I was debating whether to add the purlin plate in the middle of the rafters as NH suggests versus the structural ridge beam supported in the middle and decided that the center ridge beam was a better option for a couple of reasons:
1) The roof load would be transmitted directly to the footer through the center posts that extend from the ridge to the top of the main tie beams that run from side to side then through the posts that support the center of these main tie beams to the floor. The pulin plates would be suppported by the main tie beams midspan between the center posts and the posts on the side. Or, I would need to add additional posts to give the weight a direct path.
2) The ridge solution requires 40 ft less beam since there would be one ridge beam versus two purlin plates.
3) There would be only 2 posts in the space on the second floor versus 4
On the down side:
1) The ridge beam itself might be difficult to shape, and
2) the rafters may sag in the middle without the midspan support or I might need to add additional rafters.
The original barn was 30x76 and I'm reconstructing a 30x40 so I do have quite a bit of extra lumber. Unfortunately I am 4 ft short of having two extra plates to use as purlin plates.
I'll try to post some pictures later today.
-Dave
