Look at the sheds on www.sheds.co.uk.
Sheds usually have the same pitch, be they a pitch or pent style roof [semi flat] Check out construction at your nearest shed supplier.
Some put wood or felt shingles on the roof on top of the first 2ply plain layer of felt to give tiled effect.
What ever roof covering used start from the bottom and overlap second sheet 3" [75mm] min over first layer,so on till you reach the top strip. Felt shingles follow instructions that came with them.
Bob
August 2005