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Maximum Joist Span

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Dec 10, 2024, 4:36:37 PM
Created by
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Apr 5, 2021, 4:27:46 PM
` = "Maximum Joist Span"`
`(LL)"Live Load"`
`(oC)"On Center Spacing"`
`(bS)"Board Size"`
`(ST)"Stiffness Factor"`
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dabaec8d-962b-11eb-9e81-bc764e203090

The Maximum Joist Span function estimates the span length of a (1,200 pounds per square inch fiber stress) wood joist based on the live load, spacing, board width (e.g. 2x6 vs 2x10) and whether the joist need to be extra stiff for materials attached such as plaster below them. 

INSTRUCTIONS: Choose units and enter the following:

  • (LL) Live Load (10 lb/ft2 to 80 lb/ft2)
  • (oC) On Center spacing of joists (24" and less)
  • (bS) Board Size (2x6,2x8,2x10,2x12,2x14)
  • (sF) Stiffness Factor  (stiff for floors with plaster or other non-pliable material below them)

Max Joist Span (mxSpan): The calculator returns the max span in feet.  However, this can be automatically converted to compatible units.
/attachments/dabaec8d-962b-11eb-9e81-bc764e203090/FloorJoist.JPG

The Math / Science

The values for maximum span are based on industry tables (below) for joist lumber with a fiber stress of 1,200 pounds per square inch.  For safety purposes, values not equaling the specified live loads and on-center spacing are rounded up.  

Load Level is a vitally important factor.  Consult the local building codes for the load level required for your application.  A common load level for an outside deck is 50 pounds per square feet.  However, if the deck is going hold a hot-tub (spa), the weight of the water is greatly in excess of 50 pounds per square feet.  Again, consult the local codes for such applications. 

Note: The units for maximum span are in feet, the load level (LL) are pounds per square feet (lb/ft2), and the units for on center spacing (oC) are inches.

   mxSpan - Maximum Joist Span (non-stiff)  
LL oC 2x6 2x8 2x10 2x12 2x14  
30 12 13.417 17.667 22.167 24 24  
30 16 11.75 15.5 19.417 23.25 24  
30 24 9.667 12.833 16.167 19.417 22.5  
40 12 12 15.917 19.917 23.917 24  
40 16 10.5 13.917 17.33 20.917 21  
40 24 8.583 11.417 14.417 17.417 23.25  
50 12 10.833 14.417 18.167 21.917 24  
50 16 9.5 12.583 15.833 19.083 22.25  
50 24 7.833 10.33 13.083 15.75 18.417  
60 12 10 13.33 16.833 20.167 23.5  
60 16 8.75 11.667 14.667 17.667 20.583  
60 24 7.25 9.5 12.083 14.583 17  
70 12 9.417 12.417 15.667 18.917 22  
70 16 8.083 10.833 13.667 16.5 19.25  
70 24 6.667 8.917 11.25 13.583 15.917  
80 12 8.75 11.667 14.75 17.75 20.833  
80 16 7.667 10.167 12.833 15.5 18.25  
80 24 6.417 8.417 10.583 12.75 15  

The above table provide adequate strength to support the load limits.  However, there still may be some bend due to the flexibility of lumber.  In some cases, this is problematic if the joists are used for very stiff products such as a plaster ceiling below the joists.  In that case, the flexibility may allow movement that is harmful to the material (plaster).  In this case, the following table provide a maximum joist length with minimal flexibility.  

   mxSpan - Maximum Joist Span (stiff)  
LL oC 2x6 2x8 2x10 2x12 2x14  
10 12 12.75 16.75 21.083 24 24  
10 16 11.667 15.33 19.33 23.33 24  
10 24 10.25 14.5 17.25 20.583 24  
20 12 11.5 15.25 19.167 23 24  
20 16 10.417 13.917 17.5 21.083 24  
20 24 9.167 12.25 15.5 18.583 21.75  
30 12 10.667 14 17.75 21.33 24.75  
30 16 9.75 12.917 16.25 19.5 22.75  
30 24 8.5 11.33 14.33 17.25 20.167  
40 12 10 13.25 16.667 20.083 23.417  
40 16 9.083 12.083 15.25 18.417 21.417  
40 24 7.833 10.33 13.083 15.75 18.417  
50 12 9.5 12.583 15.833 19.083 22.33  
50 16 8.583 11.5 14.583 17.5 20.417  
50 24 7.25 9.5 12.083 14.583 17  
60 12 9 12 15.167 18.25 21.33  
60 16 8.083 10.833 13.667 16.5 19.25  
60 24 6.667 8.917 11.25 13.583 15.833  
70 12 8.583 11.5 14.5 17.5 20.5  
70 16 7.667 10.167 12.833 15.5 18.25  
70 24 6.417 8.417 10.583 12.75 15  

Joist Calculators

  • Floor Joist Count:  Floor joist framing with on center spacing.Computes the number of joists (joists and end plates) and joist ties required for a floor or deck based on the dimension of the room or deck and the on-center joist separation. 
  • Floor Joist Cost: Estimates the cost of Floor Joist boards (joists and end plates) required for a floor or deck based on the dimensions of the deck, spacing of joists and unit cost of joist boards.
  • Joist Hanger Count:Joist hanger Computes the number of joist hangers required for a deck based on the width of the deck and the spacing of joists.
  • I-Joist Count: Computes the the number of I-Joist boards required to cover a span (width) based on the size of the span and spacing of the I-Joists.
  • Maximum Joist Span: Computes the span length of a (1,200 pounds per square inch fiber stress) wood joist based on the live load, spacing, board width (e.g., 2x6 vs 2x10) and whether the joist need to be extra stiff for materials attached such as plaster below them. 
  • Joists on Beams: Computes the number of joists (2x dimensional lumber or i-joists), number of beams, number of joist hangers, and length of plate boards needed for a floor or deck with a span that might require one or more beams.
  • On Center Spacing: Computes the even spacing between a number of objects (e.g. posts, studs, joists, rafters, trusses, lights, rebars) over a specified length or distance.

Building Calculators
This equation, Maximum Joist Span, is used in 1 page
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