Finding ceiling joists in an old house?

I'm having a lot of trouble finding ceiling joists in my old house so I can mount a projector and screen.

There's one area by the stairs that I can actually see where the upstairs floorboards are nailed down, but even if I use a stud finder on the downstairs ceiling directly under where I know the joists are, I can't find anything.

I stuck a ruler down through a few cracks between the boards upstairs and found that I hit plywood about 8.5 inches below the floor. Judging by the area by the stairs where I can see the upstairs floor and the downstairs ceiling almost side by side, the ceiling is actually about 3 inches thicker than that measurement, which makes me believe there's quite a few layers between the joist and ceiling.

Any pointers figuring out where I should drill? I'd rather not do the "drill a bunch of small holes until I find a joist" method since this is a rental. Thanks!

Submitted September 26, 2015 at 11:56PM by GrandpaSquarepants http://ift.tt/1OzNQc6