Waterproofing of flat terraces is a tricky problem in building construction, mainly because the surfaces are flat. This means that the water cannot run off the structure quickly, and will move slowly or pool above the surface, creating opportunities for leakage.
The best way to waterproof a flat terrace is to use a waterproofing membrane. This is a thin layer of waterproof material that is continuous, and offers no path for water to enter the structure. The membrane is laid on top of the structural slab.