I think in brass you have to "anneal" the metal to make it easier to bend. And you just need to bend over a die with a smaller radius. But metalworking is jot my expertise.
I've done it both ways on the four electric quarry engines I built. If you anneal the brass you need to be careful because it now bends very easy. You can get a kink it real fast. With the un-annealed brass or half-hard it's sometimes difficult to get the exact curve you want. It's a technique that takes a few tries before you get one the usable and fits the roof correctly without any gaps between the roof and side walls. Practice, practice and then more practice.