A.Carl Hagstrom responds: The rule of thumb that I like to use when sizing uniformly loaded residential floor joists is "half the span plus two." First, round the clear span of the floor joist up to the nearest foot, and divide by two. Then add two to the answer. This will give you the depth (in inches) of the required floor joist.

For example, assume the clear span is 15 feet 6 inches. Round up to 16 and divide this span by 2, giving you 8. Next, add 2 to get the required joist depth. In this example, 8 + 2 = 10, so a 2x12 floor joist will be required (it’s important to use the actual lumber dimensions, not the nominal dimensions).

Typically, residential floor loads are assumed to be 50 pounds per square foot (40 pounds live plus 10 pounds dead), and this rule of thumb will work for loading conditions that don’t exceed that value. If the floor system is to be tiled or there are any other unusual loading conditions, I’d recommend having an engineer review the conditions.

Carl Hagstrom is an associate editor at the Journal of Light Construction.