Possible causes
- Heavy replacement tiles on timbers not designed for them
- Rotten or undersized rafters or purlins
- Removed or altered roof timbers
- Long-term water damage
Warning signs
- A dipping ridge line
- Uneven or wavy roof slopes
- Cracking to walls or ceilings below
Typical solutions
- Inspection of roof timbers
- Strengthening or replacing affected timbers
- Re-roofing with appropriate materials
When to call a roofer
Promptly. A sagging roof should be inspected; some cases also need a structural engineer's opinion.
This is general guidance. The cause of any roof problem can only be confirmed by inspecting the roof.