I am adding a covered porch to a Cape Cod style house. The added roof will be a butterfly style with the porch roof merging with the house roof --box gutter between the two structures. In other words one "wing" of the "butterfly" roof will be the existing house roof slope and the other "wing" will be the porch roof.

What defines the header in this style? The porch is 12'w x 6'd. The roof will be standing seam metal.

Thanks in advance,
riley

Same thing that defines a header in any other style. One must trace the load paths, deteremine what load the header will be asked to support, and design it for that load, both for moment and deflection.

A word of friendly advice: don't build a 6-foot-wide porch! You will regret it every moment that you spend on it. Make it 12 feet wide if at all possible, minimum.

It is actually a 12 feet wide by 6 feet deep landing. I am thinking I may be able to reduce the size of the beam that spans the 12' width of the structure if the side beams will carry the load.

But then your rafters will be parallel to the pitch of the roof instead of perpendicular to it as they should be. And then something has to carry the concentrated load of the side beams.