> Slate, layers of fossilized leaves, has a rough surface and frost and the weather in general will work on it and split the layers apart, seeds will find enough purchase to germinate (the handy supply of water certainly helps) and lichen and moss just love to grow on slate.
25 years ago, when I saw some roofers working to replace an old slate roof on a church outside Philadelphia with asphalt (I was horrified), I asked them why they were taking this (to me) horrible step, since my parents live and stay in homes in the UK with slate roofs that are between 300 and 500 years old.
The roofers laughed and said "yeah, that's probably welsh slate. The stuff here in PA is so much worse than that. Freeze-thaw will destroy it in 20-40 years"
So the observations you're making about slate are true but only for specific slate quarries. There are slate sources that can provide slate which could last for centuries.
The UK seems not to have much of an issue with moss & lichens causing problems with slate roofing (it grows but it isn't much of a problem).
I think the roofers would have said "oh, and the upstate NY stuff is pretty good too". The slate I've looked at in detail in PA really is pretty bad. It just isn't as dense as the welsh stuff in the UK.
25 years ago, when I saw some roofers working to replace an old slate roof on a church outside Philadelphia with asphalt (I was horrified), I asked them why they were taking this (to me) horrible step, since my parents live and stay in homes in the UK with slate roofs that are between 300 and 500 years old.
The roofers laughed and said "yeah, that's probably welsh slate. The stuff here in PA is so much worse than that. Freeze-thaw will destroy it in 20-40 years"
So the observations you're making about slate are true but only for specific slate quarries. There are slate sources that can provide slate which could last for centuries.
The UK seems not to have much of an issue with moss & lichens causing problems with slate roofing (it grows but it isn't much of a problem).