I have small roof to build, wall plate size 3.9m x 3.1m it is gable to gable ended, the rafter length is 2.1m with a 30 degree pitch, is it nessacery to use a purlin or will the 38mm x 125mm rafters be stong enough without. I have used purlins on larger roofs but didnt no if they are always nessacery. One other thing, the roof is on a side extention so will it be ok to support the pulin on the old exterior wall on one side and on the new wall on the other. Any help much appriciated