asalcedo wrote:

I need to screw 100 to 120mm, gauge 12 (5.5 mm diam) screws onto plywood
on top of hardwood (joists)


OK, I reread the other Thicknesser thread.

In the 'worst case' you have 2x18mm ply, then 27mm packer and you need
to secure this to the base joist in an asthetic manner?

I'd probably use 50-60mm screws with glue to secure the packer to the
joist, and then the same to attach the ply to the packed joists.

Will there be any/much sheer force (ie, pushing things along the top
surface of the beam) and potentially shifting the packer from the
joist? If not I think the above will do it.

If there will be much sheer force I'd consider building it up layer by
layer using 60mm screws (and possibly 40mm for the last sheet of ply).

HTH
IanC