Why is my roof sagging?
A sagging roof may indicate problems with structural timbers, prolonged moisture damage, excessive loading, previous alterations or other structural defects. Because sagging can be more serious than a simple covering problem, it should be professionally assessed rather than treated as a cosmetic issue.
Why sagging is treated differently
A dip in a roof line is a structural symptom rather than a covering symptom. The covering follows whatever it is fixed to, so when the line drops it is usually the timbers, the support beneath them or the way loads are being carried that has changed.
That is also why sagging cannot be diagnosed from a photograph alone. Establishing whether it is long-standing settlement or active movement needs access to the roof space and the timbers.
What an inspection looks at
From inside, a roofer will look at rafters, purlins, ridge and any previous alterations such as removed supports or openings formed for a conversion. Signs of prolonged moisture, splitting or deflection all inform whether the movement is historic or ongoing.
Because the possible causes range from a single failed timber to a wider structural issue, this is one of the situations where a proper assessment matters more than a quick quote.