We live in an old brick building with deep window sills.
We thought a window box herb-garden would be a nice touch for the kitchen.
To simplify the installation, we wedged the box into the opening. That way we didn't have to drill into the plaster and bricks.
Warning: This project is not toddler safe. If a little kid tries to climb this box, it could topple. We put some plants in containers to cut down on weight, but if it were filled with dirt, our box would weigh close to 100 pounds. No matter what, if this box fell on a small child it would be enough weight to kill her or him.
Step 1: Measure Twice -- the Window Opening and Then the Wood You Have Lying Around
Our window measures about 31-1/2."
the box at it's longest is 30-1/2"
The rest is taken up by the end wedges.
The bill of materials for us:
1x8 for the sides -- 2 pieces about 32" long,
1x8 for the bottom 1 piece about 30" long
3/4 plywood ends -- a little over 7" tall by the same width as the bottom.
Some glue and nails. If you want to splurge, then waterproof glue and galvanized or stainless steel nails cost more but are better.
some blocks 8"x10" x 1-1/2" for the end wedges.
We laid out the sides to taper in about 5/8" on each side, so the top was about an inch and one quarter wider than the bottom."
As for the ends and bottom, the angles are so slight (less than five degree) it wouldn't matter if you cut them straight or at an angle. We happened to have a sliding miter saw, so we cut at an angle. If we only had a circular saw, jigsaw or hand saw, we would have cut those pieces straight.
We banged the box together with some hot dipped galvanized nails so it wouldn't come apart so easy. (We used 2-1/2 inch nails which are over-kill and had to pre-drill our holes so the wood wouldn't split.) Because we have a biscuit jointer lying around, we glued and reinforced the joints to keep everything lined up and together.
Take your time on assembly, but remember that as long as you keep your plants living, you pretty much only have to worry about only one face of the box being pretty since everybody will have their eyes (or noses) on the herbs above.
Step 2: Cut the Wedges
Time to lay out and cut your wedge blocks.
I'm not going to pretend that this step is easy, but I am going to give a few different ways to get to the end product. The blocks should be big enough and flat enough to distribute the pressure to the wall structure and not just the plaster or drywall finish surface. The blocks have to MIRROR the angle of your box.
a. Best way to cut the blocks is to use a band saw. But if you have your own bandsaw, then you probably stopped reading by this point.
b. Second best is to use a hand saw -- two divergent approaches:
-- i. Start with a long board that you clamp vertically and mark three sides and try to make the cut lines meet before you cut the pieces to length.
-- ii. Alternatively you can cut the pieces to length and then just slice the piece in half. Make sure you have extra wood if you go with this bold move. It is way harder than it sounds.
c. Do not use a circular saw unless you plan on making a whole bunch of cuts and then chiseling out the waste, then sanding the pieces kind of flat.
d. If you have better things to do with your afternoon, then buy some wood shingles at the lumber yard and glue them to the back of some plywood. Cover the whole back of the plywood so you get good surface contact with the wall. Just as a caution: the shingle material could compress and shrink more over time than solid blocks, so you are taking your chances that your assembly will need some periodic tightening.
Step 3: Installation Tips for the Box
Mark the height that you want the box to sit at.
Better yet, temporarily prop up the box where you want it.
Wedge the shims in from the bottom, tapping them home with a hammer. Work from one side to the other to keep things level. Use a little piece of scrap wood to hold down on denting your creation from errant hammer strikes. Your aim is to wedge the assembly enough so that it does not slip, but not so much that your wall get smashed.
It was at this point in our project that we discovered our window opening wasn't quite square from front to back. We had to use an additional shim between the wedge block and the box end. (It was actually a chopstick not a shim and we don't recommend this solution.) Somebody smarter would have cut a compound tapered wedge. Somebody even smarter would have adjusted the box to be bigger in the back than it is in the front.
If you think of it, you can put some registration marks on the wall where the box is sitting. Over time if your box starts to slip, you'll have a head's-up that you need to wedge it in a little more firmly. Our box has not moved in the past three months. But if you are paranoid, pin each wedge into the wall with a small nail.
Step 4: Fill Installed Box With Plants.
Instead of filling the thing with dirt, we stuck with container planting.
We do not have to worry about drainage, wood rot, water spilling, or the weight of the dirt.
As a bonus, when the plants die, the swap-in is a lot quicker.
We have some sage, thyme, oregano and peppers growing right now.