I'm on chalk soil, with very poor topsoil on top of about a foot.
I decide to build a couple of beds after reading about them on this forum last year.
I built mine from floor joists, gotten from when a neighbour decided to build a new extension.
They are 3m by 6" by 2". (I know, I still go into the hardware shop and ask for "
10mill bolts, 2 inches long please"
)
It made deciding the size quite easy, just 3 lengths needed per bed, 3m x 1.5.
I screwed them together using decking screws and used some old 2 by 2 for corner posts. These I cut about 15 inches in length so that I could have 3" in the ground and 6" sticking out to use to make a deeper bed, if need be, I just need to screw another set of planks on top of the first 'bed'.
Then I layered the bottom with cardboard, the thick, corrugated stuff. Mainly scrounged from skips, neighbours and supermarkets. This was to stop the weeds coming through and help retain moisture.
On top of this I filled the beds to the top with a mix of compost, topsoil and a few bags of commercial compost, all mixed in.
It seems to have worked well, everything is growing very healthily.
The plan for next year is to add the second 'floor', lay down more cardboard and top up so that I have a good 12" deep bed. Of course, you could go as high as you like.
They are very versatile and have made my life a lot easier, in terms of keeping things organised and succession sowing and such, I wish I had done them sooner.
One of the beds I have turned over to the "
Square Foot" method, as an experiment for this season. It will be interesting to see how it works out.
My Onions, Garlic, Spinach, Lettuce, Beetroot, Carrots and Broad Beans seem to have taken to it with gusto.
At the moment it looks good and certainly makes weeding a heck of a lot easier.
I am now using the ground I have left for my spuds and stuff like Broccoli.
Having only a half plot, it does seem the raised beds make a better use of the available space.
Hope this helps. I would say, just go for it. It can always be dismantled if it doesn't work out for you.