A 4x6 beam runs along the center of the barn to further help support the hay load.  It is supported by two inner posts positioned in line with the middle end posts.  The 4x4 supports are bolted to the interior of the end-wall posts to support the ends of the beam.

(3/4" plywood for the hayloft floor)

The next step is to construct the gambrel rafter pairs.  Note they are made of two 2x4s on each side of each post.  The lower portion, below plate line, is separate and joined on later.  cut a trial set of rafter pieces and join them together temporarily with wooden cleats nailed in place.  It will probably take a bit of fiddling to get the rafters cut to the proper length and angles.  Place a short "fake" ridgeboard between the upper ends of the rafter pairs and stand them in position to try to fit them in place.  Make sure you have plenty of help and bracing as you go.

Once you're satisfied with the fit of the rafters, cut all of the rafters to the correct size and assemble in pairs.  Use 3/8" plywood gussets on each side of the center joints, nailing and gluing them with resorcinol glue.

One good method of installing the first end pair is to nail a 2x4 across their bottom edges on their outside surfaces.  Place the fake ridgeboard between their upper ends to find the correct location, then temporarily nail a collar tie across the two upper ends.  Nail a vertical 2x4 across the upper end of one rafter pair and cacross the bottom brace, extending it past the building edge a foot or so.  This helps provide vertical bracing until the first end rafter has been  installed.  Remove the fake ridgeboard and erect the rafter apir, making sure you have a brace pole both front and back to prevent it from tipping.  These rafters also have a tendency to slip off the barn edges because there is no bird's mouth cut.  Stop blocks can be used on the first pair, or have a helper at each end hold and nail as the rafters are positioned in place.  Once the first end rafter is installed, position the ridgeboard in place, noting it sticks out past the end of the front rafter pair for the overhang and "block-and-tackle" ridgepole.  Prop up the opposite end of the ridgeboard.  Note that it will have to be made in sections and must be joined in the middle of a rafter pair.

Nail the ramaining rafter pairs in place, join on the next piece of ridgeboard, and continue to the opposite end.  Because the pressure of the rafters is outward and they are held in place primarily with toenailing, knee braces are nailed to the rafters and down through the flooring into the floor joists with 16d nails.

Once all rafter pairs and both end walls have been erected, frame the remaining portion of the end walls, including the rough openings for the doors.