this is indeed a complex question.... i would hold fast on calling in the engineers yet though.... if you go to a good timber yard, they will design and warranty new roof trusses for you. these are incredibly easy to fix and as long as you dont alter tehm in any way, the warranty will guarantee the load capabilities. as a good general guage, 6"X2" at 16" (400mm) centres should be ok but see my previous note....
ivan
January 2006