Nominal Sizes Of Lumber Used For Floor Joists

Joist span and spacing is set by your local building code.

Nominal sizes of lumber used for floor joists. Lumber is identified by its nominal size which is its rough dimension before it is trimmed to its finished size at the lumber mill. The width is considerably more important than the thickness of a board. For example a joist made from doubled 2 x 6s can span a distance about 25 percent more than a single 2 x 6 but a 2 x 12 can span about 80 percent more than. Dimensional lumber is cut to a specific length width and depth.

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. 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. Rafter and joist span faq. In regard to the dimensions of lumber the term 2 x 4 in doesn t actually mean lumber of 2 inches by 4 inches but rather 1 1 2 x 3 1 2 in.

Common joist spacings are 12 inches 304 8 mm 16 inches 406 4 mm and 24 inches 609 6 mm. 1 ft 0 3048 m. That means a 1 x 4 board is actually 3 4 inches by 3 1 2 inches. Dead load weight of structure and fixed loads 10 lbs ft 2.

1 psf lb f ft 2 47 88 n m 2. 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. Updated june 29 2020. Maximum floor joist span for no.

The tables below provide both the nominal and actual sizes so many common north american lumber sizes. 2 grade of douglas fir are indicated below. You should also check your local building codes before starting construction and consult a structural engineer in unusual or extreme situations. Live load is weight of furniture wind snow and more.

The lengths and sizes of joists vary depending on the species and quality of the lumber used. 1x4 and 1x6 boards lose 1 4 inch in thickness and 1 2 inch in width before leaving the mill. For example you can span a 1 grade no knots southern yellow pine a greater distance than a 2 grade with knots southern yellow pine. The strength of a given joist board is most dramatically affected by the board s top to bottom width.

However there is a difference between the nominal size what the lumber is referred to and the actual size. This is the size that appears on the tag in store.