Step 1: Make the frame.

illustration of windowbox construction with emphasis on the frame
Illustration: Gregory Nemec
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Use a sliding compound miter saw or a circular saw to size the pieces according to the cut list. Sandwich the box's end pieces between the front and back pieces, applying PVC cement to the captured edges. Clamp the assembly together. Tack the corners with 1½-inch brads. Use a ½-inch spade bit to drill drainage holes every 6 inches along the centerline of the bottom piece, and adhere and tack the piece to the box.
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    Tools List

    • 16-foot tape measure
      Tape measure
    • miter saw
      Sliding compound miter saw or a circular saw
    • hand clamps
      Clamps
    • brad nailer
      Brad nailer
    • drill
      Drill/driver
    • spade bit
      ½-inch spade bit to make the drainage holes
    • 1/4-inch drill bit
      ¼-inch bit to drill pilot holes into a wood-frame house
    • 3/16 masonry bit
      Masonry bit to drill pilot holes into a stone or brick house
    • four-foot level
      4-foot level to make sure the brackets are level

    Shopping List

    1x10 PVC board for the box frame. Get a 12-foot length.

    1-by-1-inch PVC outside-corner molding for the vertical trim. Get an 8-foot length.

    PVC cap molding for the top and bottom. Get two 8-foot lengths.

    1½-inch brads for securing the box frame

    ¾-inch brads for securing the molding

    -by 3-inch lag screws to secure the brackets to the house

    1¼-inch deck screws to secure the window box to the brackets

    1 pair of weight-bearing window-box support brackets

    Masonry anchors for stone or brick walls

    PVC cement

    Caulk