Movement causes many cracks
Long rigid sections and tight corners are common weak points.
The chase above can leak
Good lead can still admit water if the mortar joint holding its upper edge has opened.
Temporary coatings hide detail
Bitumen and mastic can help in an emergency but repeated layers make later inspection and repair harder.
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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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