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. Common joist 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 instance, the stronger the wood, the more weight it will be able to support over a longer span.
When determining the floor joist size, measure the length of the span. The span is the inside dimension from the support on one end to the support on the opposite end. The next factor to consider is the joist spacing that will be used and the amount of weight the flooring will need to support. For example, a standard outdoor deck will not be required to support as much weight as a dining room floor, unless the deck with have a large hot tub.
For a 12 inch (304.8 mm) joist spacing, longer lengths of lesser grade lumber can be used. In this example, there will be a greater number of joists supporting the weight of the deck or flooring. So, for an 8 foot long (2.44 m) deck, a Redwood joist of grade 2 lumber would need to be 2 x 6 (50.8 x 152.4 mm ). A larger joist size would be perfectly acceptable, whereas a smaller size would not. The 2 x 6 joist size is suitable for most types of grade 2 lumber at 8 feet (2.44 m) in length.
A structure with a 10 foot (3.05 m) length, and joists set 12 inches (304.8 mm), apart would require using a floor joist size of 2 x 8 (50.8 x 203 mm). If the joist spacing will be 16 inches (406.4 mm), a joist size of 2 x 8 should be used, even if the length is only 8 feet (2.44 m). With a joist spacing of 24 inches (609.6 mm), the same 2 x 8 barely meets the minimum requirement for 8 foot (2.44 m) lengths. A better choice would be using a 2 x 10 (50.8 x 254 mm) joist size. Using grade 1 lumber will provide more structural support with a smaller joist size.
Calculations for choosing a floor joist size depend on the grade and type of the lumber, the length of the span, and the joist spacing. In addition, the weight of both live and dead loads must be considered. A dead load refers to the weight of all the materials involved in the building or deck. Live loads are the addition of furniture and people using the area.
Formulas or engineering programs are available that can be referenced when dealing with unusual span lengths or lumber varieties. Many areas are also subject to building code regulations and those requirements must be followed. When in doubt, moving to the next floor joist size up may be more expensive but worth the cost for eliminating the worry.