6" x 3" sounds way to lean for a modern roof designed purlin.
8" or 9" x 3" is fairly typical.
setting the purlin at midspan may allow you to use 100mm x 50mm rafters, but it will depend upon span and pitch.
the purlin will need to be supported off either a masonry wall or by using steel to span the walls and 100mm x 100mm props off the steel. again, this will depend upon structural layout.
the roof is unlikely to spread provided the existing rafters are fixed correctly to the existing ceiling joists. you could always prop the ridge board for peace of mind.
are the existing purlins propped off masonry?
a cut roof may need some degree of s.e. design or a lot of conversation between you and b.c.
are you planning on building the roof yourself?
8" or 9" x 3" is fairly typical.
setting the purlin at midspan may allow you to use 100mm x 50mm rafters, but it will depend upon span and pitch.
the purlin will need to be supported off either a masonry wall or by using steel to span the walls and 100mm x 100mm props off the steel. again, this will depend upon structural layout.
the roof is unlikely to spread provided the existing rafters are fixed correctly to the existing ceiling joists. you could always prop the ridge board for peace of mind.
are the existing purlins propped off masonry?
a cut roof may need some degree of s.e. design or a lot of conversation between you and b.c.
are you planning on building the roof yourself?