Choosing your window box:
You will need a box that spans the width of your window frame.
How to support your window box:
You will need stout steel angle brackets. The bracket ends need to be bent up in order to hold the box in place. You should then secure the box even further by attaching nooks to the box ends and using short lengths of metal chain or wire, connect to hooks that are screwed into the wall or window frame.
Now, if you have a wide flat window sill that can hold the box then you should still use the hooks and chain, this helps to stabilize the box during wind or bad weather. If your window sill is not horizontal, you will need to get wedge shaped pieces of wood to fit beneath the box and be sure that they are thick enough and spaced far apart enough for your box to be able to have a drip tray.
How to choose your brackets:
The brackets come in a variety of sizes and designs. Choose ones that suit your home's exterior, but are also strong enough to support your window box.
