Floor

Floors vary from simple dirt in a cave to many layered surfaces made with modern technology.

The choice of material for floor covering is affected by factors such as cost, endurance, noise insulation, comfort and cleaning effort, and sometimes concern about allergens.

[1] Some types of flooring must not be installed below grade (lower than ground level), and laminate or hardwood should be avoided where there may be moisture or condensation.

Nowadays, subfloors are generally made from at least two layers of moisture-resistant ("AC" grade, one side finished and sanded flat) plywood or composite sheeting, jointly also termed Underlayments on floor joists of 2x8, 2x10, or 2x12's (dimensional lumber) spaced generally on 16-inch (40.6 cm) centers, in the United States and Canada.

Some flooring components used solely on concrete slabs consist of a dimpled rubberized or plastic layer much like bubble wrap that provide little tiny pillars for the one-half-inch (12.7 mm) sheet material above.

At the highest end, or in select rooms of the building there might be three sheeting layers, and such stiff subflooring is necessary to prevent the cracking of large floor tiles of 9–10 inches (22.9–25.4 cm) or more on a side.

Wood clad (hardwood) and tile covered finished floors generally require a stiffer, higher-quality subfloor, especially for the later class.

Some subfloors are inset below the top surface level of surrounding flooring's joists and such subfloors and a normal height joist are joined to make a plywood box both molding and containing at least two inches (5 cm) of concrete (A mud floor" in builders' parlance).

Floors using small (4.5 in or 11.4 cm and smaller) ceramic tiles generally use only an additional 1⁄4-inch (6.4 mm) layer of plywood (if that) and substitute adhesive and substrate materials making do with both a flexible joints and semi-flexible mounting compounds and so are designed to withstand the greater flexing which large tiles cannot tolerate without breaking.

Ground level slab floors are uncommon in northern latitudes where freezing provides significant structural problems, except in heated interior spaces such as basements or for outdoor unheated structures such as a gazebo or shed where unitary temperatures are not creating pockets of troublesome meltwaters.

Once the site has reached a suitable firm inorganic base material that is graded further so that it is flat and level, and then topped by spreading a layer-cake of force dispersing sand and gravel.

Floors in wood-frame homes are usually constructed with joists centered no more than 16 inches (41 centimeters) apart, according to most building codes.

If the span between load-bearing walls is too long for joists to safely support, then a heavy crossbeam (thick or laminated wood, or a metal I-beam or H-beam) may be used.

Concrete floors are usually so massive they do not have this problem, but they are also much more expensive to construct and must meet more stringent building requirements due to their weight.