I would think anything that would smother the weeds, but wouldn't keep the water from draining would be good. In Lasagna Gardening the auther says to put down six layers of wet newspapers before layering the layers of peat moss,compost, etc. I made a mistake in my own raised beds thinking that 12" of compost, worm dirt, very old composted manure would smother the weeds, but I still have an occasional weed coming up from the bottom. The next ones I build I will put something down first.

One thing most of us don't ever consider is the millions of seeds blown around by the wind and carried by birds, etc. If these seeds land someplace hospitable, they will sprout.

One of our goals is to give traveling weed seeds no reason to sprout. That's why we
* amend our soil mix so it is rich and yummy (except to weeds--or most herbs, heh--most herbs are "useful weeds," so we keep an herb bed)
* deny weed seeds access to light and moisture
* plant good crops thickly
* mulch with 3 inches of good compost (or other good method), except right up near plant stems

BinderBees;

I have used the landscape fabric under a bed after I moved it. It did a good job of smothering the original weeds. One pesky thing came up but it is more like a nasty shrub and had very strong sprouts.

If you want to use weed free local soil I would advise digging down at least 6 inches and taking soil from there. Since there are fewer roots and no seeds that deep. But the whole point is not to have to dig so much. I often process soil through my compost pile, so everything sprouts and gets eaten up by the time I need the soil 6 month or a year later. It take time though...

I have used the newspaper method in one bed once and it worked okay, but the landscape-fabric is better IMHO.

10" & 16" is very deep, since typically you should only need 6" for most garden needs. Long carrots & parsnips usually only need 12" of Mel's Mix.

I suppose you could use some local soil in the bottom and then put a layer of newspaper over that just in case. I think the whole thing is kind of a cost to benefit ratio decision. There is a low chance you might have to muck around with weeds if you make a mistake. But weeds are part of gardening anyway and you will still have to weed the weed-seeds that blow in anyway. I think anything over 10" of Mel's mix is excessive anyway.

I have been thinking about using Trex or the like for SFG boxes. It seems like a good solution except that it's a plastic & wood composite material, I wish I knew what kind of chemicals it releases as it breaks down. Just a thought.

CloneZero

CloneZero-

Trex is completely safe for using with food crops. I know this because I was just the General Coordinator for a MASSIVE (largest in our State) playground project. Everything we used on the playground had to basically be edible in order to be allowed because kids will gnaw on anything. Most of the playground is built from Trex. There is some wood on the undersides of things (Southern Yellow Pine). The wood is sealed with a non-toxic sealer from some company in CA. It's the only sealer in existence that could be used because of the chemical issues with others.

So, anyway, just thought I'd let you know that, although expensive, Trex is perfectly safe to use. I was lucky in that I was able to buy the scraps left over from the playground project so I got a really good deal on it. That stuff is expensive! BUT it is also very dense and strong. A great material to use for raised beds. I'd recommend you call a deck building company in your area and find out if you can purchase scraps of Trex from them. Worth a shot to save some $$$.