DAB,
I'm with Jim on this one. Lowering the Tie Beam will definately add some outward thrust to the wall posts. Could you convert the material to a Principal rafter/common purlin configuration to reduce the outthrust? And with the Slate material going on this roof, I'd definately get an Engineering opinion. Sometimes you just have to spend the money...
Also, the Guild has a book available that deals with the issues discussed here (the red one). If you haven't read it yet, there might be some info in there to help you out. Also, see the downloadable articles on the website here re: Trusses... regards
