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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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Nominal vs Actual Lumbers Size – Why my 2×4 measures smaller

Construction lumber is often referred to as its nominal size or trade size. This size does not match the actual measurement of the wood you are buying. It is used in name only to identify the general/approximate size you are getting. For example, it is much easier to say “2×4” that it is to say “1-1/2×3-1/2”.

Referring to construction materials in their nominal sizes have become the industry standards for many of the products used. Other examples you will see nominal sizes used are windows and doors. Each product category has its own reasons and story for how the sizing reference came to be.

In the instance of dimensional lumber, the sizing refers to how the mill does its first cuts from the log. For an example, the mill may initially cut a 2×4 at 2”x4” but as the wood is further processed, it will end up at a final size of 1-1/2” x 3-1/2”. The reason being is that when a tree is freshly cut cell structure of the tree contains a lot of water. Once the boards are rough cut they are kiln dried, as the wood dries and loses that water it shrinks. Then the board is ran through a plainer and smoothed to its final size.

Below is a chart of nominal vs actual size of common boards used in the construction industry.

 

Nominal vs Actual Board Sizes in inches
Nominal Size Actual Size
1×2 3/4 x 1-1/2
1×3 3/4 x 2-1/2
1×4 3/4 x 3-1/2
1×6 3/4 x 5-1/2
1×8 3/4 x 7-1/4
1×10 3/4 x 9-1/4
1×12 3/4×11-1/4
5/4 x 4 1 x 3-1/2
5/4 x 6 1 x 5-1/2
2×2 1-1/2 x 1-1/2
2×4 1-1/2 x 3-1/2
2×6 1-1/2 x 5-1/2
2×8 1-1/2 x 7-1/4
2×10 1-1/2 x 9-1/4
2×12 1-1/2 x 11-1/4
4×4 3-1/2 x 3-1/2
6×6 5-1/2 x 5-1/2
8×8

7-1/4 x 7-1/4

12×12 11-1/4 x 11-1/4

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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