Do you want to add a porch roof but don't want to block the light entering your home? A clear plastic roof may be just what you need. You can get clear corrugated roofing at your local building supply store or try something a little different. The material in this photo is 1/2 inch thick honey comb plastic from Tap Plastic. I think it looks a little more upscale and diffuses the light nicely.
Here are two more examples using clear plastic sheets. Contact me if you want more specifics and where to get it.