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Im planing on taking in a corner of my finished out garage by adding two non load-bearing walls.

I wont have a problem anchoring my bottom plate to the concrete floor but I'm running into an issue on the celling. It appears as though the previous owner finished the garage ceiling with 2 layers of drywall (don't know why), this coupled with sprayed insulation in the attic is making it very difficult to locate the joists (even with my stud finder).

My question is this: is it absolutely imperative that I secure my top plate to the ceiling? Could i get away with anchoring just the bottom plate and then secure the end studs in my new wall to studs in the perpendicular walls?

I'd really like to not rip out all the drywall in the ceiling, do i have any other options?

cda

Do you have a really good electronic stud-finder? One that deep scans? They commonly cost between 25 and 50 dollars depending on the model.

We have two 1/2 inch layers of drywall all through our 1958 ranch home. (They did this for firewalls through out, noise canceling and creating a very flat wall.) And with a good stud finder, I have no problem finding studs.

Without some anchoring to the ceiling, your wall may stand ok, but be pretty wanky when trying to nail or screw into the sides attaching drywall (unless it is a very short wall section).

Some codes require double drywall, even double 5/8" drywall over garages. I ran into that in a new condo subdivision in SoCA many years ago.

I ran into that situation several times in my construction career. How I treated it depended upon the use of the wall. The last time I remember it was in an upstairs bedroom. I fastened the sole plate of the new wall to the floor easy enough. The wall was almost 12' long. There were no studs near enough to be of value and the ceiling joists were also out of the way. So I used toggle bolts on the end studs and the ceiling plate to hold it all to just the drywall (single drywall). Then taping the corners in the usual manner made it solid enough.

If the wall you are installing is going to take a beating, or if you will be hanging some weighty shelves on it, you may need additional bracing. One way to do this is you mount a couple 2x4s on the face of the wall or ceiling and fastening them to studs or joists. Butt the ends of these against the framing of the new wall.

If the two walls are joined at a 90 degree angle the wall would be very solid though if it is long it could get kind of wonky in the middle. You could toggle bolt them as Bob suggested, or if the joists are running across the wall once you know where the plate is going to be drive a drywall screw into the ceiling where it will be covered by the plate until you find one of the joists. Since you have a double drywall ceiling I'm guessing there is finished living space above the garage so looking from the attic isn't an option.