Structural floors, or subfloors, in a home generally are crafted from one of two materials. Concrete floors can be laid in basements or at ground level, while upper-level subfloors generally consist of wood. If you want to lay ¾-inch plywood as a subfloor or another ¾-inch flooring over a subfloor, you should use an appropriate size of nail to ensure the flooring stays in place.
Plywood Subfloor
Three-fourth-inch sheets are the standard in plywood subfloors. These sheets install directly over floor joists, which cross the floor every 12 to 16 inches to provide the support necessary for the flooring surface. The floor joists beneath a plywood subfloor can be a range of heights, but 6 inches is a standard. So, when attaching plywood sheets to joists, any nail size works, as long as the nail is not longer than the 6-inch joist below.
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¾-Inch Wood Floor
Finished wood flooring planks, such as hardwood and laminate flooring, install over a plywood subfloor surface. Once installed, these planks are considered the "walking floor" of the room. They do not provide structural support but serve only to enhance the room's appearance. Although the ¾-inch flooring planks do not have to be attached to the joists beneath the plywood subfloor, the floor remains more secure when the nails do go through joists, so it's best to use nails long enough to pass through the wood flooring planks, the plywood sheets and into the joists.
Nails for Subfloor
Although you can secure ¾-inch plywood sheets to the joists in a subfloor with long nails, you don't generally need extra-long nails for the floor to stay in place. Nails approximately 2 inches in length pass through the ¾-inch plywood and still provide plenty of length to secure the plywood to the joists. Realtor.com recommends an 8d nail, which is roughly 2½ inches, to pass through the plywood and bury nearly 2 inches of the nail in the joists below.
Nails for Walking Floor
When installing walking floor planks, slightly longer nails provide the necessary length to pass through the planks, plywood and into the joists. Hosking Hardwood recommends an 8-penny nail, which is roughly the equivalent of a 3-inch nail. This size allows the nail to pass through both the walking floor and the plywood and leaves 1½ inches of nail to secure to the joists. To ensure that nails in a walking floor go through the joists, you should use a stud finder during the installation process to locate the joists.