Our house is 45 years old and has had a few floors in the kitchen, mostly vinyl or linoleum, added one on top the other (not by us). It's due for a new floor so we decided to rip up the generations of previous floors and put down laminate. There were several layers of vinyl then
1/4" plywood which had started to disintegrate in a few spots. It took 2 of us ~12 hours to rip up 120 sq ft of flooring because of all the nails. The original floor was repaired and patched after a water leak and the plywood was put down to provide a smooth surface for the new linoleum. Whoever put the plywood down (I suspect it was the original homeowner) put nails every 4" in a grid across the center of the plywood, every 1" around the perimeter and half a dozen on each corner. $%&*#@$%%&*!!!!!!!!!
There were finishing nails, ring shank nails, and common nails in various sizes. Did I mention that in some places the nails were so dense that we couldn't get a crowbar under the plywood? We had to chip off
1/2" pieces of plywood, pull the nails, then chip off more plywood. The guy must have had a bucket of miscellaneous nails and couple of bored kids with hammers.
I could here this guy saying, "We'll build it to last. This floor will never squeak. This floor won't go anywhere. Put another nail in son. Build it to last."
Fortunately, the damage that we anticipated was limited to the 1/4" plywood, the floor below is rock solid, flat and level. We'll put down a vapor barrier, then some kind of underlayment, then either Shaw or Armstrong laminate tile floor. Phew! Glad the hard part is over.