Building a wooden window box is about an 8 hour process to do it right. Luckily most of that time is drying time and your actual time to gather supplies and build the box will be less than 3 hours. This article will be a step-by-step process to build a 48" long window box using plans below.
You'll need to acquire the wood to build the flower box and for this I recommend using cedar, cypruss, or redwood only which can usually be purchased at Lowes or Home Depot home improvement store. Pine and other cheaper woods will likely result in a flower box with half the lifespan. The lifespan of a well kept and well made window box is 5-10 years. There are some companies that make PVC window boxes that are lifetime guaranteed so you can check them out if you prefer to go that route. Some of these companies also make self-watering window boxes that only have to be watered 1/3 as often.
For your window box plans you need the following materials:
8' long x 3/4" thick x 7.25" wide wood x 2 pcs
#6 x 2" trim head screws and drill
caulk and caulk gun
wood filler or epoxy
paint brush, paint, and primer
tape measure, pencil, and work table
miter saw and orbital sander
exterior wood glue and a rag
1/8" countersinking pilot drill bit
Window Box Plans
The first step is to cut all your wood. Since we will be using 3/4" thick wood some parts will be cut 3/4" less than others. Just realize that if you are using 5/8" or 1" thick materials then those same parts may need to be cut by those slightly different amounts.
Start by cutting the front board of the flower window box which should be cut to the length of the flower box. In this case cut it at 48" long. For your wood boards I recommend either 7.25" wide or 9.25" wide boards that are smooth planed. After you cut the front board out the next step is to cut the side pieces. The side pieces should be the same height as the front board but cut to length to be equal to the width of the board plus the thickness of the board. In this case 7.25" + 3/4" = 8". Cut two side pieces this long and set them aside.
Now it's time to connect the side pieces to your window box. Use the countersinking pilot drill bit to drill three holes in the front board along the edge where it will butt up to the side piece. Apply the wood glue to the pieces and screw together with the trim head screws and drill. These screws will leave smaller holes to fill with the wood filler later on. Connect both side pieces this way so that it matches the above diagram.
Now you are ready to cut the bottom and back boards to your window box. These can both be cut the same at 46.5" long x 7.25" wide x 3/4' thick. You are basically cutting the boards 1.5" less than the front board to account for the two side pieces which are 3/4" thick. Connect the bottom board first. Since the side pieces are 8" long and the bottom board is 7.25" wide there will be a 3/4" leftover overhang which is exactly what you'll need for the last step when you connect the back board. Glue all components together and predrill with a countersinking bit and then finish screwing together tightly with the trim head screws.
Once the box is built you should fill all the holes with wood filler and caulk the inside seams. Set everything in your garage to dry for a few hours then come back and sand the outside of the box with an orbital sander to a smooth finish. You are now ready to prime and paint the boxes. Make sure to paint the inside of the window box to protect it from moisture, water, and insects.
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