What are you planning to raise in that bed? I made a raised bed on a on old
asphalt driveway in back that was unused, had some thick uneven blocks to
hold in the dirt and put in about 1 foot deep or less of soil. I have grown
lettuce, collards, tomatoes, parsnips, sugar snaps peas, etc in there. They
have done fine. The soil packs down with time, so I have to add two-four
bags plus manure each spring. If you are planning to grow trees or shrubs,
obviously your bed will need to be deeper, but for vegetables or annual
flowers, you certainly don't need 3 feet of soil. (However you will need to
water in times of drought, more than for your regular beds).
"Kevin Finn" wrote in message
...
Hi,

My wife and I are interested in using up an old but sturdy concrete pad
in the back of our yard for a raised bed. The pad is about 10' x 14',
and we'd like to build the beds about 3' high, in a U-shape so that it's
possible to easily reach all areas of the bed. I haven't found anything
in the archives of this newsgroup that really discussed raised beds that
were not built on top of soil, and I'm a pretty new gardener, so I have
a few questions:

1. Is 3' of soil adequate for this kind of bed? We want more than 2'
since the plants will have no chance to get into the soil beneath the
bottom of the bed.

2. Should I make special plans for drainage? I was already thinking of
some gravel to fill in a dip in the center of the pad and to make a nice
path in the center, is it also worthwhile to have a couple inches of
gravel in the bottom of the bed boxes too? Hopefully I don't have to
knock some kind of holes through this pad itself.

3. I've considered both timber and block construction. The problem
with block is that I'm uncomfortable building it three feet high without
some kind of reinforcement, and I'm a little uncertain about the best
way to anchor, say, rebar into the existing pad. I'm not sure if I can
put in support posts outside the layout of the pad, because of adjacent
structures and the way the foundation of the pad flares out below grade.
Has anybody done this?

4. I had also considered 4x4 timber. I don't want to use the treated
wood, and cedar's a little expensive around here (Chicago area). How
long would regular untreated pine 4x4 last? What would be a good
exterior coating to help extend that life? (I already plan for a liner
on the interior against the wood, although maybe not at the bottom
depending on the drainage situation.)

5. I've seen a lot of people recommend 2x lumber, but I wonder if it
can support a 3' high bed without bowing out a lot (especially at the
bottom). I had been thinking about using 2x4s laid flat, so that the 4"
thickness through the wall is still there. This would save some money
vs. 4x4 lumber but this would be double the drilling and cutting
compared to 4x4s. Also, it may look very bizarre.

Thanks for any advice that comes to mind, and sorry if some of these are
sort of off the deep end

Kevin Finn