By Shadow100
Italy 
We only hace 30cm of soil which sits above an underground garage. Consequently trees we have planted are not reaching their full potential. Would it make any difference at all to build a small wall and back fill with another 12 cm of good soil
27 Aug, 2014
Answers
My concern would be for the structural stability of the garage roof - it's unlikely that the roof was designed to have the weight of 40cm of soil (never mind 100cm). A cubic meter of soil can weigh as much as 1.5tonnes - so even a small raised bed would be heavier than a Fiat 500 :o)
27 Aug, 2014
Urbanite is right. So the weight of a metre deep of soil on a relatively small (4x6 metre) garage roof would weigh about 36 tons. Not a good idea.
What about planters or large tubs instead?
27 Aug, 2014
Previous question
That still only makes 42 cm, and if your trees are already planted, you can't raise the soil level around them because you must not cover the trunks with any depth of soil.
You should be able to grow a reasonable sized tree and larger shrubs in a metre of soil - the Kensington Roof Gardens only has a metre soil depth and they have plenty of large growing plants there.
27 Aug, 2014