Using a circular saw guided by the square, cut the ends at 45-degree miters. Repeat this process on the other three sides of the bed until you have a frame of 2x8s that sits flush with the inner edge of the bed and protrudes 2 inches over the outer edge.
Apply construction adhesive along the top of the timbers and on the board's mitered edges. Lay the boards in place. Fasten them to the timbers with 3-inch decking screws every 12 to 16 inches. Drill a horizontal pilot hole through the rails edge and across the mitered corner. Drive a 3-inch screw into the joint to prevent it from opening.
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Drill/driver with an extended 1/2-inch spade bit
to drive rebar
to cut rebar, optional
The bed’s 4 sides each need 3 courses of timbers to rise more than a foot above ground, for a total of 12 timbers. Because it’s best to have full pieces on each side, buy stock lengths at least as long as the dimensions of your bed. Make sure each timber is straight and clear of knots on at least one side.
2. 2X8 LUMBER
to make a railing to cap the bed’s walls
3. 1/2-INCH REBAR
to secure the first course of timbers to the ground. These are usually sold in 18- to 24-inch lengths. Both are long enough to secure 6x6 timbers at least a foot into the ground.
4. 10-INCH SELF-TAPPING TIMBER SCREWS
such as Timberlok and Timbrex brands, to fasten the sides together.
5. CONSTRUCTION ADHESIVE
to fasten the railing to the timbers
6. PEA GRAVEL
for drainage under the walls and at the base of the bed. Most home centers carry either 1/2- or 1-cubic-foot bags. To determine the cubic footage you’ll need for 3 inches of fill at the base, divide the square footage of your bed by 4. To figure out how much you’ll need for 2 inches of fill under the timbers, add the length and width of the bed (in feet) and divide that number by 6.
7. TOPSOIL/COMPOST MIX
Most home centers carry 2- or 3-cubic-foot bags of properly amended soil. Because the bed is just about 1 foot deep (with the gravel layer at the bottom), the square footage of the bed roughly equals the cubic footage of soil you’ll need to fill it. Throw in an extra bag to account for settling.
8. 3-INCH DECK SCREWS
For fastening 2x8 cap rails.