01
Map the line against the roof geometry
Record whether deterioration follows a slate edge, occurs below a concentrated discharge or appears at a joint. Ask for close photographs showing the surrounding roof, not just the hole. The Copper Development Association describes line corrosion where moisture remains at an overlapping roofing edge, and erosion corrosion where runoff concentrates on a small copper area. These are possible mechanisms, not a diagnosis from a photograph.
02
Expose enough to understand the assembly
Have the slater identify how adjacent pieces can be removed without unnecessary damage. The inspection should reveal the metal profile, supports and drainage arrangement. Record prior patches separately from the original detail. If only the visible hole is covered, the project may leave the conditions responsible for the damage unchanged beneath the repair.
03
Request a detail that addresses the cause
Ask the designer or sheet-metal specialist how the replacement changes contact, drainage or concentrated flow. CDA discusses separating a roofing edge from copper using appropriate detailing; the exact solution must fit the roof. Coordinate any change in the metal profile with slate support, fastening and appearance. Do not insert a generic spacer without checking the resulting assembly.
04
Inspect before the slate is reinstated
Arrange a review when the replacement metal is still visible. Photograph junctions, laps and the relevant edge condition. Keep enough context in each image that a future inspector can find the location. Include that area in subsequent maintenance observations so recurring staining or deterioration can be compared with a documented starting condition.