Step 6: Installing Cross Braces and Center Tie
The cross brace serves two purposes. First of all it helps hold the two boards that make up each side of the planter together. That way they won't move in or out from each other when the planter is filled with dirt. Secondly, they hold the planter square and prevent it from flexing while being moved. Whithout the cross braces the planter is more likely to be a parallelogram than a rectangle. It's also built with a low profile so that it will be covered with dirt once the planter is filled.
Before the cross braces are installed the planter needs to be squared. The best way to go is to check the distance between opposite corners with a tape measure. Check one pair of opposite corners and then check the other pair. The measurements will be the same if the planter is square. If they are different, go to one of the corners with the longest measurement and give it a little kick towards the center of the planter. Keep checking and kicking until you get it squared up. After that be very careful that you don't move the planter until the cross braces are installed.
To install the cross braces position them into the corner with the 5/4 boards tight up against the sides of the planter. Put four screws into each piece of 5/4 board on the ends of the cross brace; one in each corner with two screws passing into the bottom side piece and two screws passing into the top side piece (See photo 2). Each planter should have a total of two cross braces with each one in opposite corners.
The center tie holds the two opposite sides of the planter 48 inches apart. It prevents the sides from bowing outwards when the planter is filled with dirt. It's also built with a low profile so that it will be covered with dirt once the planter is filled (See photo 3).
The center tie is assembled with a 46 inch piece of 2 x 4 and two 8 1/4 inch pieces of 5/4 board. Each end of the 2 x 4 is butted into the face of the 5/4 board and two 3 1/2 inch screws are driven through the board and into the end grain of the 2 x 4. The 2 x 4 is centered on the piece of 5/4 board. Make a line offset 3/4 inches from the center of the board and align the edge of the 2 x 4 with this line.
Position the center tie in the center of the 8 foot long side of the planter. Put a screw in each corner of the 8 1/4 piece of 5/4 board with two screws passing into the bottom side board and two screws passing into the top side board (See photo 4).
Finally, take a couple extra 8 1/4 inch pieces of 5/4 board and secure them to the inside of the planter in the same manner as the cross braces and center tie (See photo 5). The most important places are on the long sides of the planter near the corners without cross braces (See photo 6). This will prevent the two boards that make up the side of the planter from moving away from each other