A header (or lintel) is a structural member commonly placed above a window, door, or other opening which spans the opening distance and spreads the overhead load of the building/structure horizontally to the nearest vertical members and on down to the foundation. Headers should be engineered by a professional for the live, dead, wind, and seismic loads they need to resist based on the span distance they are meant to cover. Most commonly, simple headers for simple structures are composed of 2x lumber and plywood or OSB (commonly 1/2″) fused or mechanically attached together to match the thickness of the wall framing for which they’ll become a part of. 
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The following are rules of thumb for the spanning capability of certain sizes of 2x lumber coupled together. 
2-2×4: 4′ (Roof Only)
2-2×6: 4-6′ (Roof Only) or 4′ (One Story) 
2-2×8:  6-8′ (Roof Only) or 4-6′ (One Story)
2-2×10: 8-10′ (Roof Only) or 6-8′ (One Story) or 4-6′ (Two Stories)
2-2×12: 10-12′ (Roof Only) or 8-10′ (One Story) or 6-8′ (Two Stories)
If a span is greater than 4′ it will typically require at least 2″ of bearing on each end. In areas where unusually heavy loads may reside above, a header system will typically be added to with cripples or blocking to create a ‘beam’ condition which is capable of transferring these heavier loads. ‘Trimmer studs’ from the floor plate to the bottom of the header support the header at each end with ‘King studs’ on either side of the trimmer studs which go from floor plate to top plate.
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10d nails are driven horizontally through the king studs into the sides of the header to keep it secured.  A ‘Box Beam’ is also a type of header commonly used that is essentially a framed beam with cripple studs which is sheathed of plywood.
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If a framed wall has a 2×6 or 2×8 header continous in place of the top plates, individual headers at each opening may not be required.
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