In reply to alvesjnr:
It's not clear what your picture shows but 30x160 is a rather odd sized timber and the timber orientation in your picture looks stranger still, I suspect your roof may be a little unusual or you've not identified/highlighted the main structure in your picture. More information needed.
In general, your roof is strong enough (or should be!) to support its own weight plus snow and wind loads so fastening a simple climbing board direct to the underside should be fine. It will however be rather steep, the conditions in most attics are horrid for one reason or another year round and you'll have to consider the strength and rigidity of what you're landing on and whether the plaster/decoration below will take the beating.
jk