It is very possible to lay turf on a terrace if you look at the pic at the
bottom of the page at this address http://www.celtic-manor.com/ you will
see one that I constructed and still maintain.

I don't think you want one that large lol but what you need to think about
is drainage and waterproofing of the structure below, however the biggest
issue is weight.

The drainage and weight issue can be resolved by using a ballast layer. we
used a substance called Vencil recycled expanding polystyrene it's like 20cm
small round volcanic rocks it works great I get good drainage over the whole
area. next place a soil separator over the ballast this is actually a filter
fabric, we used Terram 1000 fabric.

Once the fabric is completly covering the ballast you can add the top soil.

Hope this helps
Martin Heames
www.thegardenspider.com