01
Have the installer identify usable material
Ask which remaining pieces match the completed roof and are suitable to retain for future repairs. Keep different sizes and selections separate. Do not describe every leftover fragment as a spare slate. Photograph the accepted stock and retain product labels or order references with it. Clarify ownership and disposition of the remaining material according to the project agreement.
02
Create a simple inventory
Record quantities, storage location, and the roof areas each group serves. Ask for suitable handling and storage guidance rather than placing heavy stacks wherever space happens to be available. Include the inventory in the closeout documents and update it after repairs. A smaller organized reserve can be more useful than a large mixed pile whose origin and condition nobody knows.