Solid-wood strips and planks have a tongue on one side and a groove on the other, allowing them to interlock. Because the boards shrink and swell, they must be fastened to a plywood subfloor. For best results, keep your home between 60 and 80 degrees F and 30 to 50 percent relative humidity (measure both with a thermometer-hygrometer), and leave flooring in the room where it will be installed for at least a week. This allows the wood to acclimate and prevents issues like cupping and gapping down the road.
1. Strip Flooring: Face-nail the first board, then toenail the tongues of subsequent strips with a floor nailer or a stapler and a compressor (rent them from home centers for about $70 a day). Ask for the nailer's adapter plate; it prevents damage to the factory finish. Fasteners driven through the tongue are hidden by the groove of the next board. Be advised: Staples may be cheaper than nails but are more likely to fracture the flooring's tongues.
2. Plank Flooring: Wide boards move more than strips do and require additional holding power. Nail or staple planks, as above, applying beads of construction adhesive to the subfloor every foot or so. Or face-nail the boards by hand, using square-head nails for an old-world look.