1. Repeated slipped tiles
One or two slipped tiles a year is normal wear. A handful slipping every winter is the sign of widespread nail sickness — and a full strip and re-lay is more economical than repeat repairs.
2. Sagging roofline
A visible dip in the ridge line usually indicates failed purlins or rafters. Structural work is needed alongside any covering replacement.
3. Persistent stains
Ceiling stains that reappear after every heavy rain, despite repairs, suggest the underlay is perished and the roof is at end of life.
4. Delaminating slates
Slates that flake at the edges and shed layers are at end of life. Individual replacement is a stop-gap only.
5. Widespread moss and vegetation
Heavy moss growth is aesthetic — but persistent vegetation combined with any of the above suggests the roof is past economic repair.
6. Cost comparison
When repeat repair costs approach 40–50% of the cost of a full re-roof over 3–5 years, replacement almost always wins on a whole-life basis.