Roof Pressure
Stand to be corrected, but I think the pressure value of the helicopter downwash should broadly reflect it's weight, assuming no wind and a low in-ground-effect hover. Once the helicopter is above several feet, some of the downwash (and thus weight) will begin to escape or be distributed over a broader area.
Maximum pressure experienced by the roof should be as the helicopter lands: This would be equivelant to it's weight, plus the momentum of the helicopter as it is set down; but hopefully this force will not be great as it would imply a hard landing.
TT