Les Toil
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Joined: Thu Sep 24, 2009 9:25 am
Location: East Bay, Northern California

Good idea to build a raised bed over cement?

Is it wise--or completely insane--to build a raise bed directly over my cemented area in the backyard? It's a corner in my backyard cornered by two fences. The bed is 60 inches wide and 16 inches deep surrounded by a strong wall of square bricks. And the dirt wouldn't really be touching the fences to avoid water-rotting them. But my concerns are:

Drainage) Will the water be able to drain successfully?? Where will the water drain to if it's only 16 inches of dirt sitting directly on cement? Should I first put a giant sturdy piece of wood at the bottom of the bed with irrigation holes in it with a number of bricks under it to leave space between the cement and the bed for drainage?

What to plant?) Should I only plant things that are known for having shallow roots since the dirt is only 16 inches deep (and what would those plants be?)? Should I only plant bulbs as they generally don't root too deep?

Here's a quick before-and-after drawing I did on the computer to illustrate what I'm considering building. As you can see, there's already plants growing on that un-cemented strip of dirt in front of the fences.


Thanks very, very much in advance. Thus far this forum has provided me with priceless knowledge.


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Super Green Thumb
Posts: 25303
Joined: Sun Feb 15, 2009 11:04 pm
Location: TN/GA 7b

I have a couple!

When we bought our current place, pretty much the only flat sunny area for growing veggies was the concrete patio. So we built two raised bed boxes directly on top of the patio. Mine are stacked 4X4 pine fence posts, sealed against rotting. I've had them for seven years now and they are doing fine. My boxes are 20" tall. Once they were built, I put a layer of gravel in the bottom of them for drainage, just the bottom inch or so, then had a truckload of topsoil brought in to fill them (and added amendments to that once it was in the boxes).

They drain just fine, water comes out the bottom and drains off the patio. I plant tomatoes and all kinds of veggies in them and everything has done very well there.

I would not worry about putting anything under them, I think that could cause more problems than it solves. Even using concrete blocks instead of wood, as long as you don't mortar them down to the patio, it's porous enough to allow the water to escape. Just put the gravel down in the bottom of the box. If you want to be safe you could make it a little deeper than I did, but I haven't had any problems Since I was using wood and there could be spaces between the layers, I did first nail a layer of deer netting to the inside, just to help hold the dirt in place a little then the gravel, then the dirt.

I obviously have not been able to look at the concrete under the boxes since they were filled, to know if it is deteriorating at all, but I have no reason to think so. The concrete around them is not and water does not stay in the boxes, even with such a rainy season as we have had.

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Super Green Thumb
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Joined: Sun Jul 06, 2008 11:58 am
Location: Central Sand Hills South Carolina

I have several raised beds on an old car port slab. My beds are about 9 inches deep and about three feet by 12 feet long. Water drains out the bottom and runs off of the concrete with no problem. They are constructed of treated pine boards with a plastic liners to keep the soil from having full contact with the treated lumber. After discussion on this board, I may eventually remove the plastic, which is just another possible source of contamination.

As shallow as my beds are, I grow a variety of flowers, melon, lettuce and arugala, beans, squash, cucumbers, and would be comfortable planting most any garden vegetable in the beds.
Last edited by hendi_alex on Thu Sep 24, 2009 7:30 pm, edited 1 time in total.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.

Greener Thumb
Posts: 706
Joined: Mon Jun 22, 2009 7:00 pm
Location: Austin, TX

If you're concerneed about quick drainage, you can always add a drain spout in an inconspicuous spot. But as long as you don't cement the wood down or anything, and it just sits on the concrete, it should drain fine. They only way you'll have issues is if you seal the bottom by gluing or cementing it down, which isn't needed anyways.
Home Gardener from Austin, TX; by way of Iowa.

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