Make your own Concrete Garden Boxes! Build the forms and precast your own reinforced concrete panels that lock together to make long lasting and durable concrete garden boxes. See the complete build video that shows step-by-step how to make the plywood molds for the precast garden boxes. An alternative to concrete garden edging.
Background for the Cast Concrete Garden Box Project
Hi is Kent from Man about Tools and today we are building plywood moulds to precast our own concrete garden box panels.
These panels link together to create long lasting, rot proof concrete garden boxes.
Precast concrete raised garden beds.
If you’re like me then you’ve made many wooden garden boxes only to see them rot away over time. Now, I know wooden boxes have their place and they are cheap and easy to build, but they just don’t last.
I’ve used plastic liners, made them from longer lasting red cedar, bought a truckload of cheaper culled lumber and no matter what I do they still rot away.
So I decided to try making some boxes from concrete. One option is to build plywood frames, stake them into the ground and pour them in place. In a similar fashion as pouring building foundations or retaining walls. That does work well, and I like that option if you have the materials.
But instead, I want to make a more decorative, unique garden box, one that might be more suited for flowers, or closer to the house, instead of a large vegetable garden.
Poured concrete raised beds.
The panels that I make interlock in the corners in a straight line or at a 90 degree angle. I cast in plastic pipe so they lock together with rebar.
Pouring your own concrete panels gives you the flexibility to modify this simple design to your liking. You can easily change some of the dimensions to make them longer, shorter, taller, or thinner. You can also combine panels of different lengths as I’m going to do in this video.
I have two identical moulds that make a 48 inch long panel that is 8 inches tall x 2 1/2 inches thick.
I’m going to make 4 new moulds similar to that original design but, I’m going to shorten the length to 36 inches. Having a box 3 feet wide can make it easier to reach in to plant and weed.
DIY concrete raised beds.
- The PARTS
- FIRST cuts
- assemble the SIDES
- assemble the ENDS
- make the INSET
- TEST FIT and ASSEMBLY
- apply FINISH
- PIPES and WIRE
- final ASSEMBLY
- pour CONCRETE
- make the GARDEN BOX
Concrete Garden Box Sides – the Build
Here’s how I build the forms to make the concrete panels.
One sheet of 3/4″ plywood is enough for 4 forms. I’ll also need a length of 5/8″ material 3″ wide for the decorative insets (104″ in total). And a 4×4 – 32″ long that I’ll rip into a 2 1/2 x 2 1/2 inch block to hold one of the pins. These pipe holder pins are made from 5/8″ diameter doweling (16″ required).
Here’s the main parts of the mould. A base, the inset, the sides, the ends, and pipe holder pins. Cast into the concrete there’s a reinforcing stiff wire grid, PVC pipe, and wire loosely wrapped around the pipe.
I had my lumber supplier cut a 3/4″ plywood sheet into 4 strips, each 14″ x 48″. I have a small portable table saw so working with a full sheet is challenging. You can use a circular saw with a guide as an option too.
I’ll start by cutting these 4 bases down to the length of 43″. Then I lay out the design on each base. I’ll used a black marker to darken these pencil lines. I cut the rest of the 4×8 sheet roughly in half to make it more manageable.
Cut the Side parts
Once this is done start ripping the 8 Side Walls from the balance of the plywood sheet. Eight pieces are required. Then cut to length and label them part “B”.
Now rip the eight Side Hold-Down strips and also cut them to length. There’s going to be a lot of pieces to this build so label the parts as you go along. On the drawing you will find a scheme that I came up with that works well. For instance, all the Sides are B, the Hold-Downs for the Sides are labeled C, and so on.
The Sides and Hold-Downs are attached to form a right angle “L” shape with glue and screws. I pre-drill the Sides on the inside face, 3/8″ up from the bottom edge with a countersink drill bit. Then draw a line and layout the hole locations approximately spaced 8 inches apart. A clamp as a stop on my drill press speeds this up. I also mark pencil lines on this stop as a spacing guide. I want the screw heads to be recessed into the hole. I’ll fill these holes with wood filler later after assembly.
