I found some pretty reasonably priced 1x8 knotty cedar (.75" x 7.5" ), And I'm looking to make some raised beds using it and wondering If i brac it every 5 feet or so, if that will be think enough to hold back the soil.
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what kind of soil? clay, sandy, high organics? How long do you want this to last? wet soil is heavy and pushes against a retaining wall of any type– kevinskio ♦Mar 27, 2016 at 19:01
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How tall are your beds going to be?– user13638Mar 27, 2016 at 19:32
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The beds will be 15" tall ( two planks ) The soil, is a mix or organics, vermiculite, reg soil etc. So it's pretty standard, medium densiity– user379468Mar 27, 2016 at 22:02
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What's the soil like underneath the beds?– Graham ChiuMar 28, 2016 at 6:06
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2 Answers
This will work for at least five years if you take a few extra steps:
- at the corners and halfway along the length of each plank attach a 2" x 2" or similar post with galvanized or similar outdoor grade screws. The posts should at least 30" inches tall so you have 15" supporting the planks and 15" in the ground.
- for a really durable raised bed get pond liner or similar food grade impermeable plastic and lay this between the planks and the soil of the bed
The stakes will keep the planks from shifting and the liner will keep the soil away from the planks. Soil against any wood will cause rot sooner or later. The liner prevents this and stops soil from washing out.
answered Mar 28, 2016 at 0:20
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Do the posts go inside the bed, or outside? And what's to stop the posts from rotting? Mar 28, 2016 at 6:12
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I put mine on the inside, the beds look much neater, and you can use a single post per corner. Having said that I bought tantalised/treated timber, so didn't need a liner. Personally as my posts have rotted (untreated timber) and the beds falling apart, I wouldn't do it this way again. I'd put the posts on the outside, so they are more easily replaced, an although you will need more of them, I would also use some posts very tall (3ft above the bed) These can then be used as the frame for netting without purchasing extra kit, hang your cost on and as a leaning post.– user13638Mar 28, 2016 at 8:30
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The Posts go on the inside, and they do rot, slowly. As a note using treated timber in contact with the ground is not recommended as the copper compounds used to treat it leach into the ground. I see it done a lot anyway..– kevinskio ♦Mar 28, 2016 at 9:44
I think if you are raising the soil 15" as mentioned in one of your comments, that you probably would be better off using 2x lumber.
Ceder is rot resistant, but it weakens quickly when exposed. This is fine for roofing and siding because it's not providing any structural support. But in this case I think the soil will bust out the sides sooner than expected.
If you use 2x12, it may not be a full 15 inches, but I think you'll get 5 years of use out of it. Although the interior will begin to rot within a year or two, the lumber will remain flexible and so won't bust out until the last few fibers give way. Cedar when it weakens remains brittle, so when it goes it'll just split and crack from the pressure of the soil And probably in the winter from Ice heaving if you live in a freezing zone.
Also, instead of using posts in the corner, I use angle irons around the outside. This provides the greatest structural support because the pressure is perpendicular to the nails or screws. If you put angle irons on the inside or if you put posts on the inside then the pressure is "pulling out" the nails and screws and so those connections will fail much sooner.
