In a sidebar, there is a RoofMaterialResourceID and you need to assign a proper resource in it like "05m_CitizenRoof_A_RoofClose_A". Then you need to adjust Height and Tiling parameters so that the gap is properly filled.
In the few times I've tried doing this I get a lot of errors. When you do it the way the tutorial suggests by copying the wall and then changing the template to the roof I always receive an error about not having 4 walls. When I manually drag a roof from the root template directly the tiling works properly.