someone here wrote:
wrote in message
oups.com...
Someone wrote, but the attributions were munged, as is so sadly
common:
Yes I agree. The felt roof that I am about to replace has been on since
I bought the house in 1986. It never had stones on - but rather a
'silver' coloured rubberised coating - any idea what that may be?

It's a reflective layer to reflect the sun's radiation / UV.
But the way, what are the stones for that you sometimes see ?
They just mean the roof gets punctured when you walk on it.

Increase the surface area for the evaporation of rain water

Why does that help?