Check metal roof parts for corrosion
Fixings, clips, gutters and flashings can corrode over time, especially where protective finishes have failed.
Repeated slipped tiles or slates may point to failing fixings
If different slates or tiles keep slipping after windy weather, the fixings across that slope may be deteriorating rather than each failure being an isolated incident.
Use replacement materials that suit the existing roof
The repair should suit the roof covering and the level of exposure rather than simply copying whatever was fitted last time.