Using thin card, usually from a cereal box or soda case, I glue the foundation for the roof in place. It doesn’t have to be in one perfect shape, it’s going to get covered anyhow. I like to do it rough side out, glue sticks to it better. If you are building an interior, I’d go graphic side out, it’ll look better from inside the structure. I also took this chance to build the eaves, since it will be easier to get at them without the roof in place.