laying a new lawn

my friend is laying a new lawn ,on top of concrete.i have told him that it can be done with enough soil 1st but what would you do about drainage.would this be a problem or would enough soil be suffficant it is only a small area quite easy fill

 

If the concrete area currently drains to surrounding ground then it should be fine, so long as you have enough depth of soil.
If it drains to a soakaway or main drain it could be more complicated.

 

Then what happens in drought and hose pipe ban with all that concrete underneath???

 

So long as there's enough soil then it'll be no worse than other lawns.

 

To be honest if he's really serious about a good lawn I would try and at least break up the concrete a little before laying the soil; It's true that enough soil will counteract the problem, but while the roots of grass do not penetrate deep soil, moisture and nutrition is still drawn from well below the surface.