|4-mil. Black Plastic
1-1/4" Galvanized Screws
2-1/2" Galvanized Screws
1 tube Paintable Caulk
3/4" Cove Molding
Knot Sealer (such as Bin or Kilz brands)
NOTE: Hand saws can of course be used in place of power saws.
Screw Gun or screwdriver
Staple Gun (optional)
Always a nice addition to any house, flower boxes hung beneath windows add color and excitement to the exterior look of your house. Making window boxes is not a hard project but takes time to complete. I suggest you set aside a day to build the boxes and another day to paint and hang them.
Deciding how big to make each box comes first. Usually, the length should be about the same width as the window it will hang beneath. Next, decide how tall you want your boxes. Using 1 x 10 wood is common, but keep in mind that the actual size of those boards is only 3/4" x 9-1/4".
Once you have decided on a size, go pick out your lumber and other material. Hand pick the clearest pine available. The fewer knots the better, they will bleed through the paint eventually.
Making the Box
Once you've selected all your materials and have gathered the necessary tools, you can start building.
First cut the four sides to the right lengths: two short that will be the left and right box sides, and two longer that will be the box front and back. For all boards, mitre the ends that will meet in the box corners with a 45 degree mitre so that the corners will fit snugly together.
After you have all the sides cut to the correct size, you need to cut four 1 x 1 cleats (braces) to screw into the corners to hold the box together. You can use scrap wood, just make sure that it's at least 1-inch wide. Cut the length of the cleats so they are 1" shorter than the height of the box. (For example, if you're using wood that is 9-1/4" tall, cut the cleats to be 8-1/4" long.)
For each corner, place a cleat along the mitred cut of one end of a board and position it so that it is also flush with the top edge of the board, and screw it on with three 1-1/4" galvanized screws. (The other end of the cleat will be 1" short of the bottom of the box.) Then bring the two parts of that corner together, and screw the other board to that cleat with three screws, making sure the top edges are flush.
Continue doing this in all four corners until the boards are all screwed together, making one box.
Now set the box on a flat table so that the bottom opening is facing up. Use a tape measure to measure diagonally across the opening from corner to corner. Repeat for the other diagonal direction. Those two diagonal measurements should be equal. If not, twist gently on the box until the measures become equal, and retighten the screws if necessary. This is called "squaring it up."
Attaching the Bottom & Capping the Box
Measure the length and width of the bottom opening, and cut a piece of wood to be the box bottom.
Place the bottom piece in the opening so that it rests on the cleats and is recessed about a 1/4" equally all the way around. Attach the bottom by screwing through the sides of the box and into the bottom, maybe using four screws in the front and back and two in each end.
Now the next step is to take the 1 x 4 wood and rip it (cut in lengthwise) in half so you have strips about 2" wide. This will be used as a decorative cap for the box. The cap is used on only three sides, all but the back.
Cut three cap pieces to the right lengths: two for the left and right sides, and one for the front. Use mitre cuts at the front corners of the box so the pieces will fit snugly together. For the front cap piece, you will use mitre cuts at both ends. For the left and right cap pieces, use a mitre cut only at the front end of each piece. The back ends of the left and right cap pieces should be cut straight, so they are even with the box back. (See photo.)
Position the cap pieces on top of the box so they are flush with the inside of the box, and hang over the outer edge. Attach them using the 1-1/4" screws, screwing straight down through the cap into the sides of the box. Make sure your screw heads are sunk below the surface of the wood so that you can caulk them later.
Once you have finished the cap of your box you are ready to apply the decorative cove molding, which is placed below the cap on the outside of the box. Again this goes on three sides of the box, as the cap did. Mitre the front corners as you did with the cap. Attach the molding with small brads or staples, then countersink them using a nail set.
This completes the construction of the window flower box itself.
Now that the construction of your box is complete, you need to prepare it for final finishing. Sand all outer sides well. Fill the spaces over screw and nail heads with paintable caulk.
Seal any knots in the wood with a couple of coats of knot sealer and then paint your flower box the color of your choice. If you don't seal the knots first, they will soon show right through your paint. Use any quality exterior paint for your box's final finish.
After the painted box has dried, apply the 4-mil. black plastic to the inside of the box, leaving about a half-inch ridge below the top of the box so once it's filled with dirt you won't see the plastic. Next drill about a half-dozen holes into the bottom of the box for drainage.
The final step before the completion of the project is to build two brackets that will be screwed to the house to support the flower box.
This is done by ripping a 2 x 4 piece of wood in half and building an angle brace like the one in Fig. 1. The exact dimensions of the brace will vary according to the size of your box and the siding on your house. Use the longer 2-1/2" screws to construct the brackets and to attach the brackets to the house.
The piece of the bracket that butts against the house needs to be cut to the contour of the siding on your house so that it fits snugly. First scribe (draw the pattern of) the piece that will butt up against the building. Cut the correct contour (you may need to use a jig saw to cut it this piece depending on how difficult the cut is).
Paint the brackets to match or complement the color of your flower box.
Once the brackets are built, screw them to the house and mount your decorative flower box on them. Fill with dirt, plant your flowers and enjoy!
Silas Casey specializes in home remodeling and construction for Sykes Construction of Poultney, Vt.