Installing window boxes can be a fun DIY project and proper care should be taken to do it right! There are many brackets nowadays that hook to your window or railing and allow you to just sit the window box on top. This is fine if your window box is small and you don't plan on filling it with potting soil. However, if your window box is larger than 30" or it will need to support more weight than just fake flowers, it's imperative that you install your window boxes in a safe and secure manner so that it doesn't shift, fall, or break. Decorative window box brackets can be added, but should rarely be used as the go to method for supporting a window box. First off, window boxes need to be installed so that they are sturdy and level.
This is important not only so that it looks right, but also that it drains properly. This is another reason to avoid simplistic bracket systems that can bend and warp under the weight of a well planted window box. The ideal method for installing window boxes is to use the old tried and true anchor-lag bolt method. By securing and installing a window box to a home's exterior in this manner you can ensure that it is level, sturdy, and most importantly safe. And as an added feature, it will most definitely add value to the home by becoming an integral part of it. The best way to secure a window box to your home is to anchor it through the back of the box and directly into the house.
To do this you will need to drill a 3/4" hole in the back of the window box approximately every 18" of window box length. The placement of the holes should be centered vertically. Next, transfer the location of these holes by holding the window box up to the window and placing a pencil or marker through the holes. Now, take the window box down and drill a 5/8" hole into the brick or rock over each mark. These are going to be your anchor points to support the box.
Place a 5/8" expandable lead anchor into each of the holes you drilled. The anchors should be able to receive 3/8" lag bolts and work by expanding as the bolts are tightened. Now, to finish, hold the window box back up and pass each lag bolt with a washer through the back of the window box and into the lead anchors. Tighten each lag bolt down till finished and then lastly, use a level to adjust the window box if necessary.
Viola! You've successfully installed a window box.
The author is the owner of Flower Window Boxes and has been professionally manufacturing and installing window boxes for many years. He recommends this method for installation for wooden and PVC window boxes that have solid back to them http://www.flowerwindowboxes.com