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.
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.
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.