Loose mortar
Small pieces of mortar in the gutter or on the ground can be an early sign. The ridge may still look straight while the bedding underneath has cracked away.
Repointing is not always enough
If the ridge is firmly bedded and only the outer face has weathered, local pointing may be reasonable. If the tiles themselves are moving, smearing new mortar over the edge does not restore the bedding below.
Dry ridge systems
Mechanical dry-ridge systems avoid relying on mortar for restraint. They still need correct fixing and alignment. Mixing systems halfway along a ridge can be untidy, so the extent of work should be clear before starting.
