Above the concrete ceiling of my attached garage (slabs as far as I can tell from visible joints) is a 3-4 in thick concrete deck floor. 4 in at the house wall that contains the door that accesses the roof deck and 3in away from it, so that water flows away from the house into a gutter. This concrete layer looks like it contains quite a bit of gravel or alike. My problem: if there is a lot of rain I get some water dropping from the garage ceiling. How can I treat the surface of the roof deck so that it becomes water proof? What about the Thompson Concrete sealer from Home Depot? After water proofing, I'm thinking of possibly putting wood tiles on top, the kind that sits on a plastic grid and water will get below the tiles. Any suggestions are highly welcome. Thanks!