raised bed

Hi Jack. A couple of us dealt with 2 similar queries just a few days ago, but your case is slightly different in that you are going over concrete. You certainly won't need a weed screen but you must allow room for a drainage area of rubble with top soil over it. The recommendation is for at least 18" of a good top soil 24" would be even better then I would suggest on the base about 8 or 9 inches of rubble for drainage. Hope this helps. Come back if you need more or a second opinion but have a quick look down this page first about 15 questions ago. There are 2 next to each other.

John.

I would break up the garage floor with a Kango before I started, otherwise the raised bed will be about 1metre high, also the amount of soil required say 2metres wide by 4metres long equals 8cubic yards of soil and at least 1metre of rubble,
. railway sleepers also needed for the sides (expensive).

you can get concrete blocks like for building and lay them on there side staggerd .cheeper and will never rot.i would at least get someone to drill some holes through your floor or your in danger of making a bog garden

raised beds

yes as if your making a brick wall but on there sides so they dont move