Any self respecting pitched roof will want to collapse and try to be a flat roof.
<<picture of collapsed roof – Folkingham>>
Consider putting a book on a desk, opened out to mimic a roof of a house, and it is clear that it wants to slide further apart.
<<picture of book on desk – sort myself>>
In fact the chocolate box picture of a Swiss cottage with saddled ridge and wavy eaves is caused by the roof spreading and pushing the walls away. Where this happens the ridge drops and so will sag between loading points of say a load bearing wall or a timber truss.
<<picture of saggy roof – Travers village>>
Perhaps my favourite image of why roofs collapse was born out of my frustration of raised tie roofs, and the misguided belief that simply putting a tie member between the rafters will ensure the longevity of both the roof and supporting wall structure.
It all came to a head when I was talking to a builder who wanted to build a raised tie truss roof and offered the solution of putting a tie just below the ridge.