01
Identify the contributing area
Ask the roofer to map where water from higher surfaces reaches the slate and whether it is concentrated at particular points. Include gutters, downspouts, valleys, and roof-level changes in the discussion. Photograph any recurring staining, debris, or damaged pieces near those locations. The apparent problem on the slate may be connected to the roof above it.
02
Review the whole drainage route
Have the responsible professional consider the upstream detail as well as the receiving slate area. A repair confined to the lower surface may leave the same exposure unchanged. If a gutter or roof modification redirects water, review its effect before construction. Keep the drainage sketch with the maintenance record so later work does not unknowingly introduce a new concentration of runoff.