Several books come to mind that might help in your research:

The Pennsylvania Barn by Robert F. Ensminger

Old Barns in the New World/ Richard Babcock

Barns of the Midwest/ Hubert Wilhelm

The Old Barn Book/ Allen Noble

There is also a great deal of historic framing information in the back issues of Timber Framing.

If you are planning to attend the Traditional Timberframe Research and Advisory Group annual symposium March 12-14, you will meet more than a few framers with experience in the type of roof system you are studying.

This forum probably will supercede the members section BB. Thanx for using it.