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The width of the windows determines the final length of the planter. The depth and height should be large enough to hold pots or an amount of dirt that will retain moisture for a few days. In our project, the box is 7 inches deep and 7 inches high to accommodate 6-inch flower pots.
Use brackets to support the weight of the box and the dirt that will fill it. The siding on the house may prevent the brackets from evenly adjoining the house exterior. To accommodate the siding, make a cardboard template that will help to cut notches into the bracket.
Certain types of wood aren't suitable for this project. Pine rots easily and oak turns black when wet, so they are not good choices. Cyprus has a high oil content which makes it weather-resistant. Cedar is both decay-resistant and affordable; it can be left natural or can be stained, and it is available in smooth or rough finishes. Teak and mahogany are beautiful, but expensive.
After determining the full width of the planter, cut front and back boards to the appropriate length. After cutting the ends, trim 1" x 8" piece of lumber to 7 inches wide.
Mark lines with square and use a belt sander to bevel corners at a 45-degree angle -- this will reduce splintering. Remove any additional sharp or rough areas. Repeat beveling for all corners. Smooth with sanding block.
Cut a slot at the end of the two boards to adjoin sides and increase strength. Measure width of side boards for slot and mark board 1/2" from end. The slot can be cut with a handsaw and chisel, a router or a radial-arm saw. If you use a radial-arm saw, set the blade to cut a 1/4" groove. Repeat cutting until groove is completed. Use chisel to clean up slot.