The last roof I had to walk on, the guy kindly left out 2 small planks of wood. All be it said, that the roof wasn't mossy. I remember once covering custom made wood wheel jacks with carpet, this did the trick!
I did get some literature on a product specially made for this through the post, I've hunted high and low, and can't find it. It was really expensive though, if I find it I'll post it.