newshound wrote:

I have come to the conclusion that it is *only* worth using the
corrugated bitumen boards with full boarding underneath.

my shed roof is full 18mm T&G boarded, I added battens for the coroline
and used their ridge pieces, didn't get on quite so well with their end
pieces, but it looks in good shape ~8 years on, no sagging. It's had
just one very light pressure-washing.