Whilst I am planning the walls.......

If I make the rear wall 10 blocks high this is 2.250m high (enough
headroom).
If I make the front wall 11 blocks high this is 2.425m high (just within
building/planning regs exemption).

This should give a slope of roughly 1 in 16.

Is this enough for a metal roof?

If I wanted to reduce the front height (e.g by using concrete coursing
bricks above the 10 blocks) or go up another course at the back then use
coursing bricks at the front how shallow can I go and still get reasonable
drainage?

Coursing blocks seem to be 140mm high as opposed to the 225mm of a full
block which would give a 1 in 25 slope.

Alternatively I could just use wood to raise the front of the roof enough to
get a reasonable slope.

TIA

Dave R