A hardwood flooring's subfloor is vital to provide a reliable surface for the hardwood floor. The subfloor itself doesn't have to be made of hardwood.
1. Douglas Fir
The ideal subfloor is Douglas fir. It should be boards that are nailed down at 90 degrees to the joists. Having them at an angle holds everything much more firmly.
You should use #2 or a better grade of Douglas fir.
Frame-grade pine boards will also provide a good subfloor. Like Douglas fir, the wood has a tight grain. In both cases, the boards should be one-inch by six-inches (nominal).
Plywood is the most common hardwood subfloor. It’s cheaper than boards and quicker to install. The best plywood subfloor will be ¾-inch thick.
This will give ample strength to support the floor and people. A sturdy plywood subfloor is recommended most highly by hardwood associations.
It’s followed by a ¾-inch OSB, or oriented strand board, plywood. Like all subfloors, it should be attached at 90 degrees to the joists for strength and tightness.
Among the recommended subfloors, the lowest on the list is ⅝-inch plywood. This is cheaper but not as thick and, therefore, not strong enough to provide a good sturdy subfloor for hardwood flooring.