Surface weathering is not always failure
A rough joint can remain secure.
Preparation matters
New mortar needs a stable joint rather than loose powder underneath.
Old stone and brick may need a softer approach
Very hard cement-rich mortar can be unsuitable on softer traditional masonry.
Want a roofer to look at it?
Describe what you can see and the weather in which the problem appears. That gives the roofer a useful starting point.
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