Temple whether the sheeting permeable or not is going to make little difference if you have it sitting on concrete... a raised bed needs to be on soil.

I'm sorry, I'm a little confused by your answer.
Why, may I ask, does it need to be on soil if it is going to be sitting inside sheeting?
The plan is just for short rooted vegetables, so the roots won't need to go any deeper than the base (and sheeting would prevent this anyway, whether it were on soil or not).
It your issue is worms, then I can transport these myself with no problem (and again - sheeting would have been in issue here).
And if the issue is drainage ... well, that's the point of the question. If it does need to be well drained then I can poke holes, and place pebbles below the sheeting to allow for the water to flow away from the base.
I don't have an abundence of soil in my garden, so the raised bed was in order to create a place to grow some vegetables, and the sheeting was to try to contain the soil and stop it being washed out from the container.
Surely this is no different to plant pots or grow bags, is it?

I agree with Moon Growe, the point of raised beds is to create a warmer and deeper place for soil to allow plants to grow faster. A raised bed is really only supposed to be on top of soil to allow drainage and larger soil surface area. I guess you have yours on concrete with a weed suppressor or plastic sheet as a liner. It is doomed to failure as it will dry out too much in summer and be too wet in winter. It is different to grow bags and pots as it is much larger and open to the weather (less umbrella affect) and you do not have grow bags out all year.

Dear Temple,

My husband built me walls around my garden as it was all concrete, he lined it and put drainage holes in the liner, i filled them up with soil and have been successful growing all my plants and trees this way, they are deep, I have wisterias and clematis to name but a few and they have been there for 6 years with no signs of distress, I am not sure if this is the same as what your planning but if you look at my first ever blog you can see what I have.

The idea of putting pebbles in the base might be enough with some holes in your sheet to allow excess water to drain away but keep the soil from drying out. I would put your pebbles in the raised bed and not under the sheet. We don't know what the weather will be like so don't put too many holes in the bottom. If need be 30cms is not too deep to stop you adding more holes if necessary. You will be growing annuals ie veg which are sown, grown and harvested within a few months. I can't see a problem short term but to use the same soil for the same type of veg in subsequent years might give a problem of nutrition and the danger of harbouring diseases.

Thank you for your replies. Re the points made by Kildermorie (that the raised bed is doomed because it will be too dry in summer and too wet in the winter) ... I hope I don't sound too dumb here, but if it is dry in the summer, can't I water it? And why would it be too wet in the winter (any more wet than the ground) if I add drainage? I would have thought that if I place gravel beneath the liner, and puncture the liner itself then it would allow better drainage than if it were on already-drenched soil.

I know that there are ideals for gardening. But my options are limited. I don't have any soil upon which to place the bed. I'm not trying to grow prize winners. Just some home grown treats for the occasional dinner - leeks, (short) carrots, onions, lettuce etc. I don't mind if they're not record breakers. Just as long as they make it to the plate. Are you really saying that it is pointless me even trying, since the plants are doomed to die?

I would say, go for it Temple, if it were me i would worry more about drainage than any other factor, but as you can punch holes into the fabric drainage wont be a problem.
I have a raised bed (not on concrete) and as i take plants out in Autumn and sometimes spring i add fresh compost mixing it with the old and had no problems with my plants, dont we do this in our borders, chicken pellets added in spring and normal feeding throughout summer.
If i only had concrete to place raised beds on I wouldn't think twice about it, i would probably have more than one and grow as much as i possibly could, flowers included. You can only try and i dont see why you wont succeed.
Im not the sort of gardener to annalyse all the what ifs of gardening, if it works great, if it doesen't ill rethink my plans for next year :)

Like most of the rest I think that siting a raised bed on concrete is a 'second best' option. Given that I had to do this I would dispose of the liner and make a lot of 'Jerry Mouse' type holes along the bottom edge of the bed structure. Water getting trapped in the raised bed is your greatest enemy.

I am still highly doubtful that you will have success growing veggies in a raised bed sat on concrete. At the very least the soil is going to need changing every year or so as it will become impoverished and with no access to the ground below no way for nutriments to be exchanged. You'd be better off using grow bags and containers and putting the soil on your compost heap every year.

Ydd, I know it appears to be the same principle in the borders, the difference being that in the borders there are probably miles of ground underneath, for the water to drain through, if this wasn't the case, we would all only have aquatic gardens, lol, Derek.

I would put a couple of inches of gravel below the sheet - but i would use mypex ( a permeable weed suppressing sheet) in order to let water drain evenly. Then put topsoil, well-mixed with compost in the bed. Grow short carrots etc - they do well and taste great. Any quick greens will do well, and maybe strawberries if you want some fruit. Be sure to add well-rotted manure or eco-sci (council compost) each year (preferably alternately) to add 'body' to your soil. Charcoal and a sprinkle of wood ash can also be used. Feed with seaweed to ensure your plants get trace elements. This way you can keep your soil healthy - as any crop needs healthy soil. If you neglect this - the soil will not support healthy plants. It just takes a bit more care - but it will be worth it when you eat your veg!

Don't forget to let us see what you have grown. Have you heard of square foot gardening. It means you split your bed in to foot squares and plant a different veg in each. Here is a link which might interest you. http://www.gardenorganic.org.uk/organicgardening/gh_sqft.php