Safe/professional way to transport lumber with just a roof rack
Almost all of the Home Depots that I do business with have short term rental trucks available right in their parking lots for very reasonable rates. Call in and reserve one for a particular time. Drive to the Home Depot in your car and park it in the lot. Go inside and checkout the truck. Then drive it up and load all of your materials and head home with it to unload. Take back the truck, check in and head away in your own car. 

This is a lot safer option than risking some accident resulting from wood items sliding or flying off the roof of your vehicle.