Floor Joist Size In Residential Construction

In many countries the fabrication and installation of all framing members including joists must meet building code standards.

Floor joist size in residential construction.

Solid lumber joists were once the only option but modern building materials and construction technologies have rendered them less common in new homes. True floor joist span calculations can only be made by a structural engineer or contractor. Additional support for the floor joists is achieved by nailing the ends of the joists into headers. When building a house or even a deck it is important to confirm you have the correct joist sizes spans and spacing before you get started.

Considering the cross section of a typical joist the overall depth of the joist is critical in establishing a safe and stable floor or ceiling system. 2x10 to 15 feet and 2x12 to 18 feet. Putting all these factors together gives the minimum size joists needed for various types of wood grades of lumber sizes of boards spacing of joists and the load that will be placed on the floor. Choosing the correct floor joist size depends on your building application joist spacing and several other factors these include the length of the span as well as the type and grade of the lumber being used.

Joist span and spacing is set by your local building code. Floor joist span tables for surveyors floor construction surveyors and structural engineers use data from tables below to help calculate the size of timbers necessary to give adequate support to timber floors. Section r502 10 of the international residential code states that header joists can be the same size as the floor joists when the header joist span isn t greater than 4 feet but if the header joist span is more than 4 feet you ll need to double the header joist and ensure that it s capable of. The larger the deck the larger the joists.

Common joist spacings are 12 inches 304 8 mm 16 inches 406 4 mm and 24 inches 609 6 mm. While most residential construction uses 2x8 joists with 16 inch spacing there are many other factors you need to consider when determining the proper joist span length. All the figures are based on normal domestic floor loadings where the floor construction is typically 18 25mm floor boards sheets with up to 12 5mm thick plasterboard and skim underneath. You should also check your local building codes before starting construction and consult a structural engineer in unusual or extreme situations.

Joists must exhibit the strength to support the anticipated load over a long period of time. The following table gives details of allowable spans and spacing between joists for the most common timber sizes used in floor construction. Solid lumber is generally the most economical choice for joists though price will vary based on wood species board size and lumber grade.