Gates generally provide a way through a fenced-off area, but that doesn't have to be the case. A freestanding gate can be a decorative touch to a garden, or it can provide a solid passageway through a non-fence barrier like a hedgerow. When you build a freestanding gate, there is no support system other than one fence post, so it is important to make that post as secure as possible. Beyond that point, building a freestanding gate is no different than building a fence gate.
Cut two pieces of 4-by-4 lumber equal to one and a half times the desired gate height. Cut two pieces of 2-by-4 lumber equal to the desired gate width, and three pieces of 2-by-4 lumber equal to the height of the gate, minus 12 inches.
Place two of the long 2-by-4 pieces parallel to each other in front of you, lying flat and extending away from you. Place one of the two shorter pieces on the end of the 2-by-4 boards closest to you so that the edges of the board line up with the outside edges of the two long boards. Place the second short board in place just like the first one, except at the far end of the boards.
Place carpenter's glue on the four places where the boards meet. Hammer two 3-inch nails per intersection, connecting the boards together. Drive the nails diagonally through the long boards and into the short ones. Use a nail set to drive in the nails on the inner corners if you have trouble getting the nails all the way in. You will now have the basic frame for your gate.
Place a 2-by-4 board flat underneath the frame. Position the left edge of the board directly under the top left corner of the frame. Angle the board so that the right edge of the board passes underneath the bottom right corner of the frame. Make a line on the board where the inside top and bottom of the frame cross the board. This will provide you with cut lines so you can cut your brace.
Move the brace board to a table so that one of the cut lines on the board is positioned over the edge of the table. Put something heavy on the board and cut along the line with your circular saw. After that, position the other cut line over the edge of the table and cut there as well. You have now completed your brace.
Apply carpenter's glue to the top and bottom of the brace along the faces that you cut with the circular saw. Insert the brace into the frame and nail it in place, using 2-inch nails. This brace will prevent the frame from warping over time.
Cut pieces of 1-by-6 lumber equal to the height of your fence, minus 1 inch. These will become your pickets, so the number of boards you will need will depend on the width of your fence and how close you want the boards to be to each other.
Place the 1-by-6 pickets over the frame and drive 2-inch nails through them and into the horizontal top and bottom pieces of the gate frame. Make sure the pickets extend evenly over both ends of the frame.
Dig a 1-foot-wide and 2-foot-deep hole where you want the first gate post to go. Use your trowel to make the hole wider on the bottom than it is on the top. If your gate is over 6 feet tall, make this hole 3 feet deep instead.
Mix your premixed concrete with water according to the instructions on the bag. Once it is mixed, pour the concrete into the hole, up to an inch from the top.
Insert one end of one of your 4-by-4 gate posts into the hole and push it down to the bottom. Rotate the post so one side is directly facing the direction the gate will go. Place your level against the side of the post and adjust it until it is perfectly straight.
Measure a distance away from the first post equal to the width of the fence plus three inches. Dig a hole just like the first one, except you don't have to make the bottom of the hole wider than the top. Place the second post in the hole and rotate it until one side is facing the first post. Measure the distance between the posts and adjust the positioning until the gap is equal to the width of the fence plus about 1 inch.
Backfill the second post hole with dirt. After you fill in about 8 inches of dirt, place your level against the side of the post to ensure it is straight, then use a spare 2-by-4 to tamp down the dirt in the hole. Continue to do this after every 8 inches of dirt. Since this post will not bear the weight of the gate, you do not have to set it in concrete. Wait 24 hours for the cement in the first hole to solidify before you continue.
Move your gate between the posts. Align it with either the front or back face of the post, depending on which way you want the gate to swing. Place 1-inch wood spacers underneath the gate to raise it up off the ground slightly.
Place one leaf of a gate hinge against the post set in concrete and another against the top horizontal piece of the gate frame. Drill 1½-inch wood screws through the holes in the hinge to secure the hinge to the post and gate. Repeat the process with the bottom horizontal piece of the gate frame.
Place the latch mechanism on the face of the opposing gate post. Drill 1½-inch wood screws through the holes of the mechanism to secure it in place. Set the latch into the mechanism, like it will be when the gate is closed, and secure the latch to the edge of the gate using 1½-inch wood screws. If your gate is very tall, you will have to drill a hole through the gate to reach the latch from both sides.
Leave the spacers in place for another day or two to really let the concrete set before the full weight of the gate is brought down on the post.