How do I make a roof deck - cover a flat roof with a walkable tile or hard surface?

First, talk to several roofing contractors and get lots of ideas before you proceed. Secondly, beware of greenacres advice. My builder told us of people who have put down decks on top of existing flat roofs. In the fall, leaves get under the deck, someone drops a cig, and everything goes up in flames!!!

Yes, Talk to some roofers to make sure you're not going to affect the roof's integredy. it's most important. Next look into making sure your deck will be supported from the load bearing walls, and not from the roof itself, as your roof isn't meant to hold anything but itself and the weather up. Since you said it's all concrete underneath, Tiles might be a possibilty, but I'd be wary of opening leaks.