Posted: 28/02/2014 at 12:37
My raised beds are on Tarmac - school playground. The beds have no specific drainage. They in fact dry out too much when the weather is dry. If I were constructing them again (they are made from old floorboards) I would line them round the sides with damp-proof course plastic to preserve the wood a bit longer.
I also do not replace the soil, just add more compost each year + compost from pots, etc.