Plywood holds flooring nails about 80% as well as the pine subfloors and as long as you use the 3/4″ thickness (T & G is best), it should serve you well. If you are having new home built, be sure they use a good quality urethane based construction adhesive on each joist before they nail it down. When builders try to use thinner plywood, say 5/8″ they are really defeating the whole purpose of the flooring nails. The standard flooring nails (for 3/4″ hardwood floors) are 2″ long and go into the subfloor at a 45-degree angle. They penetrate the subfloor 7/8″. It’s really dismaying to see flooring nails poking through a thin plywood subfloor by 1/4″ just to save the builder a few bucks. If you find you have a 5/8″ or thinner plywood subfloor you really should beef it up to a total 7/8″ at least. Even with a plywood subfloor you should install the hardwood a right angles to the joists. But when you cannot, add another layer of 1/2″ underlay if possible to prevent floor deflection. If you find that you can’t raise the level of the floor any more and you still have to lay the hardwood parallel to the joists, try this. Install 2 by 6 blocking under the subfloor between the joists every 16″, this will act as a stiffener for the whole floor.
I often encounter modern buildings with plywood subfloors that are nailed with little 2″ smooth common nails every foot or so. In these cases I will renail the subfloor with 3″ spiral nails every 6-8″ before beginning my strip floor installation.
Now that you know what best to nail a strip floor to, what’s the best fastener to nail it with? Most flooring contractors and even D.I.Y’ers use some sort of mechanical fastening device to accurately drive and set the 2″ nails into the subfloor. For the past 50 years the Power Nail Co., and now Primatech has been making a flat, serrated edged flooring cleats, that imitate the old cut nail that have had centuries of use. You can still buy square cut nails, which by their constant thickness, but tapered width, bend the wood fibers down in the nail channel. The square tip shears the fibers to prevent splitting. The result is a nail that is held strongly by the wood fibers and prevents the nail from easy withdrawal.
Primatech and Power Nail co. have done this one better, in that they have added serrated edges to the nail shaft so the wood fibers will hold the nail even more securely. These flooring nails or cleats (as they are also called) were designed specifically for wood flooring and have a good half-century track record. In the past these flooring cleats were designed to be driven from mechanical (not pneumatic) nailing tools. But as the wood flooring industry grew, the staple making industry saw a opportunity to make some fast money. Staples have been driven pneumatically into lots of light duty factory applications, so why not wood floors? So one enterprising company simply modified it’s roofing nailer to shoot these 2″ staples on an angle and voila’, a pneumatic flooring stapler. Well, sort of. (As a side note, even my roofer refuses to use these staples).