Joist composition and deterioration
I have a house that is coming on 30 years old which I have been living in for around 25. Right after we moved in I put in a suspended ceiling that covered about 2/3 of the ceiling, and there were no problems. Recently, I started to put a ceiling in for some of the rest of the area and found what seemed like an odd situation. The joists are 8" and appear to be a yellow pine. When I drilled holes into the bottom of the bottom edge of the joist, I got a strange response from the drill. The wood would drill for about an inch and suddenly jump forward like I had hit an air space or layer of deteriorated wood, then I would hit another layer of wood, and an inch or less later hit another air space and clunk deeper into the wood without any drilling taking place. I have found this in the three or four joists that I have tried so far. I have never had any indication of insect damage, filings, or external exits and entrances that would indicate any kind of termite presence.
Can this type of wood some how dry out and shrink internally. Would this be and indication of any type of danger structurally?