raised beds on an existing concrete foundation

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 snipped-for-privacy@speakeasy.net
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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).

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gregpresley wrote:

It's going to be a vegetable and herb garden, no trees or shrubs. To be honest, we mostly thought about 3' because it would be less bending over when planting or weeding. But I'm not sure if I want to do a lot more effort in constructing and filling the boxes, just for that :).
Thanks for the feedback; as a newbie this is the kind of thing I really didn't know about.
Kevin
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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

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.)?
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.

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 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.

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?
James
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Thanks for the detailed response, see comments below.
JNJ wrote:

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.

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).

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.

A fence, and the garage back wall.
I guess I could go the hole-drilling route. That makes sense.

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.

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
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Don't forget to provide drainage. Otherwise you could drown everything. I would place gravel on the bottom, then sand and then soil. Also, the sides would have places where the water could run out. Usually that will not be a problem.
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3 feet is massive overkill. I'd go with closer to 18-20 inches -- 3 inches of gravel followed by screen and landscaping fabric then a rich blend of soil, compost, organic matter, etc.. I'd also toss in a few different types of worms as well including red worms and earthworms (not likely they'll be working their way through the concrete into the bed)

This is a problem. You do NOT want water draining on either side. Are the front and/or back graded in such a way that water will drain away? If so, I'd use cement block on either side, mortared onto the pad and each other. To keep them firm to the pad you can use some short lengths of ribar inserted into holes you drill in the pad -- the other end goes through the hole in the cement block and you put cement in the hole to complete the picture.

Your best solution here would be to remove the pad -- it's near structures and that can pose issues. At the least you need to build a drainage system into the front and back to accomodate any excess then of course run it off somewhere productive. The simplest system is to use perforated plastic tubing, covered with a piece of landscape fabric to prevent clogs, that is completely ensconced in gravel/pebbles. This gives the water a place to go and then you can put holes in the front and back walls to get the runoff.

Can't have wet feet, that's for sure. :)

Be sure to create drainage at the front and rear still.

Probably about 3 years assuming decent drainage. That's really no biggie though -- it's dirt cheap. If you did the sides in cement block and the front/back in a pair of 2x8's you'd probably be looking at less than 30 bucks for the whole thing.

Think of it as an experiment -- go with less expensive pine this time around and if all goes well, you like it, etc. then move to cedar.
We'll be doing a similar project here although on a slightly lesser scale. I have a side yard area that's about 10 feet wide and 20 feet long. Part of that is a sidewalk that is lined by a concrete wall that meets up to soil -- it's also all uphill. I'm going to build a small form along the sidewalk, extend it at the bottom of the decline across to the fence, tap ribar into the existing concrete and pour the form. At the bottom it will be about 3 feet tall, at the top it will be about 18 inches. The only complication is a tree that is there -- I'll extend the form around that as well. When all is said and done, I'll have a plantable bed that will be level although it edges an incline, and it will have about 4 feet of plantable area. I've debated turning it into a water feature rather than a planting area (it's just outside a bedroom window) but who knows -- have to see what I fancy when I start work on it next spring. :)
James
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