01
Start with an assessment
Natural color variation, weathering, biological growth, and deposited dirt are not the same problem. Ask a qualified slate professional to identify the condition and explain any need for treatment. A dramatic cleaning photograph does not show whether a method disturbed the roofing assembly or affected its long-term condition. Do not approve aggressive cleaning solely to make an older roof look uniformly new.
02
Review a specific method
If cleaning is justified, request a written method addressing access, the slate, adjacent metals, runoff, and protection of the building. Ask whether a small trial area is appropriate before wider work. Avoid treating pressure washing as a routine maintenance requirement. Keep the assessment and treatment record so future contractors know what was applied and why the work was undertaken.