how far can 2×4 span as floor joist

The maximum span for a 2×4 floor joist depends on several factors, including:

1. Load: The weight that the floor will be supporting, including people, furniture, and any additional loads.
2. Grade: The type of lumber used, with higher grades having fewer knots and imperfections.
3. Spacing: The distance between joists, with closer spacing allowing for longer spans.
4. Species: Different types of wood have varying strengths and stiffness.

According to the International Residential Code (IRC) and the American Society of Civil Engineers (ASCE), here are some general guidelines for 2×4 floor joist spans:

* For a 2×4 pressure-treated Southern Pine joist, with a 16-inch on-center spacing, and a 40 psf (pounds per square foot) live load:
+ Maximum span: 8 feet 1 inch (simple span), 7 feet 1 inch (continuous span)
* For a 2×4 #2 grade Spruce-Pine-Fir joist, with a 16-inch on-center spacing, and a 40 psf live load:
+ Maximum span: 7 feet 9 inches (simple span), 6 feet 11 inches (continuous span)
* For a 2×4 #2 grade Douglas Fir-Larch joist, with a 16-inch on-center spacing, and a 40 psf live load:
+ Maximum span: 8 feet 5 inches (simple span), 7 feet 5 inches (continuous span)

Keep in mind that these are general guidelines and may not apply to your specific situation. It’s always best to consult with a structural engineer or a qualified builder to determine the maximum span for your specific floor joist application.

Additionally, it’s worth noting that 2x4s are not typically used as floor joists in modern construction, as they are not strong enough to support the loads required by most building codes. 2×6, 2×8, or 2×10 joists are more commonly used for floor framing.