We found this artifact under the porch where I had dropped a drill bit through the cracks. It's a piece cut out of the roof when the wood burning stove chimney was installed. Don't know when that is. Here you can see what the roof is made of.
First some strange shaped asphalt shingles:
Then nice big fat lengths of tongue and groove roof decking. They are a true two inches thick. They are also the ceiling inside the cabin: