Where are the ceiling joists?

[PoorOwner]

the ceiling seams are 4 foot apart as shown in picture..

-I thought they are joists there but does not pick up on the detector.
-between the 4 feet I can see nail dimples for the drywall and they run perpendicular to the seams, like every 16" apart
-The weird thing is I tried to install a shop light along the dimples, they may been furing strips because the drill bit poked through thin wood and it's hollow beyond it. the screw doesn't back out but it's not great.
-Yet, the bracket for the garage door is lagged to one of those dimple runs perpendicular to the seams.
-part of the ceiling is a roof and part of the ceiling is room upstairs.

OK so I am trying to install some overhead racks and bike hooks and dust collector and not sure how to do this.Are there joists sitting higher than the ceiling itself for the lag bolt to grab onto? The stud detector pick up what I think is furing strips reliably. But seems to be thin wood when I drill a pilot hole

 

[PoorOwner]

Another pic. The dimples run across between the seams. Too small to show in picture like this

 

[ItsNemo]

Probably metal studding for bulkheads that drop lower than the floor joists to handle getting enough R value up there.

Cut a small hole and have a look what's going on up there.

 

[Cyberbear]

Cut a small observation hole, use a mirror and light to actually see what you have. The observation hole can be easily patched, or install a crawl hole cover.

 

[evildky]

Those dimple runs are most likely your joists. the fact that your one attempt found a weak spot, or the edge of a joist doesn't mean the whole thing is like that, drill a couple of 1/4" holes along one of those dimple runs and suspect you'll find the joists are present and properly spaced.

 

[pjboy]

you could put a permanent access hole?12by 12" or larger just a thougt?

 

[MarkWilliams60]

Does it have an attic ? Go look.
If not ****** out the electrical box in the ceiling and see what you can see. If the truss-joist is wood hammer it with your fist along the seam.You'll get a solid thud as opposed to the dull flex of a hi-hat. Take a 3/16 bit and pop a hole. See if it comes out woody. Lots of options here.