I am not sure what, if anything you should put under the bed. I would think the water would drain out the sides at the bottom if it is on concrete, as it certainly wouldn't be water-tight. Do a search using the search box at the bottom of the SFG forum home page for cement and for concrete. You will find others who have put their beds over cement as well.

If anything, I would sit the bed right on the concrete, but line the bottom, going up the sides, with landscape fabric so the soil won't leak out the bottom which would be messy.

Here is one that talks about height of the bed over cement ...
http://forums.gardenweb.com/forums/load/sqfoot/msg041954057656.html

I'm going to do what you said about putting landscape fabric in the box. However, what can I use to keep the fabric in place besides using a staple gun?

The purpose of the garden cloth/cardboard is to keep weeds from growing up from the soil the raised bed is placed upon. Plastic, cardboard, garden cloth, newspapers all work to name a few.
That being said, I like cardboard best, because it attracts worms which eventually break the cardboard down,then work the soil above. Worm composting in the ground. After the first year it doesn't seem to matter as the weeds don't seem to come back.

If my SFG bed was on soil i would follow your advice. However, my bed is going to have to be on cement.

Jose,

I think a staple gun would be your best bet. Also, since the bed is going to be sitting on concrete rather than soil, it will act more like a container. You won't get the wicking effect having it on soil would give you. As such, you may need to take a slightly different approach than just following the rules straight out of the sfg book. It should drain just fine without raising it though, if you are concerned about the drainage, you could always drill a few holes around the base.

Don't let my words discourage you though, you will still be able to have a greatly successful garden, it just may take a slightly different approach than a standard raised bed on soil would.

You may want to search the container gardening forum for advice from others growing in situations like yours.

Good luck:)

Mike

Okay I'll look there. Thank you for the help everybody.

You won't need to staple it in. The weight of the soil will hold it in place.

Lots of people build SFGs on concrete, definitely do a search here and you will find lots of posts where it comes up in the conversation. The thread I linked for you above Includes a guy who is friends with Mel Bartholomew and talks about experiments they did, especially involving carrots.

These boxes were built to be placed on asphalt. They have landscape fabric stapled in place to keep the soil from washing out.

Here is a link that might be useful: How to make a simple raised bed

When my wife and I lived in Phoenix, we had a concrete back yard. I made a square foot garden out of a kiddie pool. I just cut a few holes in the bottom for drainage. It worked great!