01
Look at the actual assembly
Slate type, material condition, fastening, flashings, drainage, and previous interventions all matter. Ask the assessor to explain which components need work and what evidence supports the recommendation. A building's age may also differ from the age of some roof areas.
02
Treat predictions as conditional
Request the assumptions behind any remaining-service-life estimate and identify areas that could not be assessed. Avoid converting a general statement about long-lived slate into a guarantee for an individual roof. Keep the inspection findings and repair history together. They offer a more useful basis for ownership decisions than either a reassuring age comparison or a replacement recommendation based on age alone.