I have always wondered why it has never been considered that Stonehenge stone circle was a building with a roof. Maybe a large temple or even a living space for the people who built it. If the roof was made of wood with thatch nothing of that would remain. I am sure there are very good reasons but why would that be discounted?
Actually it was not discounted as a possibility. Avebury is also considered as potentially having had a roof of some kind. But as you can well imagine the evidence of this is no longer present. Had the roof burned and fallen then subsequently covered, then this may have been found in one of the many excavations of the site but unfortunately this has not occurred. Thus, it is not known definitively if did or did not have a roof. Since it is an observatory, however, having a roof may have diminished its value in the study of the stars by its builders.
The study of Archaeology and the cultures we study is not definitive because of the nature of the remains found. We can speculate about some things but often, it can go no further than speculation.