Slate may be very brittle and split easily into
sheets, but it is actually a tough rock that is
very resistant to the weather. This is why it
is often seen in craggy outcrops in mountain
regions, standing out darkly, almost black
when it rains. The combination of slate's
weather resistance, and the ease with which
it can be broken into flat sheets makes it a
superb light and durable roofing material.
Buildings have been roofed with slate for
thousands of years. The famous slates of
North Wales have been used since at least
Roman times. The Roman fort of Segontium
(modern Caernarfon) had its tiles replaced
with slate in the 4th century. while at nearby
Caer Lugwy a fort had a slate roof two
centuries earlier.