said by VioletVenom:
Going by your picture the problem is using 3 tab shingles on a pitch shallower than a 3/12. I would be surprised if you have any roof deck under that moss.
The wood underneath the tar paper is coated with poly.
There's a long story to the slope -- suffice to say that making it significantly steeper would've made the space underneath much less usable.
Any pitch less than 3/12 needs another product, like a rolled roof.
Wacky Races 2012!