These corrugated steel garden boxes are a nice addition to our back yard. We have a pretty big garden area, so these are fairly large (4 ft W x 10 ft L x 3 ft H), but they could be scaled to about whatever size you want.
The only limitation is the manufactured dimensions of the corrugated steel. The metal I got through Home Depot is 24 inches wide and can be ordered in any whole foot length of at least 4 feet. This limited me to a minimum box width of 4 feet. You could cut the pieces shorter with tin snips, but then you expose the interior of the material to corrosion and rusting. Make sure to use the special metal roofing screws with rubber washers to ensure a good seal, which will also help prevent rusting.
I used rough sawn untreated cedar for a more natural look. All of the structure is on the outside of the box. This minimizes the amount of wood in contact with the soil. I applied a transparent oil stain to the outside surfaces that don’t touch the metal panels (the galvanized metal is designed to be attached to untreated lumber only) in order to preserve the color of the cedar and prevent rotting.
Unless you can build the box in place, you’ll have to build it in pieces and assemble it at the final location. I put together each of the long walls individually, which were easy enough to move on their own. I started with the two longer sides.
The extra 2x4s on the left side are just there as bracing until the metal is attached. Here’s a closer view of how the pieces fit together in the corner.
And the two shorter sides.
Attach the metal panels to the inside of the walls (remember the structure is all on the outside), making sure the top sheet overlaps the bottom sheet on the inside of the box. This will keep it sealed well and your soil won’t leak out through the side.
I put screws every couple feet, which seemed to do well enough to secure it.
On the long, back walls of the box, you have to leave enough space for the metal on the opposite wall to fit together. So leave a 5/8 inch gap between the corner and the edge of the metal on one side, so the pieces fit well.
Note also it pays to spend some extra time making sure the lumber you’re using is nice and straight and not warped. The second box I built had some pretty warped and twisted pieces, which resulted in a lot of things not being very square. That threw everything off quite a bit when I was assembling the box. You can see here the large gap I ended up with in the back corner:
Now move the all pieces into place and start assembling them. Make sure the ground is level and you have a plan for where the box should be placed. It doesn’t really work to just eyeball it, because all the walls need to fit together squarely. I’d also recommend using stakes to hold the walls in place while you’re assembling and filling it. I had some issues with the walls moving on me by a couple inches, which again causes it not to be square.
Now is the time to run any irrigation lines you want inside the boxes as well.
Make sure the first short wall is placed very accurately so everything fits together properly. On the short ends of the boxes, I just used two 2x4s to attach the long back wall and the short front wall. Then you have the surface to attach the sheet metal to.
At this point you can also begin filling the box to some extent. It’s a lot easier to do some of it now with a wheelbarrow, rather than once the box is completed and you have to shovel it all in manually.
I had a lot of issues with the sheet metal bowing out due to the weight of the soil, so I attached short 2x4s (or 2x6s ripped in half) to each of the acute corners. This helps support the corner and prevents the metal from bowing out.
Finally, attach the last metal pieces to the inside of the top level box.
Now that the main structure of the box is complete, I added 2×6 trim across the top edges. This looks pretty nice and it covers up the sharp edges of the sheet metal.
On the top box, some of the sheet metal pieces were protruding above the 2×4 (mainly due to the box not being perfectly square.) I used a hammer to flatten the edge down on the 2×4 so there was a good surface to attach the trim.
Finally, cut the 2×6 trim to length, as well as the 45º angles for the corners. I also notched out the ends of the lower trim on the back sides to wrap around the 4×4, which I think looks a little nicer.
On each of the corners, I also drove one screw through the angle joint just to keep the corners more snug.
You can attach most of these trim pieces from the bottom, by driving a longer screw up through the 2×4 that they sit on. This keeps the top surface nice and flush without any screw holes. The exception is on the short ends of the boxes, where the 2x4s are placed the “tall” way. So unless you have wood screws longer than 3-3/4 inches, you’ll still need to attach those from the top.
Again, remember the stain these pieces before attaching them, it’s much easier that way!