Anchoring tongue and groove roof decking?

[stavskar]

I have a ceiling on the downstairs that consists of 22 mm raw boards closest to the ceiling joists (cc 60) that are attached to these with thin nails. On top of the raw boards is thin 6 mm gypsum that has been plastered and painted. I am now planning to screw up double layers of 13 mm gypsum in the ceiling to improve the sound insulation between the floors, since this cannot be done from the upstairs floor for several reasons.

I plan to anchor the existing ceiling to the ceiling joists with long screws so that they can handle the weight from the gypsum. The problem is that I cannot see how the raw boards are positioned behind the existing gypsum.

I can locate the joists with a stud finder that works with ultrasound and then mark them on the gypsum. If I could see the raw boards, I would drive a solid screw in the middle of every 4-5 raw boards at each ceiling joist. Since I cannot see the raw boards, I cannot determine sideways where to place it. Maximally bad luck means I hit the joint.

How can I solve this? Should I cut/freze a groove in the current ceiling gypsum along the ceiling joists so that I can then place the long screws in the middle of the raw boards, or do I dare to screw in the anchor screws randomly along the ceiling joists?

 

[SBH]

I think you should skip that and screw the new drywall a little closer if you are worried. Mark where you have the studs and screw the drywall into them.

 

[pbengtsson]

You can also add battens 21x70 or 28x70 30cm cc, fasten these securely, and it won't matter if you hit the joint. It adds a bit more, but the air gap that forms also helps to dampen sound and you can run some extra wiring in the space.