I just did a home remodel using 2X12's and 2x10 headers.
There are engineering tables with the type of header you will need, depending on the span of your opening.
For my remodel, we had three openings, the first one is 10ft. We used 3-2x12's bolted together, seated on a 6x6 treated lumber post.
The second opening was only 8ft, so we use 3-2x10's bolted together and seated on the post.
The third opening was 14ft, we had to use 4-2x12's seated on the 6x6 post.
Since we did not want to see the header, we used 8ft posts and the entire header was then in the attic. We used joist hangers on the header to hold up the roof trusses.
It really turned out beautiful. The owner loves it.
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