to the wall. The only tricky measuring you have to do is to calculate how far below the bottom of the windowsill or window frame to attach the piece to the house.

Typically, the top of the window box, once installed, is a inch or so below the window sill or the outer frame of the window. It’s easy to do the math to see where the bottom of the house piece needs to be in order for the box to be at the right height. If you started with a piece of lumber 3-inches wide, the square bottom of the house piece needs to be 4 inches below the bottom of the windowsill or window frame.

The pieces of treated lumber that are screwed to the house as well as the window box need to be attached using galvanized or stainless-steel fasteners. These will not rust.

When you attach the one half of the cleat to the house wall, be sure it’s parallel with the bottom of the windowsill or the window frame. You’ll be tempted to use a level for this task, but that could be a mistake. You don’t want the window box level if the bottom of the window itself is not level. It will look like you made a mistake after you’re done.

You may have to install one or two 3/4-inch blocks of wood on the lower rear corners of the window box to ensure the box doesn’t tip. The French cleat will hold the back of the window box away from the wall, so the blocks ensure that the top of the box is level front to back. These blocks will not be seen when you look straight on the box.

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