This is interesting what you did to stop the rain from doing the damage. Is there a possibility of see some pictures of your creation? I am having the same problem, even though my roof isn't quite as steep as yours (12/6).
Sorry, no pictures available at this time. If I can borrow a friend's digital I'll take some and post them or email them to you. I don't have a head for design, so I'm sure mine is as simple as it gets: Stacked rocks that form a semi-circle. If design is an important factor, you could build it from any material available, even, as Howard says about compost, just "piling it up on the ground". That's how I started. I was just looking for a way to keep the ground where the water hit from being eroded. So far, I haven't planted anything to cover up the sight of the ring. Because it is stone it has a sturdy look to it, and I don't mind the visuals of the ring.
I should also clarify a bit here. In my original post I used the word "compost", but in reality it was mulch I added to the center of the ring. Without the wall around it the mulch tended to bleed out and become part of the landscape after a few days of rain. With the rock in place the water would still drill a hole in the mulch, but the composting process causes a lot of it to fall back into the center. If there is any problem at all, it is that the mulch breaks down so quickly I need to fill the ring several times a year. This year I did some tree trimming and cut the branches into a size that would fit the ring. I put the hardwood material at the bottom of the pile and added mulch on top of that. I've already noticed the lighter organic matter breaking down and the hardwood sticking up through the pile.
The above may be due to the mulch material I use. I freely admit to being a scavenger, and I frequently scam the bags of yard clippings and raked leaves from the curbs of my neighbors for use in the compost pile. I know leaves aren't the greatest mulch material, but I have found that they are a good insulator in cold weather, break up the force of rain and break down quickly.