Thanks for the detailed response, see comments below.

JNJ wrote:
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.


Depending on what you plan on planting, a 3' depth is more than adequate.
If you're looking to put in trees and shrubs you might have some issues
though. For veges, you could probably get away with much less -- many beds
laid on top of soil are just 2x6 or 2x8 pine boards nailed together

Definitely no trees/shrubs, so maybe we will consider less than 3'. The
biggest plant in there would probably be tomatoes or squash. And I
might just grow tomatoes in the regular 'ol ground anyway.

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.


Question: How close to your house is this pad? What is the layout of the
area behind the house (sloped towards/away, level, cut-out in a hill, etc.)?

It is a level pad that used to have a smokehouse or a lean-to on it. It
is bordered on one side by the garage (which used to have a door opening
onto the pad area) and on another side by the neighbor's fence. So on
two sides it will be difficult to have bracing that is outside the
footprint of the pad (although I guess the garage wall is enough bracing
on that side).

Basically what you're building here is an oversized pot, so you should think
along the same lines as container gardening. Drainage is an obvious
issue -- you don't want the plants to have wet feet or they'll likely rot
out and/or suffer a variety of diseases. You mention a dip in the center of
the pad -- water WILL collect at this point.

You'll want to measure how level the pad is all over to see how water will
drain. Given this information, you can design a drainage system. Gravel
and screen with some sort of weep system along the walls is likely your best
bet -- be sure to have somewhere for the excess water to go though, and keep
it away from the foundation.

That's a really good point as far as container gardening; I hadn't
thought of it that way before. The pad has been exposed to the weather
for many years apparently, and it is cracked and sunken in the center
although the raised lip around the outside (6" wide, 2" above the pad
surface) is still all there. There is apparently some drainage in the
center fairly large crack, because even after heavy rain I never seen
standing water in it. I could probably break out the center if I needed
to, but taking out the whole pad would be a bit of work, which is why
we'd rather just build something useful on it instead.

In the worst case I guess I'd knock some notches into the surrounding
lip, so that the water would drain before it backed up too far into the
beds.

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?


What adjacent structures?

You can mount brackets into the concrete; lay cement blocks, drill holes for
rebar, then fill the holes in the cement blocks (where the rebar sticks out)
with cement; use landscape block or stone.... My immediate concern though
would be these adjacent structures.

A fence, and the garage back wall.

I guess I could go the hole-drilling route. That makes sense.

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.)


A liner will cause issues with drainage. Pine will last a few years
untreated before it becomes part of the soil in the bed. Treated lumber
will last longer obviously. Anything you use will have the bottommost
pieces in direct contact with the soil, liner or not, so water has a place
to drain. Anything you coat the wood with may leach back into the soil.

Right. For pine, are we talking 2 years, or 5 years, or 7 years, or
what? If I went with treated but lined wood, I suppose I'd be in fairly
good shape as far as any leaching from the chemicals - they would only
be able to leach at the very bottom of the bed where the liner stops.

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.


You could just as easily get away with using landscaping timbers.

You might be able to make this job a little easier by reducing the height of
the beds. What kinds of things are you looking to plant?

For some reason (possibly the time of year) I have not found landscape
timber around here. I've tried Home Depot, Menards, local lumber
stores, etc. Maybe it is strange to build something like this in the
fall, but in my case:
- I had other projects that took most of the spring,
- I already have the dirt and compost that's going into the beds, and
- I need to move the dirt pronto so I have a place to store this
year's leaves for next year's mulch

Reducing the height sounds like a good plan - that way, I could still
build it out of cedar without the cost being too prohibitive. And I
guess I could always add on the extra foot at some point in the future,
as long as I secure it properly. I'm not sure if I have enough dirt to
fill up a 3' high bed that will work out to about 30' long, anyway.

Thanks for the ideas,

Kevin