01
Ask for the actual selection
Request the source identification, dimensions, thickness range, relevant test documentation, and representative samples. Compare the information against the proposed use. A broad statement about slate cannot establish that two offered selections are interchangeable in an existing roof or a new design.
02
Compare relevant differences
Have the designer and slater identify which differences affect layout, appearance, handling, or suitability. Document the accepted product and any approved substitutions. The purpose is not to assume that one source is always superior; it is to make a decision using evidence about the material actually supplied. Clear specifications are more useful than either blanket equivalence or blanket dismissal of an origin.