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American Wood Council

The American Lumber Standard Committee (ALSC) Board of Review has approved changes to design values for all grades and all sizes of visually-graded Southern Pine and Mixed Southern Pine lumber with a recommended effective date of June 1, 2013 to allow for their orderly implementation. For more information, visit www.spib.org and www.southernpine.com.

AWC has revised the Maximum Span Calculator for Wood Joists and Rafters to use design values for Southern Pine/Mixed Southern Pine prior to June 1, 2012 (pre 6/1/12); interim design values that became effective June 1, 2012 (Eff. 6/1/12); and new design values for Southern Pine/Mixed Southern Pine that became effective June 1, 2013 (Eff. 6/1/13).


            Maximum horizontal spans for joists and rafters are calculated on the basis of allowable design values for dimension lumber provided in the 2012 National Design Specification® for Wood Construction (NDS®) Supplement: Design Values for Wood Construction.  The calculated spans are based on the following assumptions and conditions:

  • Joists and rafters are oriented to support vertical loads applied to the narrow (1.5 in. thick) face of the member, resulting in strong-axis bending.
  • At least three joists or rafters are installed parallel to one another and spaced not more than 24” o.c., with properly designed sheathing attached on the top (compression) edge of each joist or rafter.
  • Joists and rafters are single-span with adequate structural support at each end.  The on-line Span Calculator does notaddress cantilevered joists and rafters, multiple-span joists and rafters, or spliced joists and rafters.
  • For joists and rafters, most building codes specify a minimum bearing length of 1.5 inches on wood or metal and not less than 3 inches on masonry or concrete. These minimum bearing lengths are to be used in the absence of calculated bearing lengths. Consult the local building code for specific requirements.
  • Joists and rafters carry only combinations of dead load plus live load (D + L), dead load plus snow load (D + S), or dead load plus roof live load (D + Lr).  The on-line Span Calculator does not address wind load or seismic load.
  • Buildings in regions with ground snow loads greater than 70 psf should not use the prescriptive provisions of the governing building code without approval of the authority having jurisdiction. Buildings in these high snow zones should be designed in accordance with accepted engineering practice.

The quality of structural wood products and fasteners and the design of load-supporting members and connections shall conform to provisions of the NDS.  All members shall be framed, anchored, tied, and braced to provide necessary strength and rigidity.  Adequate bracing and bridging to resist wind, seismic, and other applicable lateral forces shall be provided.

Adequate support shall be provided at the ends of all joists and rafters.  For roof slopes less than 3 in 12, properly designed ridge beams shall be installed at roof peaks and rafters shall bear directly on the ridge beam or be supported by adequately designed hangers or framing anchors.  Ceiling joists are not required when properly designed ridge beams are used.  Properly designed ridge beams are required for cathedral ceilings, regardless of roof slope. 

When roof slope equals or exceeds 3 in 12, ridge boards may be substituted for properly designed ridge beams.  Ridge boards shall be at least 1 inch nominal in thickness and not less than the depth of the cut end of the rafter.  Rafters shall be attached to the ridge board directly opposite each other, and ceiling joists shall be installed parallel with rafters to provide a continuous tie between exterior walls.


            Calculated spans apply to undamaged lumber identified by the grade stamp of, or certificate of inspection issued by, a lumber grading or inspection bureau or agency recognized by the Board of Review of the American Lumber Standards Committee or the Canadian Lumber Standards Association.  Calculated spans apply to surfaced (S4S) lumber having dimensions that conform to the American Softwood Lumber Standard PS 20-10.  The actual lumber sizes are:

Minimum Dressed Dimensions
Nominal Lumber Size                           for Dry Lumber

            2×4                                          1.5 in. × 3.5 in.

            2×6                                          1.5 in. × 5.5 in.

            2×8                                          1.5 in. × 7.25 in.

            2×10                                        1.5 in. × 9.25 in.

            2×12                                        1.5 in. × 11.25 in.



While every effort has been made to insure the accuracy of the information presented, and special effort has been made to assure that the information reflects the state-of-the-art, neither the American Wood Council nor its members assume any responsibility for any particular design prepared from this on-line Span Calculator. Those using this on-line Span Calculator assume all liability from its use.


WARNING: Netscape 4.xx is no longer supported by the web community. As such, there are some capabilities of AWC's Calculator Tools that will not work correctly on these and older versions of the Netscape browser. AWC Calculator users are cautioned that such use may result in incorrect results.

Last update: 05/16/2013

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Leesburg, VA 20175

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