No Spam wrote:
Keeping the roof wet stops degradation of the felt in the first place.
A wet roof is much better than one that is kept dry, bakes in the sun,
expands, contracts and subsequently cracks. If its is properly done
in the first place, not covered in sharp stones and not regularly
walked on they can easily last 40-50 years.

Covered in chippings. AH! You mention that! Why the D- do they do that
in the first place? Grr.