In some places it is illegal to drive with snow on your roof so it's a good idea to go ahead and remove all the snow off your roof if possible before driving the vehicle.
The snow could fly off your roof and onto someone's windshield which could cause someone to lose vision in the vehicle and then lose control of their vehicle as well.
So even if it were not illegal you should still remove the snow off your roof so it doesn't prevent a hazard on the road.
But most times if you do get stopped because of snow on your roof the police will have you remove it and give you a warning ticket or just a verbal warning to not leave snow on the roof again.
Then the next time you could get a $100.00 fine or more depending on the state.