Grow Your Own Magazine

Im all set to build my own beds, but i may have to put them onto a graveled area of my garden, so my question is do i need to put a membrane of sorts down first with holes in it ? and if im able to build onto my grass do i have to do the same thing?
Thanks

How deep are they going to be, and what do you intend filling them with. If its the grass option, I would skim the top layer off, fork the ground below to improve drainage, turn the grass upside down, and fill with whatever.

They wont be more than 15 inch deep as im a newbie and dont want to "over face" myself!!!
But pots and troughs just arent enough anymore, i started with pots etc... to see if i could actually manage to grow anything and i have had loads of success so off i go!!!

Putting them on top of a path doesn't sound very good to me - the plants are going to need a good depth of soil. My thinking about a raised bed is they get X depth of the original soil and about 1/2 X more on top!