How to prevent concrete roof from Absorbing water
Concrete absorbs water. That’s why it can never be finishing layer in this type of situations. Even if it is properly tilted that wouldn't solve your problem completely. You must protect it. Now, if you want to do it right it's a bit more complex than just putting tiles. If you are short on money you can do just that but it's not proper solution. Firstly you should apply some hydro coating. It should be something that is meant for outside usage. Pay special attention to the edges; places where your slab connects with surrounding walls etc. Than, since under the slab is a living space (which is being heated) you should put some thermal insulation to prevent condensation which also can cause humidity problems. If you use thermal insulation which soaks water you must wrap it around with a foil. Then you can put some metal sheets on top of the roof. Keep in mind that sheets cannot be HORIZONTAL. There MUST be some slope. Again, pay attention on the edges, they should be placed so water cannot find the way in. If you want I can draw some sketches how to do it, but I will need a picture for that.