Both skill and technique is essential. You can't have one without the other, especially when it comes to avoiding leakage.



The above picture shows 2 grade 25 concrete sections after several drops of water was dripped onto it. Within minutes, the one on the left absorbs. As you can see on the right, the droplets remains. That's because that block of concrete was added with waterproofing compound.

I would not say that concrete overtime will leak. It is porous by nature. The picture only shows droplets of water. Imagine a slab which is expose to rain water and without water proofing. The concrete will absorb water bit by bit continuously. Where does it go? It goes into pores of the concrete. Now before it even has time to dry, it rains again next day. Eventually, the concrete will be so full of water, it has to go somewhere. That's when you will start seeing moisture and drips of leaking water from wherever the concrete is the weakest or have signs of shrinkage cracks.

But if your flat roof is well graded for water to fall towards the gutter or rain water down pipe, the risk for leakage is minimal. That's my experience with flat roof and also my opinion