Posted By Mark Preston I can't help you directly - but if you go to the US Safety list here http://siri.org/mail/index.html and search using truss as your term in the lower right search box you'll get plenty discussion including some from people in the roof truss manufacturing industry - the first hit I got answers your question

Posted By Allan St.John Holt Word of warning - be a little careful of accepting US arguments for or more likely against fall protection. OSHA allows employers not to provide fall protection in curcumstances where it can be shown to be 'unfeasible'. This is often/usually taken to mean 'awkward'! Having said that, there are several answers available to the situation involving nets and also harness protection for banksmen unavoidably standing on open decking near edges. Allan