What's the maximum amount of weight that can be stored on the roof of the cab?

I'm very grateful someone may be gifting me a very nice roof rack soon, but then it got me thinking. How much weight can the cab support? Not talking about the rack capacity, but the actual body of the truck.

Just a curious question.

 

Hmm, I would imagine quite a bit as some people have tents there, but again not sure if they looked at weight capacity

 

Yea definitely, just curious about the amount though. It's not necessarily a steel frame that holds the truck together so I'm surprised it can handle so much. Just couldn't help but wonder what that number is.