google "free standing lean to roof plans".
Instead of making it free standing, you could attach it to your existing run. I would raise the front at least 12 inches and run the rafters down and attach them to the top of your existing back wall. You could probably use hurricane clips to attach your rafters, to make it easy. Use plastic panels for the roof, there light weight and will serve your purpose. I used tuftex panels from Lowes, for my run roof.
Here's a pic I found, that might help you see what I am talking about.
Instead of making it free standing, you could attach it to your existing run. I would raise the front at least 12 inches and run the rafters down and attach them to the top of your existing back wall. You could probably use hurricane clips to attach your rafters, to make it easy. Use plastic panels for the roof, there light weight and will serve your purpose. I used tuftex panels from Lowes, for my run roof.
Here's a pic I found, that might help you see what I am talking about.