I’ll also pre-drill the Side Hold-Down strips with a one eighth inch bit along the center line spaced about 8 inches apart. Later, during assembly, I’ll use pan head screws to attach the Sides to the Base, (part A on the drawings).
Mark the sides and drill a 5/8″ hole with a Forster bit in each for the Pipe Support Dowel.
Set aside parts B and C.
Cut the End parts
Now I’ll rip and cut to length the End Walls. They are 2.5 x 8 inches and labeled E. These have their own Hold-Downs. Rip and cut these to length and label F.
The Ends and End Hold-Downs get pre-drilled in the same manner as the sides and their accompanying Hold-Downs.
I make the Pipe Holder End Blocks, part G, from a fir 4×4. Then rip this on my saw using several passes from both sides down to 2.5 x 2.5 inches. I then cut these to 4 inch lengths using a stop on my miter saw.
I inspect these blocks, label the UP side then with a square I mark the center of one end.
And use a punch to make a small divot that will held guide the drill bit.
I chuck a 5/8 Forstner bit in the drill press. I have stop blocks clamped to the table to help secure the block during this processes. You can also use a vice to hold the block if that works better for you. Drilling into end grain like this can sometimes cause the bit to wander so go slow and allow the chips to clear. Drill this hole 1 inch deep.
Assemble the SIDES
Now assemble the Sides with glue and screws.
I attached a stop strip to my workbench and onto it I attached two hold down clamps. The clamps quickly and securely hold part C to the table. I also attached an end stop to align the ends of B and C.
Then apply a small bead of exterior wood glue to the side and press it into place. I drill a pilot hole into the hold-down strip then run in a screw.Then drill pilots thru the side all the way down and secure it with screws. Check each assembly that the L shape is 90 degrees.
Set these aside for the glue to set up.
The Sides have a small square Stop Block to hold the ends in place. Rip and cut these to length.
Then pre-drill these as well.
Assemble the ENDS
Now assemble the ends parts E and F.
I marked the center of each to help with alignment and also used the hold down clamps to secure the pieces during assembly. Like before, apply glue, drill, and attach with screws.
Drill pilot holes into E and attach the Blocks with glue and screws.
Mark the side assemblies and attach the small square Stop Blocks with glue and short 1″ screws.
Make the INSET
Next we will make the angled Inset.
I used 5/8″ material, and beveled the sides 30 degrees on the table saw. And cut to length on my miter saw also at 30 degrees.
I attach the Inset part to the base by pre-drilling with a countersink, then attaching it with glue and short screws.
Lastly, I’ll cut the 5/8″ Dowels to length and chamfer the end on the disc sander, or you can round off one end with coarse sandpaper.
Glue these into place and let the glue fully setup.
Cut a short test piece of half inch PVC electrical conduit.
Use this to test fit the Dowel Pins. Use some sandpaper to slightly reduce the diameter of these dowels so the pipe slips on easily. The inside diameter of the pipe is bit less than 5/8″.
Fill the screw holes with wood-filler, let it dry completely, then sand smooth. Also fill any gaps in the plywood and add some filler to the inset where it meets the base. Give everything a light sanding to break any sharp edges.
Test Fit and Assembly
With pan head screws, assemble the form using the layout lines as a guide. Check that everything is square, fits tight, and there are no gaps.
If there are any variations with each assembly, you may need to try different part combinations to see what fits together best.
Label each mould assembly one through four. And also label each part assemblies with the corresponding base so you reassemble them the exact same way each time you use them.
I apply several coats of Polyurethane Finish, lightly sanding between coats.
I used four coats of this durable flooring finish. Wet cement is very corrosive so you want to protect the wood to get the most life from your concrete forms.
