I understand the "approved" subfloor issue, and I've searched archives to no avail in finding an answer to the following question.

Would a 1/4" underlayment of OSB be thin enough for cleats to penetrate through and hold sufficiently into a Ply subfloor

Has anyone done this with satifactory results?

What size cleats are you using? What is the thickness of the flooring?

2" cleats...3/4" ply.

so the cleat would be going through the flooring, 1/4" underlayment (OSB), and into the ply.

I have no doubt the cleat will reach the ply, but the uncertainty is whether or not enough of the cleat will reach the ply.

The question is why is the 1/4" OSB being used? Is it because you do not wish to remove it or you need to raise the height of the floor? If the latter, 1/4" plywood would be preferable..

The 1/4"OSB was used only in the kitchen to raise the floor to be level with the carpet in the rest of the house. So the new flooring will be on an area with the 1/4" OSB underlayment as well as an area with only the subfloor (where carpet used to be). Laying down new underlayment to raise the new floor to be level with the kitchen will be easiest. So the options are:

1- lay down 1/4"OSB to make the entire new floor the same level

2- lay down 1/4"ply to make it all level & hope for the best nailing through the OSB underlayment in the kitchen (not really an option if #1 is ruled out)

3- demo the OSB in the kitchen (which of course is under the cabinets & the owner does not want to take them out and put them back in) so the demo would not remove all of the OSB currently in the kitchen.

Which brings us back to question of the cleats holding power if driven through the flooring, 1/4' OSB underlayment, and 3/4" ply subfloor. If the installed floor will be fine through the OSB this will be very pleasing to the owner as it ill be easiest and cheapest...if not, demo needs to happen.

Any further question??
Recommendations/opinions???

I am assuming you are installing 3/4" solid. If that is the case then 2" cleats will be just fine. I use 2" staples and they penetrate 3/4" hardwood and 3/4" plywood with about a quarter inch penetrating the bottom of the plywood. Don't forget to check the moisture content of the plywood you are adding.

Yes, it is solid prefinished heart pine. That is about what I was expecting, but I want more to tell the owner than just "I would expect..."

any other input anyone?

What else is there to tell him. The 2" cleats are perfectly acceptable. Write it up.