This year we'll replace the wood framing on the last four of our beds (hooray!) that we've also used for the last 20 years.
The above two pictures show some of
the rotting that has occurred
in the wood framing.
It's a project we've been working on for about the past 3-4 years, replacing a few each year. When these last four are done, all the wood framing of all our raised beds will be "new" and hopefully good for another twenty years of raising delicious, nutritious food.
Our first frames were made of two 2" x 6" boards stacked one on top of the other to equal a height of 12". We used the 6" boards because we had them, but it wasn't the wisest move. They didn't hold up nearly as well as the 2" x 12" boards we've used since then.
Initially we nailed them together into a 4' x 8' rectangle, but found with the soil freezing and thawing over the years, the joints started to pull apart, and we had to reinforce the corners with metal "L" brackets.
Using screws as fasteners works much, much better.
The screws which seem to do the best job are these.
The type of wood we've experimented with has included local rough-cut white cedar, tamarack and pine. The longevity of each has been about the same.
For the ones we've been replacing recently, we've used pine or fir from the local lumber yard with two applications of tung oil, thinking this would prevent rotting longer than unfinished boards. Time will tell if this proves out to be true.
When questioned, my husband stated that if we have to replace raised bed framing again after this time around, he'd like to go with a good quality western red cedar . . . probably with the coating of tung oil as a preservative again.
Although there's no question that pressure treated lumber for the raised bed framing would last longer, and supposedly the chemicals used in the pressure treating process are not as toxic as they once were, we wouldn't feel comfortable using any pressure treated product that comes in direct contact with the soil in which we grow our food.
Hope this gives an overview of the construction of our raised beds, and answers your questions, Karen.