i agree with TT, as the rotorwash would become relitively insignifigant after about 1 rotor diameter, As it states in the Australian HLS criteria. . roof top HLS must be able to with stand the static loads of the helicopter...so if you have say around 2000kg of helicopter coming and going on a hospital HLS. . this would have more of an imapact on the surface.
If i understand you correctly parts of the roof have debonded enough to warrant a rebuild. . is it concrete ? (i'm not a construction wizz) or is it a synthetic surface ?
JF