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What Size of Floor Joists do I need

Floor joists are calculated by how much deflection they will give when placed under a load. If you use to small of a floor joist or space them too far apart you will end up with a bouncy floor or worse, a floor that could fail.

The deflection is typically calculated using 10 psf, (pounds per square foot) dead load and 40 psf live load; combined to a 50 psf. The table below is figured using a deflection between 3/8” and 1/2″. Meaning, if you place 50 psf on the center of a board supported only on its ends it will deflect between 3/8” and 1/2”.

It is also important to know what species of wood you are using and what grade it is. Each wood species naturally has a different strength. Lower grade boards are weaker because of the knots and defects in the wood weaken the boards.

Below you will find a general guide for selecting what size of floor joists to use. Be sure to consult an engineer/architect and your local building department before implementing. Other factors could require a different configuration and larger joist. For example, placement of interior walls or bathtubs, those soaking tubs get heavy when full of water.

Floor Joist Span Ratings
Species Grade 2″x8″ Board
16″ O.C
2″x8″ Board
24″ O.C
2″x10″ Board
16″ O.C
2″x10″ Board
24″ O.C
2″x12″ Board
16″ O.C
2″x12″ Board
24″ O.C
Douglas Fir-Larch 2 13′ 1" 11′ 3" 16′ 9" 14′ 5" 20′ 5" 17′ 6"
Douglas Fir-Larch 3 10′ 7" 8′ 8" 13′ 6" 11′ 16′ 5" 13′ 5"
Douglas Fir South 2 12′ 10′ 6" 15′ 3" 13′ 4" 18′ 7" 16′ 3"
Douglas Fir South 3 10′ 3" 8′ 4" 13′ 1" 10′ 8" 15′ 11" 13′
Hemlock 2 12′ 3" 10′ 15′ 8" 12′ 10" 19′ 1" 15′ 7"
Hemlock 3 9′ 5" 7′ 8" 12′ 9′ 10" 14′ 7" 11′ 11"
Mountain Hemlock 2 11′ 4" 9′ 11" 14′ 6" 14′ 8" 17′ 7" 15′ 4"
Mountain Hemlock 3 9′ 7" 7′ 10" 12′ 3" 10′ 14′ 11" 12′ 2"
Western Hemlock 2 12′ 3" 10′ 6" 15′ 8" 13′ 4" 19′ 1" 16′ 3"
Western Hemlock 3 9′ 11" 8′ 1" 12′ 8" 10′ 4" 15′ 5" 12′ 7"
Engelmann Spruce/Alpine Fir 2 11′ 2" 9′ 1" 14′ 3" 11′ 7" 17′ 3" 14′ 2"
Engelmann Spruce/Alpine Fir 3 8′ 6" 6′ 11" 10′ 10" 8′ 10" 13′ 2" 10′ 9"
Lodgepole Pine 2 11′ 8" 9′ 7" 14′ 11" 12′ 3" 18′ 1" 14′ 11"
Lodgepole Pine 3 9′ 1" 7′ 5" 11′ 7" 9′ 5" 14′ 1" 11′ 6"
Ponderosa Pine/Sugar Pine 2 11′ 4" 9′ 3" 14′ 5" 11′ 9" 17′ 7" 14′ 4"
Ponderosa Pine/Sugar Pine 3 8′ 8" 7′ 7" 11′ 1" 9′ 1" 13′ 6" 11′

Notes:
O.C = On Center
Strength – 10 PSF Dead Load plus 40 PSF Live Load
Deflection – Limited to span in inches divided by 360 live load only.

The information provided here is only intended to be used for general knowledge. Before implementing into your project consult an architect/engineer, your local building department, and the manufacture of the products you are using.

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