Q&A / 

Window Box Installation Tips

DEAR TIM: My wife wants me to install a window box or flower box, depending on what you call them. I’m pretty sure that if I’m successful, I’ll be installing a few more.

The boxes, when full of potting soil, plants and watered are pretty heavy. I’m very concerned that the box is going to fall and crash to the ground.

What have you found to work best when installing these decorative items. I sure need your help, as I don’t want to upset my bride. - Don B., Staten Island, NY

DEAR DON: Oh I’m quite aware of the wrath you might feel if your window box plummets to the earth scattering the plants and flowers.

Plants are Children - Beware!

My wife treats her plants like they are her children, and if I ever step on one or otherwise mistakenly cut one off with the weed trimmer, I’m in the dog house for quite some time.

Window boxes full of flowers can be used under windows or to add color to deck railings. They are sure to please. PHOTO CREDIT: Tim Carter

You do want to succeed at this task for any number of reasons. First and foremost is the safety of anyone who might walk under the window.

You sure don’t want a window flower box to detach from a wall and fall on someone’s head. It’s happened before and surely will happen again.

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There are several ways to support a flower box, the most common being metal brackets that are screwed to the wall. The challenge when using a bracket is to make sure the screws go into solid lumber. The only problem with this approach is that the placement of the brackets might not be centered under the flower box. Centering brackets on a masonry wall is easy, and it’s easy to anchor them for solid support. However, many houses have wood or vinyl siding with the wall studs undoubtedly off center.

This is why I usually don’t use brackets choosing to use hidden french cleats instead. A french cleat is a time-tested method to secure pictures, mirrors, mantles, and even a window box to a wall surface.

The french cleat is a two-piece connection system where the piece of wood or metal that’s attached to the window box interlocks with the mating piece that’s attached to the wall. The best part is that once installed the window box appears to be floating in mid-air with no visible means of support.


It’s easiest to use lumber to create a french cleat. I highly recommend using treated lumber in your case since the wood will be subjected to water. You don’t want the french cleat to rot over time and fail causing the window box to crash to the ground.

The french cleat can be made from 3/4-inch-thick material. You need a piece of lumber that’s about 3-inches wide and as long as the window box.

The magic happens when you cut the single piece of lumber into two pieces along its length. You need either a circular hand saw, or better yet, a table saw with the blade set at a 45 degree angle.

By splitting the piece of wood down the middle with the saw set at the angle, you create two pieces of lumber that look identical, but one actually interlocks with the other when mated together. This is what’s going to hold the window box to the wall.

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The piece of lumber that attaches to the window box is screwed to the window box so that the angled cut points to the ground and the long tip of the cut is not touching the back of the window box. The flat or square edge of this piece is usually flush with the top of the rear of the window box.

The other piece of lumber gets screwed to the wall of the house with the angled cut pointing up to the sky and the long point not touching the wall.

I’m sure you can see how when you bring the window box over to the wall and allow the two pieces of lumber to interlock that the box will be securely attached to the wall. The only tricky measuring you have to do is to calculate how far below the bottom of the window sill 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 window sill 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 window sill 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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