Use an experienced timber frame engineer. Many beautiful looking hammer beams fail over the years, the room spreads, the hammer beam buckles out of plane, the supporting post fails, to name just a few. Keep in mind the old cathedrals had massive stone buttresses on the outside to resist the outward force, and these buttresses climbed quite high because the foreces were at the top where the truss landed. Modern day trusses tend to be sitting on long posts, the posts undergoing significant horizontal bending, putting the connection at the bottom of the post into lateral tension. And then if you get all of that right, without all the heavy ornamentation you see in the ancient cathedrals the hammerbeam and post now tend to buckle under the load since they are frequently left floating in space and not properly braced perpendicular to the ridge. Be afraid....
