Dan from Winnipeg, Manitoba writes: "We have an older manufactured home with a basement. Years ago a new roof was put on top of the existing one. It is post & beam with trusses, sheeted in with shiplap and a metal roof put on. The problem is the original metal roof now seems to sweat in the winter causing leaks inside. Any idea how to solve this problem?"
Leaving an old roof on when making a much higher new roof is common because it avoids the costs of taking it apart and disposing of the scrap and protects the house during the building of the new roof. However it can often cause problems, especially if it is a metal roof because it can so easily block the proper movement of air and moisture in the attic.
The graphics show working on improving the performance of this attic by cutting many holes through that old roof to assure an air flow from the soffits to the roof vents, both under and over that old roof inside the attic.
The real key to stopping condensation problems in an attic is to air seal the moisture paths from the house below. Working either from inside the attic (probably very difficult in this case) or from inside the house (lots of cutting open of ceilings and patching), you need to find and seal off every path through which warm moist air is finding it's way into the attic. Then the presence of that old metal roof is far less important. See the keyword "Air Sealing" in this database.