Corrugated Raised Bed

Our raised beds are built with corrugated tin and 2x4s. It includes a 6 inch wide bench around the top which gives a comfortable seat while working the bed. Below the jump is instructions for building one.

The design was driven by a couple of factors. First, we're used to working 4x8 beds. We can comfortably work about 2 ft across the bed, so if it's 4 ft wide then you can work the whole bed.  8 feet is a good length for us - it fits the space in the garden, and all the materials to build it are easy to carry in our little pickup.

We wanted the beds to be about  2 ft high. As we get older, our backs and knees make us less enthusiastic about reaching for the ground. We also wanted a wider bench-like top to be comfortable siting on while working the garden. With the design we came up with, the frame for the bed is made entirely of 8 ft 2x4s. The top is made with 6 inch wide deck boards. It takes 3 sheets of 2x4 corrugated sheet metal for the sides.

Setup

Before we start, here are the tools I used:

• Miter saw
• Circular saw
• Cordless electric drill
• Phillips bit
• 1/8" drill bit
• 3/16" drill bit
• 5/16" hex socket
• Tape measure
• Dead blow mallet
• Claw hammer
• Speed square
• Framing square
• Level
• Pipe clamps
• Spring clamps
• Paint brush
• Carpenter's pencil
• Sharpie
• Safety glasses
• Earmuffs

Over the years I've built various shop aids - they're not pretty, but they're pretty useful. I built a platform for my miter saw to make it easier to hold long boards while I cut. 

I also have a large assembly table made from two sawhorses with some 2x4's running between them. Usually I'll throw some plywood over it for a table, sometimes I just lay my work across it. In this case I set the miter saw on it while cutting the longer 2x4's, and put a tarp under it so drips from the deck sealant wouldn't stain the concrete floor of the shop.

Each raised bed requires the following material:

• Treated 2x4x8 - 12 each
• 18 of them 26 inches long
• 4 of them 94 inches
• 4 of them 39 inches
• Treated 5/4 x 6 deck boards - 3 each
• two of them mitered at 96 inches on the long side
• cut the other into two mitered 48 inch boards
• Corrugated metal 2x4 ft - 3 each
• two cut to 88 inches
• one cut into two at 39 inches each
• 1 lb 2 1/2 inch  #9 deck screws
• 1/4 x 2 1/2 lag bolts galvanized -  8 each
• 1/4 x 4 lag bolts galvanized - 8 each
• Dry wall nails, or some other galvanized short big-headed nails - about 40
• 1 1/2 inch #9 deck screws - about 30. (Optional - could use the last of the  2 1/2 inch screws)
• Deck sealant - about 2 quarts. I got a gallon and covered 2 raised beds and had a little left over.

Material Prep

Each of the 4 sides consist of two horizontal rails (top and bottom) separated by vertical stiles which are notched to hold the rails. It turns out that one sheet of tin is about 26 inches wide, so the stiles are all cut to 26 1/2 inches.  Altogether, we need to cut 18 stiles.

I clamped a board 26 1/2 inches from the miter saw blade, that way I could quickly slide the 2x4 up to the board and cut it without having to measure for each cut. It turned out that my setup wasn't exactly rock solid, so I ended up with a variance up to about 1/8 inch on some boards. Next time I'll rig up something a little more stable.  Also, since the boards are so long, I could only get 3 stiles per 2x4. I saved the offcuts for a later project (more on that later).

It's now time to  cut the notches in the end of the stiles to hold the rails. First, set the circular saw to cut to 1 1/2 inches - the thickness of a 2x4.

Now clamp 9 of the boards together side-by-side.  Note that you could do more or less - My pipe clamps aren't long enough to do all of them at once, so I did two batches.  Clamp lightly, then use a mallet or your fist to line all the boards up flush with each other (It was at this step that I realized my cutting jig wasn't 100% accurate - I was so distraught I forgot to take a picture...).  

The notches should be the width of a 2x4 - 3 1/2 inches.  I clamped a scrap piece of 1x4 to serve as a guide for the circular saw and made multiple cuts about 1/4 inch apart. You should be wearing eye and ear protecting while doing this. The sawdust is flying and the saw is pretty loud.

As you can see, it makes a lot of sawdust. Knock out all the chips with a claw hammer. 

Clean up the cuts with a chisel (the widest I had was 1 inch, which worked pretty good).  You can check to be sure the notches are the right size by laying in a piece of 2x4 and make sure it doesn't sit too proud.

Repeat these steps until you've notched both ends of all 18 stiles. 

