The car was not overloaded how ever the roof was DOH
Current Meriva depending on engine gross vehicle weight 1785-2046kg (You know what the mordel is and the first page of "operation, safety and maintenance" in the Manual.
Kerb Weight of car around 1400kg - Again you know which model it is
Roof rack - 10kg
Driver say 70kg
Rough count 30 sheets @ 28 kg = approx 900kg (You know how many you bought)
And you think the car wasn't overloaded!
RTFM BEFORE YOU PUT ANYONE ELSE AT RISK AGAIN.
And it doesn't matter how well you fasten it to the roofrack to the car the roofrack will come off the car in an accident or heavyt braking.