In article ,
says...

Ian Clowes Wrote:
asalcedo wrote:

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

Why such long screws? I'd use 50mm or so (and probably an 8 or 10
gauge) for this.

All the load is being held by the joists. The screws are mainly
stopping the ply floating around. Slightly different if you're
building
a box section, but even then I can't see that longer screws add much.

Or am I missing something?

IanC

36mm for the top plywood board (two 18mm panels)
28mm for a suplement on top of the joist to level it out
36mm into the joist (the same as the nails fixing the floorboards
penetrated into the joists).

That adds up to 100mm

I was thinking of 120mm because the first 36mm into the joists has
already many holes and the screws may not grab as well.

The joist is about 230mm tall and is certainly not pine, it is harder
and redder.

Does it make sense?

If so, should I drill the pilot down to the full depth?

I'd have thought it would be better to glue and screw the packing pieces
first, then add the ply as a separate operation (but I suppose you'd
have to be careful to avoid the screws you just fitted). Otherwise you
could put everything in place, drill with the bit you have then remove
the ply and deepen the holes.