Jeff Wisnia wrote:

I'm puzzled by your question, but maybe I'm not reading it correctly.

I presume that if you're asking that question it's because the "ceiling
drywalls" are already in place so you can't measure their thickness
directly.

If that's the case, why do you need screws? Is it sagging down in some
places and you want to try and correct that with some more screws?

I made the mistake of stepping onto the "floor" while in the attic and
as a result got a few popped-out nails.