Our deck was rotting so my husband was going to replace a few boards and fix the post holding up the roof, but when he opened up the deck he saw the most amazing thing - stones holding up the rotting boards. Yes. Stones.
The black goo seams to be some kind of roofing tar. The joy, and we are never sure when he attempts one of these house exploration projects if it is joy, of living in the Ozarks in a 110+ year old house is that we never know what we are going to find.