Currently rendering a building=roof seems to be rendered the same way as other buildings. That makes them appear as a solid blocks raising from the ground as long as no min_height is assigned (most of the cases). Assuming a default of _minheight=(height-0.1) will render them much more realistic.
OSM2World has special treatment for building=roof and building:part=roof built in, the relevant code is here. The default thickness is currently 30 cm, but that number is ultimately rather arbitrary.
Currently rendering a
building=roof
seems to be rendered the same way as other buildings. That makes them appear as a solid blocks raising from the ground as long as no min_height is assigned (most of the cases). Assuming a default of _minheight=(height-0.1) will render them much more realistic.