i have hade a structural survey conserning roof purlings causing sagging to roof i have been told that the roof requires high level and low level purlings fitting with 100mm props were load bearing walls are . would anybody know if the roof will need to come off to do this job and how much roughley would it all cost to do if the roof does not need to come off it will need about 60 meters of new purlins with around 8* 1.5 meter props to support them . thanks to anybody that can help much appreciated