Floor framing includes all structural components and fasteners used to construct floors in a house or commercial building. This term is typically only used to describe floors made from wood, as opposed to those constructed using concrete slabs. While a number of techniques may be used during floor framing, the basic structure and framing members remain the same in the majority of applications.
Standard floor framing starts with a concrete foundation wall, which represents the footprint of the building. Wooden sill plates are anchored into this concrete wall using a series of bolts or masonry anchors. Next, rows of wooden beams, often called “joists,” are laid out over the entire surface. The end of each beam rests on the sill plate, and is fastened in place using nails or screws. The exact size and spacing of the beams is determined by local building codes based on the overall design of the structure.
Smaller wooden framing members, or cross beams are placed in between each row of joists. The cross beams form an “X” shape to provide additional support to each floor joist. In some applications, the span of the floor is so long that a single beam can't reach all the way from one foundation wall to the opposite wall. In this situation, an intermediate beam, or girder, is placed at the center of the foundation. The end of each beam can rest on either the girder or the sill plate, depending on the length of the beam.
Floor framing members are often made from pressure-treated lumber, though some buildings may use rough-sawed timber for a more rustic look. Others substitute laminated beams, which are coated with plastic to reduce warping and swelling caused by humidity. Depending on the building code, it may be permissible to join several small layers of lumber together with nails in order to construct floor framing members.
Once floor framing is complete, it is covered with plywood sheathing to form a subfloor. Walls are built over the subfloor and connected to the floor framing using nails or screws. The floor framing not only supports the walls from below, but also helps them stay erect by reducing lateral forces. Once the majority of construction is complete, floor finishes can be used to cover the plywood sheathing and create the desired finish.
Some builders may choose pre-manufactured floor framing, or trusses, in place of traditional stick framing. Trusses feature an open web design, similar to the design used for roof trusses. The wood is pre-cut and joined at the factory, making it less susceptible to moisture or humidity damage once it's on-site. Trusses allow installers to complete floor framing more quickly, and often result in a more stable floor structure. The added cost of truss construction may be offset by labor savings during installation as well as reduced maintenance and repairs in the future.