system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do Homes with grass roofs not rot

I see photos from Northern Europe of homes with grass roofs. As awesome as it is, How does the retainage of moisture not rot the material beneath?
gpt
There's a waterproof barrier underneath, either layers of plastic/rubber sheets or concrete.

Which is more suitable depends on the climate and how likely you are to get repeated freeze/thaw cycles.  Concrete is more durable, but susceptible to damage from freezing moisture in cold climates.