 3/16 inch above the bottom of your upper door jamb board.

Set your miter saw to 45 degrees on the left side of the zero mark. Place your trim board flat on the cutting surface of your miter saw. Make sure that the pencil mark on your trim board is facing you. Use your miter saw to cut your trim board. Repeat steps 3 and 4 for your right trim board, except set your miter saw to 45 degrees on the right side of the zero mark.

Place your left trim board next to the pencil marks on your left door jamb. Use your compressor and finish nail gun to secure your trim board to your door jamb. Repeat this step for your right trim board.

Set your miter saw to the right side of the zero mark, place the left end of your trim board on the cutting surface of your miter saw with the thin edge facing out, and cut your trim board.

Measure across your top door jamb from the lowest point on each side jamb. This is the length that your top trim board has to be. Mark this length on your top trim board from the shortest point on the left side that you previously cut, across to the right side. Place your pencil mark on the thin edge of your trim board.

Set your miter saw to the left side of the zero mark. Set your top trim board on the cutting surface of your miter saw with the thin edge of your trim board facing you. Cut the right end of your trim board.

Set your top trim board in place. The miter angles on your top trim board should meet and be parallel to the miter angles on your side trim boards. Secure your top trim board in place with your finish nail gun. Caulk all edges, filling your nail holes with painter's putty, and painting a topcoat.