01
Follow the drainage path
Streaks below chimneys, fasteners, gutters or rooftop equipment often point to the source. Compare the stain with adjacent metal, mortar joints and vegetation.
02
Distinguish weathering from contamination
Some slate intentionally develops buff or brown tones. Other deposits sit on the surface. Product history and older project examples help avoid mislabeling normal weathering as failure.
03
Test cleaning cautiously
Identify the deposit and confirm compatibility with slate, flashings and masonry. Start with a small inconspicuous area and avoid methods that roughen the surface or force water under the roof.