Pipes and Wire
Cut the PVC conduit down to length.
Also cut a 30″ length of stiff wire and wrap it loosely around the pipe. This will reinforce the concrete and give it strength where it is the thinnest.
Cut a section of Concrete Reinforcing Mesh to size with small bold cutters and have it ready to lay in the wet cement.
This will make the concrete panels very strong.
This is where I remembered that not all 3/4″ plywood is fully 3/4″ thick. Most is now slightly undersized and I laid out the parts for actual 3/4″ plywood. It’s an easy fix by adding a few slim shims.
Once assembled, the molds need to be made water tight. I use a small bead of Latex Caulking to do this.
Latex works well as it’s not overly strong and will allow you to disassemble the mold easily once the concrete sets up. Don’t use regular silicone for this. It’s too strong and will be difficult to remove later. Latex caulk is all that’s needed.
Place the moulds on a strong and secure surface and level them in both directions.
Once the caulking is dry, spray the mould with a Releasent Agent to keep the concrete from sticking. I used Vegetable Oil Cooking Spray for this.
Pour Concrete for the Concrete Garden Box
The 48 inch mould I made takes one bag of cement to fill. These new shorter moulds will take approximately 3/4 of a bag.
Have everything ready as you need to work quickly before the concrete sets up. I like to add a small additional amount of Portland Cement to my mix to give it a bit more strength.
I mix the concrete in a wheelbarrow and use cold water.
Try to not make your mix overly wet. Too much water reduces the strength of the concrete and this extra water may soak into the wood forma and shorten their life.
It can take a while to get a feel for just the right mix.
Shovel the wet cement into a mould and fill it half way, lay in the Wire Mesh, and fill the rest of the way.
Gently tap the mould with a hammer to help settle the wet cement and allow bubbles to come to the surface. Also, any tool that vibrates can work well to settle the cement. I have a drill with an impact setting that I have used for this.
Now cover the wet cement with a plastic sheet and leave for at least 2 days.
Remove the Hold-Down screws and gently pry off the Sides and Ends. Gently lift the panel off the base. The concrete is still soft and will not cure to full strength for a few weeks so be careful handling them.
I use a paint scraper to remove the old latex caulking and a cloth rag to wipe down and clean the mould parts.
Make the Concrete Garden Boxes
I cut rebar into one foot lengths with a cutoff blade in my angle grinder. I clamp the long length of rebar across sawhorses. A hacksaw will work but it takes longer.
I level a spot in my garden and overlap two panels, and check the corners for square then drive in the rebar thru the corner into the ground to secure them.
I repeat this on each corner to complete the Garden Box. For this box I used 2 – 48″ panels and 2 – 36″ panels.
For deeper boxes you can stack and alternate the panels if you like. You will need longer rebar pins.
We put a layer of cardboard down over the grass then fill the box with compost and soil before planting.
It’s pretty rewarding to remove the panels from the forms. I get a kick out of that every time. I hope this tutorial has informed and inspired you to give concrete forming a try!
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- Dewalt Compact Job-Site Table Saw (DW745)
- Dewalt 12″ Sliding Compound Miter Saw (DWS779)
- 10″ Bench Drill Press with Laser
- Dewalt 20V MAX Compact Drill/Driver set
- Miter-Saw Workstation Tool Mounting Brackets – Dewalt(DW7231)
- Dewalt Heavy Duty Miter Saw Stand (DWX723)
- Dewalt 20V Battery Charger
- Bosch 1375A 4-1/2-Inch Angle Grinder similar to mine
- Excellent Hammer
- Socket Set
- Center Punch
- Angle Grinder
- Sanding Blocks
- Wood Filler
- Small Speed Square
- Compound Square
- Mini Square
- Hold-Down Clamps
- Bolt Cutters
- Quick Clamps
- Aluminum Ruler
- Center Punch
Adhesive & Finish
- I’m sure there’s something I forgot