The first one is a sloped one that you can find in section Construction > Stairs > Roof. In order it to look ok, you have to adjust its measurements well or drag corners in such a way so that it covers the whole building. As for the second way, a flat roof covers your project automatically once you turn to 3D mode.
If none of those ways don't satisfy you, then you may build the roof on your own, using magic cube. That's the common practice of our users. Learn how to use magic cube: https://planner5d.com/forum/post/24211/
Have more ideas on roof, don't hesitate to share with us!