Step 3: Install the brackets.

illustration of windowbox construction with emphasis on the brackets
Illustration: Gregory Nemec
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Position a support bracket on the wall just below the window, to catch the sill framing, 3 to 6 inches in from the window's sides. Drill ¼-inch pilot holes through the bracket and into the house, squeeze caulk into the pilot holes, and fasten the bracket with -by-3-inch lag screws. For brick or stone, use a masonry bit and masonry anchors. Use a 4-foot level to position the second bracket before installing it.
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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