I can't actually give critisism on the stuff at hand since i am not that good at modelling. But it at least looks good to me
What i can say though is that you are doing a good, working through it and learning and actually trying to get better, and thats something i can respect, so good job and keep it up!
Oh also about that moss thing, i really don't know but a thought went through my mind when i saw it at first, that the lower part of the roof had a bit more moss on it and i thought, hmm thats rather logical, since the rain and moist is going downwards, making the bottom more moist and therefore it would grow more there. But then i read all saying more on top so i got unsure.