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Floor Joist Spans

Figure Out Floor Joist Spans with Wood From Your Home Improvement Store

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Floor joist spans: more is better? With no other factors in place, that may be true. But you need to think about matters of cost and size. For instance, 2" x 10" joists might be stronger--but you only have 8" tall maximum space to work with. What to do? This entails tighter spacing of the joists.

These floor joist span scenarios are meant to illustrate basic dimensions of timber and to link them up with actual products you can easily buy. Also, I hope to show how joist sizing changes the instant you change one of the factors, such as joist spacing or span length.

Live Vs. Dead Loads

Below are scenarios for floor joist timber (not engineered wood) you can buy at your local Lowe's or Home Depot. These floor joist spans cover minimum needs. 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.

Note, too, that 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.

And in our last scenario, we discuss a short span of 8 feet. Yet this is for a workshop. So you would need to take into account very heavy items such as drill presses and power saws. That short span might be deceptive in the span tables: you may actually need heftier floor joists.

Wood Species: Hem-Fir

Because Lowe's does not offer a great variety of wood species, all of 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.

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. And you won't find the same species of lumber in every Lowe's store: it's a local thing. Hem-fir shows up in West Coast stores; whitewood in East Coast stores.

Check Span Tables

A more 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.

Scenario #1: You are building an addition, and one of the rooms is 12 feet. You need to span a distance of 11.5 feet for a live load.

You Will Need to Purchase This Item:
  • 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

Scenario #2: 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.

You Will Need to Purchase This Item:
  • 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

Scenario #3: You are building a one-room addition to your house which is sixteen feet. This means that you need to span a distance a distance of 15.5 feet for a live load.

You Will Need to Purchase This Item:
  • 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

Scenario #4: You are building a workshop. Your floor joists need to span a distance a distance of 8 feet for a live load.

You Will Need to Purchase This Item:
  • 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
Will these 2x6s 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

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