How to Build a Double Mailbox Post

Mark the 4-inch by 4-inch post to a length of 5 feet, 6 inches, using your tape measure and a pen or pencil. Cut the post to length with your miter saw.

Dig a hole for the mailbox post that measures approximately 6 inches in diameter and 18 inches deep. Use a set of posthole diggers by pressing the blades of the tool into the ground and pushing the handles of the digger tool closed to scoop up the soil. Collect the displaced soil in a bucket to be removed from the area.

Set one end of your cut post into the hole. Place a level on one side of the post to ensure it stands perfectly vertical; the level's bubble will be in the center of the window when the post is level. Have an assistant hold the post in this position.

Mix a bag of ready-mix concrete with water in a wheelbarrow or bucket, following the package directions. Use a shovel to stir the mixture. Shovel or pour the concrete into the hole around the mailbox post. Hold the post upright for 10 to 15 minutes until the concrete hardens enough to support the post without your assistance.

Determine the correct height for mounting your box to the post by asking your mail carrier or local post office what postal regulations require. Mark this location on your post by measuring up from the ground with a tape measure and marking a line with a pen or pencil.

Mark your two-by-four board into four sections, each measuring 18 inches long. Cut the board into four pieces with your miter saw. Stand two of these pieces on their 2-inch-wide edges, parallel to each other and 18 inches apart. These are the bottom pieces of your mailbox frame.

Turn the remaining two pieces onto their 4-inch sides. Place them flat on top of the bottom pieces, running them in the opposite direction to span the distance between the bottom boards. This creates a square frame. Align the edges of the top pieces with the sides and ends of the bottom pieces.

Drive a 3-inch wood screw through each corner of the frame, using your power screwdriver. Drive the screw through the top pieces and into the edges of the boards below.

Locate the center of one of the frame's bottom boards with your measuring tape and mark it with your pencil. Place this board against the street side of the mailbox post, aligning the frame at the post height you marked in Step 5. The frame will be horizontal and extend outward from the post. Drive four 3-inch wood screws through the 4-inch side of the bottom board and into post.

Measure the distance from the front of the frame at a 45-degree angle downward toward the mailbox post. Mark this measurement on the remaining section of two-by-four board with a pencil. Cut this board to length by mitering the ends of the two-by-four with 45-degree angles.

Set this final board into place at an angle between the post and the front of the frame. Drive a 2-inch wood screw with your power screwdriver, going through the end of the mitered board and into the bottom front of the mailbox support frame. Drive a 3-inch wood screw through the mitered board and into the mailbox post.

Mount your two mailboxes by sitting them on top of the support frame, facing the road. Screw or nail each box to the horizontal 4-inch board faces, placing the fasteners in the box's mounting holes.