I'm building a carport that is 30x24 ft including a 20x24 gable and 10x24 lean to. It currently has 2x4 roof rafters spaced 24" with collar ties about a foot below the gable peak, so 5-6 ft long ties. Seems a 2x4 SPF #2 should really only span about 6 ft horizontally, using the American Wood Council online calculator. Why on earth do I have 2x4 rafters 24" on center? It's a long story.
The rafters on the project are spanning about 9.5 ft horizontally, both on a gable with almost 7/12 pitch (19 ft total gable span) and spanning the same 9.5ft on a side lean to with about 2/12 pitch.
This does not seem structurally sound.
My question is: Will adding 2x6 rafters in between the 2x4's make it structurally sound? So final spacing will be 12" between rafters, but alternate 2x4, 2x6. I would put 10 ft collar ties on the new 2x6 rafters, so about at the halfway mark down from the gable peak to the top plate on the posts.
Of course the answer to the question depends on the overall structural elements, but just assume that everything else is ok. Any other issues are a separate question, and you might see another from me regarding this project. I just want to know thoughts on 12" spaced rafters alternating 2x4 and 2x6.
Other info: On top of the rafters will go 0.419 OSB, felt, and metal roofing. There are other issues like adding additional bracing to the rafters, but that may or may not be relevant to this question. I'll give more details if anyone requires.