Goedjn wrote:

hasn't touched it since[in any serious way anyway]

He would probably be due a roof by now if he had put asphalt

SO I will go with slate over plastic, his slate is by an large 150 years
old now, and doing just fine.


Not all slate is the same.

yeah some is harder than others or weathers away differently.

sadly nothing lasts forever even heavy copper is rated at about a 100
years.

some rain has more acids etc leading to faster detoriation