I am trying to determine if I have an active termite problem, or if my problem is an old one which has been hidden until now.

The structure in question is a two story brick building built in the early 1920's. A non-bearing wall, which carries hot and cold water to the second story, as well as a 4" cast iron waste pipe, has been sagging for years. When we bought the building 40 years ago, a doorway on the first floor in this wall had visibly sagged. Lately, the sag became more pronounced and the drainage in the waste pipes became difficult.

When we removed the ceiling in the basement, we found that the joist under this wall was totally shot - it can be removed by hand. There is evidence of termites in this joist (tunnels, etc.), and in two joists on each side of it. The integrity of the joists on each side is not compromised.

We suspect that the basement was refinished to hide the termite infestation. This included 3/4" wood paneling and a tile ceiling. Other the joists in question, there is no other evidence of termites. Given the facts above, am I safe in assuming that the termite problem occurred in the past? Thanks for any help.