How To Determine Floor Joist Span Accurately

We often think that the closer the joist spacing, the better it carries a load. Although it is commonly true, determining a joist span is much more than that.

Since each wood species has its unique characteristic, the determination of joist spacing is definitely needed to pay attention to several points.

It is the engineer’s job in calculating the maximum span allowed according to the current designed load. Here are some points that engineers need to look at:

  1. Wood Species
    It is obvious that each wood species has many variations in strength characteristics. Some have a higher bending strength, sear strength, and compressive strength. All of that mainly depends on the weight density of the wood.
  2. Grade of the lumber
    It is highly recommended to use a 1 or 2 grade of lumber. Using 3 or 4-grade lumber is a big mistake since flooring joist has very long term usage.
  3. Width and thickness of the boards
    Most of the lumbering strength comes from its width. Although the ratio between the width and thickness also important. For example, a joist made from doubled 2 x 6s can span a distance about 25 percent more than a single 2 x 6, but a 2 x 12 can span about 80 percent more than a 2 x 6, even though it has the same amount of wood as a doubled 2 x 6.
  4. The load placed on the floor
    Wood material has a limited value of load that allowed to be supported. In order to get a precise calculating, you need to separate between live-load and dead-load.
  5. Length of the joist span
    Joist span is the length between 2 supporting structures where the joist will be placed. The following table shows us how long the maximum joist span allowed based on 3 points. Which is the wood species, the joist spacing is it 16″ or 24″, and the joist size.
Joist calculation table / image via

With that basic knowledge, contractors can choose the right size and the wood species used to build their clients house safely. Flooring joist span is a vital item of the entire house, therefore a careful calculation will be important to the entire house durability.

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