On a 2x6 exterior wall, how thick do you make the header?
The header is the same thickness as a 2x4 wall. Don't add a 1/2" spacer, just nail the two 2x10s together as usual. Nail the header in place with the outside of the header flush with the outside of the wall.
Nail a 2x6 (on the flat) under the header and against the two cripples. This gives the thickness of the wall below the header for drywall (or any other type of wallboard) on the inside of the wall. Make sure the 2x6 trim, on the flat, is not nailed on under the cripples. The cripples should be supporting the header directly as in this drawing:
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