BUILDING CEMENT BOARD RAISED BEDS

Raised garden beds are a proven method for producing bountiful harvest. Although you can actually make raised bed gardens by simply building mounds, it is much more efficient to build walls to hold the soil in. Traditionally, raised bed planters are made out of wood or cement blocks. Wood is great but it will rot over time and can be very expensive if you want to use rot resistant redwood or cedar, especially when you are building high beds (higher than 8"s,) You don't want to ever use old railroad ties or pressure treated lumber for vegetable gardens as they contain chemicals which will leach into soil. Cement blocks will work but they are expensive and heavy to move around. I was looking for a cost-effective alternative and I decided to try and build raised bed frames out of cement backer board (brand names: Wonderboard, Durorock, etc.) cement board is usually used as underlayment material for tile floors and walls.

Cement board is basically cement with a fiberglass mesh on each side. It is available in 1/4" or 1/2" thickness, usually in 3' X 5' pieces. It is fairly easy to cut by scoring and snapping, similar to sheetrock. Using the concrete board, I built a series raised beds that were 20' long and 3' wide. So to build a 20' X 3' X 12" bed (4 - 3'X5' sections), you'll need five 3' X 5' boards. (you'll have a partial board left over.) Obviously, if you want to make smaller or shorter beds, you'll need less material. To connect the cement board together I utilized inexpensive hurricane ties and concrete board screws although you can also use more costly L brackets or other similar strapping material. Since I have problems with pocket gophers and voles I decided to layout 1/4" hardware cloth at the bottom of the the beds to help keep them out. Hardware cloth comes in 3' widths so that works out pretty well with the 3" beds. So in summary, here's a list of materials you'll need to build a 12" X 3' X 20' bed and the approximate costs of the material.

5 - 3' X 5' X 1/2" Cement Board (I used Wonderboard brand) @ $8.95 ea
16 - Hurricane Straps @ .29 ea
1 - box of 1-1/4" Cement Board Screws @ approx. $6
1 - Cement Board scoring tool @approx. $6
20' - Hardware Cloth (optional) @ approx. $20
8 -Flat framing straps (optional) for joint support between sections.
You'll also need a cordless drill with a square head screw bit (some concrete board screw boxes come with one in package with screws.)

Okay here's a step-by-step on building the frames, One person can do it but it'll be easier with a helper.

1) Score and split your cement boards into 1' X 3' sections (fig A). If my math is right;) you'll get of these sections our of a 3' X 5' board. TIP- It works well use a T-square to guide your scoring tool and it is easier to split the boards over a the edge of a solid even surface like the edge of a deck.

2) Prepare a level area to accommodate whatever size bed you are building. I dug down a few inches as my garden area has a slight grade. If you are building 12" high beds, you may want to dig down a couple inches so the sides have a little more support (fig B).

3) Begin assembling the corners by placing the metal straps on the first two outside corners, Place the the straps roughly centered between the top and the bottom of the beds. Or if you dug the beds down into the ground a few inches, adjust them up so they are a couple inches form top top of the frame (fig. D).

4) Finish assembling one (3' X 5') by repeating step 3, screwing in the 3' end piece with straps. If you are going to add another 3' X 5' section, place the straps on the inside corners. If you're going to be adding more sections, you will need to offset the straps so you can screw another set on the other side. At this point, double check to be sure all the sides are level (fig. C).
5) Lift the up sides and slip in layers of damp newspaper (about 1/4" thick) or cardboard so that it extends an inch or so outside the bed frame (fig. E). Then fill the entire bottom of the garden bed section with newspaper or cardboard. This will help keep weeds or grass from coming up into the bed from the bottom.

6) (Optional) Roll out hardware cloth and cut to length of the section leaving an inch or so extra on each end. If you're using 3' wide hardware cloth, you won't have any extra on the sides so it will have to fit perfectly.Next, drill in a cement board screw about every 2', approximately 1" from the bottom of the frame. Don't drive the screws all the way in so you can use some bailing wire to loop under the hardware cloth and twist around the screw (fig. F). This will keep the hardware cloth from shifting around.

7) If you're building more sections that will butt next to one another (fig. G), you can save money by utilizing one common end piece for two sections. So you'll have to add the side sections to the existing end. If your doing several sections end to end, you may find it necessary to add some addition support on the outside by adding some inexpensive flat metal framing straps over the joints.

8) Once you've completed the boxes and checked to make sure they are level, it's time to fill the box with soil. I like the lasagna method (see lasagna gardening post) for doing this or you can purchase clean top soil or top soil mixes to fill the boxes. The lasagna layers are built with a layer of peat moss between each element such as soiled barn litter, crushed leaves, compost, grass clippings, composted steer manure, etc. The lasagna method creates very light loamy soil which will be less pressure on the sides of the boxes. Be careful not to use heavy soil mixes in or you may experience bulging sides. Fill the boxes to the tops as you'll get a lot of settling.

There you have it. You may want to snip off any screws that are protruding out of the boxes to avoid being scratched. Also note that the cement board makes an excellent base for stone if you'd like to add a more finished look to your garden beds.

-HG

GLUMPISM- "Instead of planting seeds of discontent, plant seeds of carrots-they are much better for your vision."

10 comments:

Anonymous said...

Gret idea!
I will use this inside my landscape timbers as a barrier to potential leaching.

Bob

Gonzo1011 said...

Did you really use 5/8" cement board screws? I can't find those online or at my local hardware stores - they only seem to carry 1 1/4" or 1 5/8".

What kind of snips did you use for the screws?

Horace Glump said...

5/8" cement board screws was a typo. I've corrected to 1-1/4". The shorter the better. I did not snip my protruding screws off as most stick out inside the beds but a heavy wire cutter would work to snip off the pointed end of the screw. - HG

Anonymous said...

i've been wanting to try this, glad to see your excellent posting. now that you had them a couple years, any thing you would do differently? Thanks-

Anonymous said...

Do you have to seal or paint the cement board?

Horace Glump said...

I didn't seal the cement boards and they've held up nicely through 3 winters so far. I wouldn't want to add a seal anyway for fear that the chemicals would leach into my vegetables.

Anonymous said...

It's now 2014 are your beds holding up and are the sides still good and straight? Thanks

Horace Glump said...

As of January 2014, the cement board beds are still holding up. There is some deterioration along the top edges and I may need to add some additional bracing this spring but they've held up remarkable well.

Anonymous said...

I am considering building some large wicking raised bed in Zone 7. Looks like you have had great success with yours. So I have a sense of how much the weather affects the cement board, can you let me know what Zone your beds have lived in for the last four or so years?

Thanks.......mh

Horace Glump said...

MH- I live in a zone 4 climate and the cement boards have held up pretty well. Just some slight deterioration on the edges.