The roof slab is layed with clean shuttering plates and some places probably due

           There are so many method to level the surface as described by you. All are effective for 5mm thick depression.

       If you are more particular than mark the depression portion first than roughened the depressed portion by chiseling after that cure the portion for 24 hours than apply the cement shullery with painting brush than apply thin required coat of cement mortar 1;4 and after setting it cure it for 3 days. After it you will do any finishing on the surface which will be as good as you want.

        Thanks