I've had raised beds for about 8 years and often struggled with larger annual vegetables really getting established and flourishing. My housemate says it's the hardest garden she's ever worked in. We got the soil in the raised beds tested a couple of years ago and everything looked good: high amount of organic matter and enough of all the nutrients (we add homemade and commercial compost and homemade wood chips periodically, and the initial soil was a commercial vegetable garden mix since the soil here is heavy clay). I initially buried wood under two of the four beds when I built them, and that doesn't seem to make a huge difference right now in productivity. We used untreated wood to build the beds initially, and they've reached the end of their life and I've been replacing them with modular metal beds one at a time. As I've dug out the soil for each bed, I notice a lot of tree roots, and I'm starting to think that this is causing our problems. The garden is in the front yard and there's also a 40' tall very established western red cedar, a small apple tree that we've nursed back to health from a 2' stick after most of it was cut down by the previous owner, and a young fig tree that's now about 6'x8'. The bed right next to the fig tree had a nearly 1" root wrapping around the edge inside it, which I was able to leave in the ground but outside the bed when I rebuilt it, but all the other beds have some level of small roots. I don't want to get rid of the fruit trees, or the cedar which is so well established and beautiful.
The garden is in front of the house because that's where the only full sun on the property is, it's closer to irrigation water (only things we can grow without irrigation are overwintering veggies started inside, planted out in Sept/Oct, and harvested by May, zone 8b with dry summers), and the back yard is inconvenient to access because of the topography of the property. Moving all the veggie gardening doesn't seem like an option, especially because the open area of the back yard is also bordered by two huge spruce trees that divide my property from the neighbors. When I've dug back there, there are roots all through it too.
After looking up what other people have done and the challenges of trying to create a barrier against the roots that still lets water drain, I'm thinking about lifting the beds up and making elevated beds that are off the ground -- basically turning them into container gardens. Since I already have invested in the metal frames that go around the beds, I think it makes sense to continue using them. I was thinking creating some kind of platform with drainage to go underneath the beds, and then setting them on landscape blocks like 6" above the ground. I've seen a lot of plans and instructions for building all-wood elevated bed structures with legs, but I don't care about raising them up very high, the ground is somewhat sloped and I can make it flat by stacking/burying blocks, and I figure the wood in contact with the dirt would be the first to break down anyway.
Does this make sense? Would wooden platforms made of a more durable species, with gaps between the wood for drainage, lined with landscape fabric to keep the dirt in, work? Any other ideas for materials or items I could repurpose?