- 1). Measure the width of the windows that you would like to install planter boxes under from the outside of the home. The length of the planter boxes will be determined by the length of the windows, though typically the length of the boxes will be 4 to 6 inches shorter on each side than the windows. For example, if you have 36-inch windows, the planter box should be around 24 to 28 inches in total length. The width of the planter box should be a total of 7 inches (inside will be about 6 1/2-inches wide). The depth of the planter box will also be 7 inches. So the total measurement for a 36-inch window would be 28 by 7 by 7 inches.
- 2). Cut two pieces of lumber (cedar or redwood) to 28 inches. You can use 2-by 6-inch redwood or 1-by 6-inch cedar fencing. Using 1-by 6-inch cedar fencing is more appropriate for building window planter boxes. Three more pieces of lumber will be needed soon. Cutting all lumber at the same time will make the building process go faster. The other three pieces of lumber should be cut to 7 1/2-inches (two pieces) and approximately 24 1/2-inches long.
- 3). Cut one dado groove 1/4-inch deep into each end of the two 28-inch boards one inch from the ends of each board with a table saw. A dado groove is a rectangular groove cut across the grain of a board into which another board is inserted. A dado groove creates a very strong joint between two boards. The boards that will be cut to fit into the dado groove will create the ends of the planter box.
- 4). Insert the ends of the 7 1/2-inch boards into the dado grooves after squeezing weatherproof wood glue into the dado grooves. Use a pneumatic nailer or stapler to hold the joint together tightly.
- 5). Measure the internal dimension of the box lengthwise. Cut another piece of lumber to this measurement. Slide this piece of lumber into the box and tack it into place with the nailer or stapler to create the bottom of the box. Drill one hole for every 2 feet of length for drainage with an electric drill and a 1/2-inch drill bit.
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