What size are floor joists in residential construction?
The most common lumber sizes for floor joists are 2x6, 2x8, 2x10, and 2x12. Larger joists can span across wider areas. For example, a 2x6 joist would have a 12-foot, 6-inch maximum span, and a 2x10 joist would have a 21-foot maximum span.
Can you use 2x4 for floor joists?
For average storage use, 2x4 16" O/C is a standard floor joists with good strength. For customers looking to increase their load capacity with standard 2x4 floor joists, then consider spacing your floor joists 12" O/C.
How far can 2X8 floor joist span?
It's going to depend on how far apart the are, and assuming you don't have any crazy dead loads to account for the general rule is 8′ max. Rule of thumb say X 1.5 of board width 8″ = 12″ convert to feet. So, 8″ × 1.5 in feet = 12' of span.