Maximum Joist Span

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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.
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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: 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: 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.

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