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Hi Alex
From what you have described it is not a roof leak. Roof leaks will manifest as a wet patch generally limited to a small area of around 1 to 2 square metres. Multiple roof leaks will appear as small isolated patches. A large area of moisture covering the underside of the roof is condensation, caused by the moisture content of the air in the roof space being too high.
Often this problem is caused by a number of different sources which combined overload the roof space air with moisture. You need to be a bit of a detective as sometimes the source of the problem is not obvious.
To cover it all in detail would take quite a bit of time but here is a quick snap shot of some of the things to consider:
First check the usual suspects such as range hoods and bathroom fans are not venting into the roof space but are ducted to outside the building. Sometimes there are passive vents installed in ceilings in laundries and cupboards that duct straight into the roof space.
Ceiling access hatches in bathrooms and laundries can let a lot of moisture laden air past as they usually don’t seal very well.
Downlights in ceilings are another big problem, especially in bathrooms and laundries but even in living areas, especially if they are drying clothes inside or using a un-flued gas heater (either of which they should not be doing!!!)
Then you get into the problem of moisture transfer from other areas, sub-floor spaces are a big problem, but because it is under the house, most people don’t release the link to the roof space moisture issue.
The hub is developing a moisture tool to assist home advisors work through this issue and come up with potential solutions, so watch this space.
I am also right in the middle of writing a paper on this subject to deliver a presentation on moisture problems in New Zealand homes, at the Build A Better New Zealand Conference in September, with some case studies illustrating exactly the problem you have encountered, the causes and the solutions to rectify these problems. It is looking like a great conference and I would recommend all advisors attend if they are able to.
I have come across this problem in a lot of homes, not just old ones but also homes built to the current building code. It will continue to be an ongoing issue until we are able to educate both industry and home occupiers about how we should be not only building our homes, but also how we operate them.
Cheers
Richard