I have the same problem. I put thick cardboard under the bed and that lasted only 1 to 2 years and had to be redone. This fall and winter I am going to raise the beds up a little and put a bottom on the growing bed. It can be raised by putting cinder blocks, bricks or any kind of wood to raise the bed off the ground. I plan to use 2" thick by 12" high pine boards as a base. Then I will put the growing bed directly on top of the base. Since the base will not be in contact with the soil, I plan to use pressure treated lumber for the base. This will also raise the bed up a little so I don't have to bend over as far.

Interesting Yolos. I like the idea of using cinder blocks... But why not natural cedar instead of treated wood?

If you can find it in two inch thick wood, it is terribly expensive. Nobody around here stocks it. You have to special order it from a lumber yard. One time I saw it at Home Depot but that was a few years ago and it was only 2x4's.. I have used the thin cedar fence boards before but I don't like the thin wood and don't think it would work well as a base for another box to sit on top of the base.

Do you mean that the regular 1 1/2 inch thick lumber is not thick enough? (by that I mean that 2X4's are not really 2" X 4") ? Or do you mean that you can't find it in stores where you are located?

The 2 inch thick (1-1/2 actually) is what I use and it is a good thickness. The cedar fence boards we get here are about 1/2 inch thick and not appropriate (in my opinion) for stacking and holding another bed on top of a base. I have only seen the 2x4 cedar (or 1-1/2 x 3-1/2) one time in home depot.

Wow..Here, they are very easy to get... You can get them basically anywhere...

Unless that maple trees branches spreads out to about 100-150 feet, I think those are roots from another tree. You won't be able to help that if your property is full of trees.

You could try an elevated raised bed. Just elevate it slightly with the cinder blocks that you are able to get. Any kind of strong bottom with drainage should suffice, even some strong wood.

No, I am 100% sure they come from the maple. I could show you nighmare pictures and you would understand (and have nightmares as well ;), so I'm not gonna). Anyway, the raised beds would not be where the compost bin was, but almost under that maple. Thanks for the advice about cinder blocks.