Window boxes come in a variety of shapes and styles, but the one thing they all have in common is that they sit in front of a window. Getting them in that position is often a challenge--particularly if the box is a large one that will be heavier still when filled with soil and plants. Fortunately, it is no more complicated to mount heavy-duty brackets as to use less-sturdy methods to support your window box.
Use a stud finder to locate the framing timbers within the house walls beneath the window where you want to install the window box.
Measure your window box to determine the number of brackets needed--figuring one bracket for each end of the box, and additional brackets for every 16 to 24 inches between (depending upon the spacing of your house's framework).
Measure the height of the window box to determine the distance below the window for placing the shelf brackets.
Measure the same distance down from the bottom of the window sill at each of the corner points and draw a straight line across the wall between them.
Position one heavy-duty steel shelf bracket against the house wall just beneath and touching the pencil line, at the spot where you located the first stud on the left side beneath the window. Use a pencil to mark points through the holes in the bracket for future placement of screws. Repeat for each bracket working left to right.
Drill pilot holes at the marked points (sized slightly smaller than the diameter of the screws you will be using).
Screw the brackets to the wall using pilot holes as guides. (Screws should be long enough to penetrate the siding on the house plus at least 1 1/2 inches into the wall stud.)
Set the window box on the shelf brackets and use a spirit level to make sure it is level. Adjust the position of brackets if necessary.
Mark the locations for screw holes on the bottom of the window box (while it is in place on the brackets). Remove the box and turn it upside down on a flat work surface so you see the marks.
Choose a drill bit the same diameter as the holes in the bracket, and drill through the box at the marked points.
Choose bolts of the same diameter as the holes, and of a length to penetrate the bracket, window box and enough distance beyond for the nut and washer to fasten.
Put the box back on top the brackets, and line up the holes in the box with those of the brackets. Insert one bolt into each hole from beneath the bracket and up through the bottom of the window box, fastening them in place with washers and matching nuts. Tighten the nuts.
Fill the window box with soil and plant as desired.