The current percived knowledge indicates that roofs do not stay waterproof for long. (often due to bad design - not taking into consideration the location of the building and its exposure.)
The result is that the woodwork gets wet and goes rotten, roof leaks more problems grow and the roof is a danger to the occupants.
Or the water vapour from inside the house makes its way into the loft and condenses onto/into the woodwork and it goes rotten, with same result.
Both of these problems are recognised and the solutions, careful construction and design, attention to detail are ignored, by the builders.
As dannyppp says its all about moisture control. The moisture rising through the vapour permeable ceiling into the roof space. The roof itself is supposed to be weather tight and so if it leaks you get it fixed.
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