I had a hell of a time finding joists in my room with both the stud finder and the knocking method. It turned out that our ceiling has sagged slightly away from the joists so there was a gap between them, which we found when Dad drilled into the ceiling to find them (he was going to drill in there anyhow, we were putting up a piece of wood onto the ceiling). Anyways with that found my Dad drilled the wood into the ceiling joists and now I can put my pole up on there and the weight will be spread between the two joists