good point, Jim.
if you aren't trying to clearspan, then you can support the rafters at the peak with a post (as Jim was illustrating) which if done correctly eliminates thrust on the plate, or you prop them up somewhere along their span with a post and reduce the thrust on the plate. In this instance, you need to tie the posts together to contain the (somewhat) reduced thrust. Either way, you don't need a scarf.
I would not put a scarf in tension without going to a pretty beefy scarf.
Here are some beefy tie scarfs --
http://books.google.com/books?id=PRdDAAAAIAAJ&pg=PA465&dq=timber+framing&as_brr=1#PPA454,M1
It goes without saying that timber frame designs should be reviewed and approved by a structural engineer. (doesn't it?)
gabel
