Mike,

I'm not in a position to answer your specific question, but I would like to offer a little perspective about the truss you are attempting to design.

Traditionally the hammer beam truss was used in structures which either had relatively thick masonry walls, or reinforcing butresses to counteract the spreading of the truss. This failure is due primarily to the lack of a continuos tying beam or chord in the hammer beam truss.

In modern applications the hammer beam is all to often seen perched atop hinged walls constructed of stick framing or timber posts and panel enclosures. Since these walls have the ability to rotate around their bases, the truss is free to spread as much as it needs to. Although many of these roof frames have been effected with some degree of success in modern timber framing, many have failed or are doing so slowly.

My personal recommendation would be to consider a queen post or king post truss. They are quite beautiful and much less prone to failure. If a hammer beam is unavoidable, consider reinforcing the plate connections with wings in the floor plan, or shed construction of some sort. I have seen wonderful examples of shed porches which do a fine job of absorbing the thrust at the plate level.

Truthfully, if this is your first truss, I would recommend at least having the design reviewed by a qualified structural engineer. Trusses are complex and confusing, but quite a joy to behold when effected correctly.

Rudy R Christian