How do I choose the correct floor joist size? (with photos)

View of the rafters on the second floor seen from below.

Choosing the correct floor joist size depends on your construction application, joist spacing, and many other factors. This includes the length of the span, as well as the type and grade of wood being used. Common beam spacings are 12 inches (304.8 mm), 16 inches (406.4 mm), and 24 inches (609.6 mm). The amount of weight that needs to be supported can also be a significant factor. For example, the stronger the wood, the more weight it will be able to support over a long span.

Floor beams in new construction.

When determining the floor joist size, measure the span length. The span is the internal dimension from the support at one end to the support at the opposite end. The next factor to consider is the spacing of the joists that will be used and the amount of weight the floor will need to support. For example, a standard outdoor deck will not need to support as much weight as a dining room floor unless the deck has a large hot tub.

For 12 inch (304.8 mm) joist spacing, longer lengths of lower grade lumber can be used. In this example, there will be a greater number of beams supporting the weight of the deck or floor. Therefore, for a deck length of 2.44 m, a Grade 2 wood Redwood beam would need to be 2 x 6 (50.8 x 152.4 mm). A larger beam size would be perfectly acceptable, while a smaller size would not. The 2 x 6 joist size is suitable for most types of Grade 2 lumber that are 8 ft (2.44 m) long.

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A structure with a length of 10 feet (3.05 m) and beams that are 12 inches (304.8 mm) apart would require the use of a 2 x 8 (50.8 x 203 mm) floor beam. If the beam spacing is 16 inches (406.4 mm), a beam size of 2 x 8 must be used, even if the length is only 2.44 m (8 ft). With a beam spacing of 24 inches (609.6 mm), the same 2 x 8 barely meets the minimum requirement for 8 ft (2.44 m) lengths. A better choice would be to use a beam size of 2 x 10 (50.8 x 254 mm). Using Grade 1 lumber will provide more structural support with a smaller beam size.

The calculations for choosing the floor joist size depend on the type and type of lumber, span length and joist spacing. In addition, the weight of live and dead loads must be considered. A dead load refers to the weight of all materials involved in the construction or deck. Live loads are the addition of furniture and people using the area.

Formulas or engineering programs are available that can be consulted when dealing with unusual span lengths or lumber varieties. Many areas are also subject to building code regulations and these requirements must be followed. When in doubt, moving to the next floor and increasing the size of the rafter may be more expensive, but it is worth eliminating the worry.

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