Before we talk about cutting the horizontal rails, take a look at the corner detail. 

The rail for the long side is on the left. It runs to the 8 ft edge of the bed. The seat on the top is 96 inches long, with a 1 inch overhang. So the rails for the long side should be 96 - 2 = 94 inches.'

The seat for the short sides are 48 inches long, with a 1 inch overhang. the rails butt up to the corner styles, which are 3 1/2 inches wide. So the short rails should be 48 - 2 - 7 = 39 inches.

Cut 4 boards at 94 inches, and 4 boards at 39 inches. Now would also be a good time to pre-drill for the lag bolts that hold the bed together.  Drill a 3/16 inch hole in both ends of all 4 long boards. Drill about 1 3/4 inches from the end.  You should also drill 2 holes in 4 selected stiles. 

The seat boards are 5/4 x 6 deck boards. My little miter saw won't cut a 6 inch wide board so I raise it using short pieces of 2x4 as riser blocks. Unfortunately, it still won't make 45 degree miters, so I cut as much as I could with the chop saw, then finished it up with a hand saw.

By the way, this board had a split in the end - you can see in the picture above how a notch fell out. Be sure and take time at the lumber yard to pick out good boards. 

Once all the boards are cut, treat them with the deck stain. It's a lot easier to do it now than after everything is assembled.

Panel Assembly

Each 4 ft end panel consists of two 39 inch rails separated by 3 vertical stiles.  On a flat surface, lay the three stiles out, notched side up.  Lay the two rails in the notches. The two stiles on the ends should have 3/16 holes drilled in them. They should be flush with the ends of the rails, with the third stile centered in the panel.

I used the pipe clamps to hold everything together, and the speed square to verify that everything was square. Pre-drill holes with the 1/8 drill bit, then screw the frame together with the 2 1/2 inch screws, two in each end of the stiles. Repeat for the other end.

The 8 foot side panels have two stiles doubled up at each end, then 2 more 1/3 of the way from each end. Clamp two of the stiles together, then drill and screw them.

Lay the end assemblies and the other two stiles out with the notch side up, then lay the long rails into the notches.  The end stiles should be 3 1/2 inches from the end of the rail. 

Square them up, clamp them together, then drill and screw it together.

If you forgot to pre-drill for the lag bolts, do it now.

Once the panel frames are built, you can add the corrugated sheet metal. Cut them to length, and nail them to the inside of the frame.  I used ring shanked drywall nails, since that's what I had handy. Mostly you want big headed nails that won't pull through the metal. It doesn't have to be particularly strong, since the dirt in the beds will hold the sheet metal in place.

Be sure the last corrugation bends toward the frame. This ensures you don't have a sharp edge exposed.

Final Assembly

I built the panels in the shop, then carried them one at a time out to the garden.

Our garden has some slope to it, so I had to dig trenches to level it. I clamped the corners together with spring clamps and shifted things around until everything was square and level.  Once I was happy with it, I screwed in the lag bolts. The shorter ones went through the rails for the long sides. The longer ones go through the end stiles of the end panels into the doubled up stiles of the long sides.

Now to fill it up. 4 x 8 x 2 is 64 cubic feet, more than 2 cubic yards of material. As it happens, I had some logs around the place from a previous owner cutting down some trees. I used these logs to fill the bottom 12 inches or so.  My hope is that some of the benefits of Hügelkultur. As the logs rot they will provide valuable nutrients to the soil, and hopefully the wood will act as a sponge to soak up water when it rains, then release it when it's dry.  We'll see next July...

I then added partially composted leaf mulch. The Huntsville Botanical Garden has mountains of it that's available for free. I filled the back of the truck - roughly a cubic yard of material.

I packed it in pretty good around the logs, thinking the composted material will help the logs to rot faster.

Our neighbor Susan works at a horse boarding facility. I followed her over there and loaded a dozen old feed sacks with a mixture of old stall bedding and horse manure. Half of them went in this bed, the other half in another raised bed I had built earlier.

After topping it off with a bit more leaf mulch (mostly so flies won't be attracted to the manure) I put the top boards on. This was a pain in the ass to get the miter joints to close, and I'm still not real happy with it. If any one knows how to do this better I'd love to hear from you.

The Botanical Garden also has free wood chips. I've lost count how many truck loads I've gotten from them. I got one more load and spread it around the beds to give it a nice look. I plan to build two more beds next month. They'll sit through the winter to allow everything to compost and settle. I'll top it off with some good garden soil and compost and plant next spring.