Window boxes add a charming touch to your house. However, modern plastic boxes are flimsy, and the more substantial metal grill styles are exorbitantly expensive. Fortunately, there are many free plans for wooden boxes that are sturdy and easy to build at home. This box requires only one naturally rot-resistant cedar board, inexpensive hardware and a few basic hand tools. It is sized for an average 36-inch-wide window, but the board lengths may be adjusted to suit any window size.
Cut the cedar board into five pieces and label as follows: two pieces 36 inches long (label one "front" and the other "back"), one piece 34 1/2 inches long (label "bottom") and two pieces 4 1/2 inches long (label "sides").
Lay the front and back pieces on a flat surface. Measure and draw a light pencil line ¼ inch up from the bottom on each piece, then measure and draw a similar line ¼ inch in from the edge on each end of both pieces.
Make a dot at the point where the penciled lines cross on each corner. Between those two dots on each board, mark additional points approximately every 3 inches along the bottom lines and at the center and ¼ inch from the top along the side lines.
Use the 1/16-inch drill bit to drill pilot holes for screws at all the marked points on each board.
Place the bottom board on a flat surface and draw a pencil line all the way across each end ¼ inch from the edge.
Make a dot ¼ inch in from each corner and in the center along the line on each end, and drill as before.
Run a bead of carpenter's glue along the bottom edge of each of the two end pieces.
Hold the end pieces, bottom edge up, on the work surface and align the bottom piece on top of them. Wiggle each piece a bit to ensure the glue spreads and the surfaces of the pieces are in good contact.
Using the pilot holes in the bottom board as a guide, screw the bottom to both end pieces with a screwdriver.
Turn the joined bottom and side pieces over on its side and run a narrow bead of glue along the edges of both sides and the bottom.
Screw the front piece to the sides and bottom using the pilot holes as a guide.
Repeat the previous two steps for the other side to complete the box.
Turn the box over and drill six to eight ¼-inch drainage holes spaced equally across the bottom.
Put angle brackets on the back of each end of the box in such a way that one leg of the bracket rests flush against the back of the box and the other leg is at a right angle facing away from the box.
Seal or paint the box if desired, or leave as is--cedar weathers to a soft silvery gray color.
Position the box so that the bracket legs lie directly on top of the windowsill, and then screw the box in place through holes in the bracket.
Place flower pots or a plastic window box liner inside, if desired, or fill the box itself with dirt and plant your favorite flowers.