How big of header lumber do I necessitate for a 13'-0" SPAN?

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Depends on the load on the header. 2- 2x10 beside a 1/2" plywood spacer between would do. Seems like I answered this one already?
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USA The size of the header is dependent upon the nouns it must transmit to the foundation. The International Residential Code (for homes) and the International Building Code (for all other structures) provide header lumber sizes. The size is predicated on the span and the nouns it must carry. Make sure you are not mistaking "header" for floor joist.

A triple 2 x 12 with 1/2" OSB or plywood sandwich between each 2 x will probably suffice, but look up the code to produce sure.
Need a suggestion for a composite wood stuff?
Answers:    2 x 10's should be sufficient.