need help with ceiling mounts

[Ross]

Are you sure certain that the joist are running in the direction that you are scanning and not the other direction?

A method that I have used in the past is fairly simply and requires a metal coat hanger.
But this only works in uninsulated roofs and homes without knob and tube wiring.

Take and flatten a metal coat hanger into a straight line.

Bend a pair of 90° bends into it to form a Z shape.
(The middle section should be about 2" long and the forks to be 12" or so.)

Insert one end into a small hole (or push the wire really hard to produce a small hole.

Rotate the wire in the hole so that the small section of the Z is vertical in the hole.
The end will be reseting parallel to the roof.
Rotate the wire around untill it stops.

That is the edge of a joist.
Use a push pin or tape to mark the angle of the coat hanger.

Rotate the coat hanger around the other way to confirm this wasn't an anomaly.

Gently retract the wire from the hole.
Use the wire to determine where the end was based on the angles marked out previously.

Good luck