First sawmill

[Don P]

I'm not saying it wouldn't work, but I wouldn't know where to begin. A truss of some form would work.
I'd start by scouring scrap piles for something with those W12x19 dimensions or larger.

[Nebraska]

I actually have a similar I beam, and a sawmill. Just where my mind went....an "I could do it this way" thought...A truss would be far more practical as would a big enough beam.  My ancestry  shows through,  my brain remembered one in my little pile of red steel....

[OldRedFord]

A very easy alternative has come up. 

In the photo say I wanted a open shed in front of my shop. I build the support structure of the roof to be higher then the peak of the building, gable end is facing me. 

Do the exact same thing for the sawmill building. The lower section over the track, edger etc and the higher, front facing gable over the log deck. 

 

[Don P]

Making the gable end the log deck does give the potential for a building size opening 

[moodnacreek]

Log deck openings on a sawmill building are always worth a glance. Mine is a steel 6x8 arched [box] gard rail under a 10x12 wood beam and it is only 21' opening. Often they are 25' and wood with cracks and starting to sag. In the old days they would put a truss on the roof with a center pull up rod. Not often seen is the cable under the beam with center fulcrum and the cable pulled tight with turn buckle or eye bolt. They key to this trick is the strength of the post/beam connection. If done right this is a very strong support for the span with a light beam.

[47sawdust]

A lot of the granite sheds in Barre Vermont are built in the manner described in the previous post.
I find the design very pleasing as well as functional.