The use of slate roofing stems back to sometime around 700 AD. The roman empire had also increased the use of Welsh Slate, quarried in Northern Wales.
Slate roofing is incredibly durable and typically lasts over a century. When considering the minimal maintenance applied to these stone shingle roofs, if one should apply modern coating technologies to the natural stone, the slate, in all theoretical and practical terms, should last longer now.
When looking to restore a stone monument or structure that is aging and in need of repair, one must assess several key factors.
First and foremost is public safety. The sandstone soffit face shows that this is still quite a sizeable stone. In saying this, you can see the visible exfoliation underneath from water damage. The outermost edge also looks quite thin so in this case it would be wise to indent stone.
Other cases may not be so risky and you will be able to patch with mortar repair. You can expect a good lime-based mortar patch to last quite a long time. Obviously, just not as long as replacement with new stone.