Builders/roofers question

mutation, Jul 24, 2:37pm
Hi. Inside my roof cavity, I've noticed perfect virtical lines of condensation seeping through the building paper, that align with the roofing iron corogations. Does that mean the paper is stuffed? Only seems to be on the shaded side of the roof, and not the sunny side. Cheers. any comments appreciated. Roof was put on around 1989-1990
gabbysnana, Jul 24, 9:05pm
yeap.
johotech, Jul 24, 10:50pm
It probably just condensation from the inside, not a leak.
Are there any fans from the house exhausting into the ceiling?

It usually wouldn't cause any problems (and nobody generally notices), but maybe adding some vents around the soffit might help.
hydroplane, Jul 25, 12:51am
The building paper is there to catch the condensation that drips off the underside of the iron in winter, if the temp difference is a large then the amount of water can be excessive, over time the tar in the building paper washes out and becomes less effective, usually the time when the most condensation is when the sun hits the roof after a big frost and the frozen condensate melts on the under side of the iron. In older houses they used 25mm- 1" sarking which did a better job and lasted longer but more expensive with the cost of timber and labour these days.
In colder climates, 10mm thick tar treated soft board or plywood sarking is used along with building paper, I tend to use the heavy self supporting building paper because it is stronger and will last longer. by using sarking the temp difference is less - so less condensation and a stronger roof in wind and earthquake, less leaks and a better bracing element.
mutation, Jul 25, 9:47am
There are no fans exiting into the ceiling space. I get what you are saying about the condensation, the cold spot on the paper is the corrogation touching it? I might look into putting a couple of vents at the end, through each gable. The ceiling cavity is not currently vented. Hopefully the paper is OK, looks like the old black tar paper.
tweake, Oct 11, 9:44pm
vents can make it worse. so be careful.

is it dripping enough to cause a problem?
if ceiling insulation is not getting damp, wet spots etc then don't worry about it.