Though some floors are built on a concrete slab—flat on the ground—most floors are raised above the ground. Raised floors are more resilient under foot and provide access for heating equipment, insulation, plumbing, wiring, and other mechanical equipment.

A raised floor is constructed with a wooden framework that bridges from one exterior wall to another. This framework may or may not be supported intermediately by girders, beams, or walls.

On upper levels of a house, the underside of the floor framing generally serves to back ceiling materials. Ceilings are usually built just like floors, only they may be constructed of lighter materials because they’re not intended to carry the same loads.

A floor’s framework is made up mostly of wooden joists that run parallel to one another at regular intervals. Floor joists are typically 2 by 8s, 2 by 10s, or 2 by 12s; ceiling joists are usually 2 by 6s or even 2 by 4s in older homes. Some newer homes have manufactured, I-beam-shaped joists.

Floor joists, spaced on regular intervals, span the areas between supports such as walls, foundations, girders, and beams. Normal spacing is 16 inches “on center” (from center to center), though some floors may have joists on 12-inch or 24-inch centers. Joist sizing and spacing are determined by building codes, which are based on engineering requirements. Joist headers run perpendicular to the joists, capping their ends.

At the foundation level, floor joists rest directly on a sill that is treated with preservative so that contact with the foundation will not encourage termites or rot. Their exact construction and connection with the wall studs all depends on the method of framing that is utilized.

Subflooring provides a base for finish flooring and also serves as a platform during construction. It may be made of boards laid either at right angles or diagonally across joists. Or the subfloor may be made of plywood or other panel products that are laid perpendicular to the joists.

Joists are spliced over beams or other supports. They may be butted end-to-end and connected with plywood gusset plates or lapped. Solid blocking or metal bridging prevents joists from twisting and helps distribute loads evenly.

Wherever an opening occurs, such as for a staircase, joists are doubled up at the perimeter and capped with perpendicular headers.