Floor and Roof 'Wood Joist' Table
Based on 1000 psi fiber stress and L/360 deflection
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Nominal
Size
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Spacing
(inches)
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Maximum Allowable Span for Uniform Loading
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67 psf
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57psf
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47 psf
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37 psf
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22 psf
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2 x 6
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12
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8'-9"
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9'-5"
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10'-4"
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11'-8"
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15'-1"
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16
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7'-6"
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8'-1"
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9'-0"
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10'-1"
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13'-2"
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24
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6'-0"
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6'-8"
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7'-4"
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8'-4"
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10'-9"
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2 x 8
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12
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11'-6"
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12'-5"
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13'-8"
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15'-4"
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20'-0"
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16
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10'-0"
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10'-9"
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11'-10"
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13'-4"
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17'-4"
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24
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8'-1"
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8'-9"
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9'-7"
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10'-11"
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14'-1"
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2 x 10
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12
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14'-7"
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15'-10"
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17'-5"
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19'-7
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25'-6"
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16
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12'-8"
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13'-9"
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15'-1"
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17'-0"
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22'-0"
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24
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10'-3"
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11'-2"
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12'-4"
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13'-11"
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18'-0"
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2 x 12
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12
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17'-9"
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19'-2"
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21'-2"
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23'-10"
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---
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16
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15'-4"
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16'-7"
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18'-4"
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20'-7"
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26'-10"
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24
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12'-6"
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13'-7"
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15'-0"
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16'-11"
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21'-10"
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Caution!
Care needs to be taken when using the tables. You must be sure of what type of
loading you are going to place over the joists. Refer to the table below. To
be sure, ask your Engineer or Architect.
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Definitions:
Nominal Size = Size used to order material, such as, a 2 x 4, but which is
Actually 1 1/2" x 3 1/2". Therefore, when an Engineer or Architect perform the
calculations on a wood member they use the Actual dimension (which is smaller
than the nominal size).
Spacing = How far apart each member is installed in the horizontal plane.
Fiber Stress = The Bending Fiber Stress which determines the capacity of the
member to support the load placed on it.
Maximum Allowable Span for Uniform Loading = The Distance between the end
supports of the Joists or Rafters.
Allowable Deflections:
Floor and Ceilings Allowable Deflection is L/360 = 0.4 inches
Roofs Allowable Deflection for less than 3 in 12 slope is L/240 = 0.6 inches
Roofs Allowable Deflection for greater than 3 in 12 slope is L/180 = 0.8 inches
(Note: the L distance needs to be in inches)
Example for Floors and Ceiling Deflection:
L/360 = the amount of deflection permitted in the Wood Joist, where L = length
of the Joist in inches.
For example: 12' = 144", therefore, L/360 = 144/360 = .4" of deflection
permitted (these values are set by the Codes).
Loading Conditions: (where, psf = Pounds per Square Feet)
NOTE: These Loading Conditions do not take into account any additional loads,
such as, Wind Loads, Snow Loads or Rain Loads and are limited to interior,
non-exposed loading conditions. Visit your Architect or Engineer to determine
the final Size of your Structural members.
67 psf --- Floor Joists with plaster under, 50 psf Live Load, 17 psf Dead Load
57 psf --- Floor Joists with plaster under, 40 psf Live Load, 17 psf Dead Load
57 psf --- Roof Joists having a slope of 2 1/2 to 12 or more, 30 psf Live Load,
27 psf Dead Load
47 psf --- Floor Joists without plaster under, 40 psf Live Load, 7 psf Dead Load
47 psf --- Roof Joists with plaster under, 30 psf Live Load, 17 psf Dead Load
37 psf --- Ceiling Joists over Living Rooms and under Attic Spaces, 25 psf Live
Load, 12 psf Dead Load
37 psf --- Roof Joists without plaster under, 30 psf Live Load, 7 psf Dead Load
22 psf --- Ceiling Joists without usable Attic Space, 10 psf Live Load, 12 psf
Dead Load
The Span Tables have been Designed Assuming that the Wood Members have a
Bending Stress = 1000 psi. Also check that the Wood Members are in Good
Condition, Avoid Splits, Checked or Cracked Lumber. These Tables are to be
used to Estimate the Lumber Needed for Construction and
MAY ONLY BE USED FOR DESIGN BY SOMEONE WHO HAS THE KNOWLEDGE AND EXPERIENCE IN
CONSTRUCTION AND SHALL TAKE INTO ACCOUNT ANY ADDITIONAL LOADING, SUCH AS, WIND
LOADS, SNOW LOAD AND/OR RAIN LOADS.
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Example of How to Do the Calculations:
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