just becareful 6" is low for supporting your weight in the middle of that span if it has large knots in it, it will snap on youYou must have some heavy roofs, slate, brick to mandate such large dimensions? I've seen spans like that on 4x4s and 6x6s on similar buildings and I consulted with an engineer to get the 6-8 inches I plan to aim for.
I plan to use sheet metal roofs with a big enough angle that snow won't collect on it. I do plan to add in a 2nd floor so a roof truss in the middle is possible to add later.
as you might be installing rafters and also laths on top to hold your metal roof on