How do i add snow load to a portal frame?

Good day, I am currently working on a portal frame. when i apply the snow load on it. it gives me 0. but when o make it flat it gives me result. 

this is when i model it as a flat roof.

This is when i model it as a portal frame and i apply snow load. 0 reaction

this is when i model it as a portal frame. 0 reaction