How much weight the prius c roof can handle?

A lot depends on the hardware you use to attach your platform.

I don't know the specific ratings for the c, but most small cars can handle about 110lbs when folded up/driving, and closer to 400lb when stationary/fully deployed camp setup.

Good luck!

 

The roof itself will handle the whole weight of the car to prevent occupants being crushed in a rollover situation. obviously you don't want to be driving around with that much weight.

It depends on how you attach the system as to the rating, but most off the shelf racks are rated around 60kg and I wouldn't go higher than that while moving.

 

Where do you find the maximum roof load at rest? I did a number of searches and didn't find it specified on any material online, including my car's manual. Everything I find relates to the value when the car is moving.
If 400 lb, then the car won't hold more than 2 people and in some case just 1 person. Tent (100 to 150 lbs) + 2 people (150 lbs each) = 400 to 450 lbs.

I'm also considering purchasing a rooftop tent but need to make sure I can use it before dropping a lot of dough.

 

If you are still looking, this roof rack for the prius c will hold a rtt.