How to Build a Window Box Planter – Easy Beginner DIY

Do you love the look of flowers blooming out of window planter boxes? I do. Which is why we decided to add some to our home this year, to add a bit of color at above-ground level.

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This is a pretty straightforward DIY project, great for beginner or advanced builders. (With my boards cut, each box took me about 20 minutes to build, start to finish.) So even if you’ve never built anything before, I think you can do this one!

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Materials needed:

  • 3 – 1×8 cedar boards, cut to desired length (for example, the width of your window)
  • 2 – 1×6 cedar boards, cut to 7-1/16” (the true width of a 1×8 board)
  • 2” exterior decking screws

Step 1:

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Attach corners of front board to bottom board. Cedar planks come with one side smoother than the other. Out of your three window-width-cut boards, choose the one you want on the front of your box and hold it upright. Lay another board, smooth-side-down, next to this one to form an “L” shape. Check for alignment on both ends. At the bottom of your box-front board (and, thus, through the side of the bottom board), predrill about 2” in from the board end. Put in a screw. Do the same on the other end.

Step 2:

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Secure front and bottom board attachment with more screws. Flip the L-shaped boards over (to form a tent) and continue screwing the two boards together. Place a screw about every 6”-8” apart.

Tip: Cedar is very prone to splitting, so you can predrill these if you’d like. (My boards were wet from the morning’s rain, so I did not predrill during this step.)

Step 3:

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Attach back plank. Repeat these steps to attach the back plank to your bottom board. Because the interior face of the back plank is the one that will be seen in your window box (the top few inches, anyway), make sure the smooth side is facing the inside of the box. In other words, the smooth faces of both your front and back boards should face the same direction. Predrill and insert your two corner screws of your back plank, then complete the attachment with additional screws.

Tip: Cedar planks can be slightly warped; check for plumb and hold securely in place while you screw to force the plank in place.

Would you believe you’re almost done? Because you are. Nice work.

Step 4:

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Position the sides. Position a 1×6 side piece where you want it in your box. This example shows the sides inset about ½”.

Step 5:

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Attach side piece to bottom of box. Holding the side piece in place, predrill then screw through the bottom plank into the side piece. If your cedar is a bit warped, this will help to pull things more squarely into place before you attach the sides.

Step 6:

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Secure sides completely. Predrill, then screw into the holes to attach the side piece to the front and back boards. Tip: Predrilling is almost always necessary for preventing or minimizing splitting the wood when you’re working closely to the end of a plank.

Step 7:

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Step back and give yourself a high five. Of course, you can stain, paint, and/or seal the boxes at this point. Although cedar is a great exterior wood in and of itself, it will grey more quickly without protection.To hang the boxes, screw two L brackets into your house’s framing studs under the window, checking for level often, and then screw the bottom of the box to the bottom of the L brackets.

I hope you enjoy building your own window boxes!