So, the door in my room appears to be hollow. I found that out after moving some furniture and bumping a corner into the door. It broke so easily, and now there's a huge hole in it. Looks like 2 sheets of think cardboard-like "wooden" material were nailed to the wooden rectangle(door's frame), and painted white. My question is, what are these sheets called? I want to tear off one side with the hole and just replaced it with a new one, and painted over. Please let me know. It isn't plywood. This is really easy to brake, and it's only like 4mm thick. Looks like cardboard or some compressed sawdust. Whats are these sheets called?..