Floor Joist Spans: 4 Scenarios That Help You Calculate Your Own

Floor Joists
Getty / Hans Hansen

In the case of floor joist spans, more wood is not always better. For example, joists that are 12" deep are, logically, stronger than joists that are 10" deep. When dealing with a two-story house, you want to maximize ceiling height for the level below. So, shallower joists provide that lower floor with a couple of valuable inches of headroom.

These floor joist span scenarios illustrate basic dimensions of sawn lumber (as opposed to laminated veneer lumber) and link them to actual products you can easily buy at your local home improvement store.

Pay special attention to the way that joist sizing changes the instant you change one of the factors, such as joist spacing or span length.

Live vs. Dead Loads

  • Live load means that the joists are carrying the weight from occupancy--people and things.
  • Dead load refers only to stuff that does not move around, such as things you might store in an attic.

There are different types of live and dead loads. First-floor live loads have higher requirements than second-floor live loads (40 pounds per square ft. vs. 30 psf). A room used solely for sleeping might need to carry only 30 psf, whereas a garage would need 50 psf or higher.

Wood Species: Hem-Fir

These joist spans are based on hem-fir wood. Hem-fir is a common type of lumber and is a species combination of hemlock and fir. This is the type of species you will find at your local Lowe's, Home Depot, or Menards. In this way, species does matter. For example, 2x10 inch hem-fir can span 15 feet under certain conditions.

Yet Austrian Spruce, an imported species, could accommodate an additional half-foot of span under the same conditions.

Finally, you will not find the same species of lumber in every Lowe's or Home Depot, as different areas will stock different kinds of local wood. Hem-fir shows up in West Coast stores; whitewood in East Coast stores.

1. Addition With 12 Ft. Floors and 16 In. On-Center (OC) Joists

Scenario:

You are building an addition. One of the rooms is 12 feet. You need to span a distance of 11.5 feet for a live load.

Purchase This:

  • Dimension (Inches): 2x8
  • Length (Feet): 12
  • Wood Species: Hem-Fir
  • Retailer: Lowe's
  • Actual Product Name: Top Choice 2 x 8 x 12 Kiln-Dried Hem-Fir S4S Lumber

Installation Spacing (Inches, On-Center): 16

2. Addition With 12 Ft. Floors and Joists That Are Spaced Wider

Scenario:

You need to span a distance of the same distance, 11.5 feet, for a live load, but circumstances dictate that you can only space the joists every 24 inches.

Purchase This:

  • Dimension (Inches): 2x10
  • Length (Feet): 12
  • Wood Species: Hem-Fir
  • Retailer: Lowe's
  • Actual Product Name: Top Choice 2 x 10 x 12 Kiln-Dried Hem-Fir S4S Lumber

Installation Spacing (Inches, On-Center): 24

3. Addition With 16 Ft. Floors

Scenario:

You are building a one-room addition to your house which is 16 feet long. This means that you need to span a distance a distance of 15.5 feet for a live load.

Purchase This:

  • Dimension (Inches): 2x12
  • Length (Feet): 16
  • Wood Species: Hem-Fir
  • Retailer: Lowe's
  • Actual Product Name: Top Choice 2 x 12 x 16 Kiln-Dried Hem-Fir S4S Lumber

Installation Spacing (Inches, On-Center): 16

4. Short Workshop Floor, 8 Ft. Long

Scenario:

You are building a workshop. Your floor joists need to span a distance a distance of 8 feet for a live load.

Purchase This:

  • Dimension (Inches): 2x6
  • Length (Feet): 10
  • Wood Species: Hem-Fir
  • Retailer: Lowe's
  • Actual Product Name: Top Choice 2 x 6 x 10 Kiln-Dried Hem-Fir S4S Lumber

While 8 feet is a short span, remember that this is a workshop, so you would need to take into account very heavy items such as drill presses and power saws.

Will these 2x6's be adequate if you have very heavy items in the workshop, or will this just be used for light hobby work? These are factors to take into consideration with floor joist spans.

Installation Spacing (Inches, On-Center): 16

Check Span Tables

A nuanced look at floor joist spans can be found from span calculators or tables, a few of which are listed below.

True floor joist span calculations can only be made by a structural engineer or contractor.

Intended room functions, deflection of the floor joists, and bearing points are additional factors that the engineer or contractor can look at to determine joist sizing.