The major building components of any home are the floors, ceilings, walls, and roof. Floors can either be built flat on the ground on a concrete slab or above the ground. Raised flooring is the most common method employed, because it allows space for heating equipment, insulation, plumbing, wiring, and other mechanical equipment. Floors are constructed of wooden joists--horizontal roof and floor-framing components--that run parallel at regular intervals. The joists are generally two-by-eight, two-by-10, or two-by-12. Ceilings are built like floors, except that they're typically made with lighter materials because they're not intended to support heavy loads like floors. Walls are built using two-by-four or two-by-six wall studs, which can be metal, but are more commonly wooden. Two-by-six walls provide more strength and allow room for insulation and large plumbing pipes. When openings must be made for windows and doors, wall studs are replaced by small beams, called headers. Most roofs are made by stick or truss framing. Stick-framed roofs consist of rafters that span from the top of exterior walls to the ridge, and they're generally built at the construction site. Truss-framed roofs are triangle units that are shipped to the construction site ready to install. Both methods require 16- to 24-inch spacing from center to center. However, rafters are generally positioned directly above wall studs, while a truss is one adjoining double rafter/ceiling joist unit.
