To understand the problems associated with hammer beam trusses I reckon its important to consider the origins of the form, and the historic buildings that hammer beam trusses were typically associated with; namely large masonry buildings with massive walls. There's a good reason for this: the trusses are lousy. OK, so they're kinda cute, but they just don't work very well.
Taking these trusses out of context and specifying them in a timber house can cause all sorts of problems...and that's why so many of the contemporary hammer beam roofs out there are either dependant upon their stress-skin panels or metalwork to make them stand up to November storms and February snows.
It can certainly be done, and one solution (rather than building 6 to 8 ft thick stone walls all over the place)is to position the trusses at locations in plan, where adjoining wings can provide some resistance to the thrust. Works a treat with a cruciform plan! BUT...and this is for real: get a decent structural engineer on board early in the design process if hammer beam trusses are your kinda thing.
Enthusiasts might want to check out Timber Framing No 47. More good stuff in Brunskill's 'Timber Building in Britain', Hewitt's 'English Historic Carpentry' and 'English Cathedral and Monastic Carpentry' as well as Brandon's 'Open Timber Roofs of the Middle Ages'.
