There are actually two aspects to environmentally friendliness when it comes to building materials, so it’s not such a simple matter.The two aspects you need to concern yourself with before you reach a decision on which one to use are how much impact the extraction and production of the materials has on the environment and how well it is able to reduce your energy consumption. For roofing materials, which clearly has a lot of impact on energy consumption (and the materials generally need to be replaced more regularly than other parts of a building), you can see how this discussion is very important.
The pros and cons of the different materials need to be carefully examined in order to make sure you choose one which has the least environmental impact. Choosing naturally sourced materials is usually the best option, as even if you can find man-made materials which provide greater insulation, that difference is often offset by the amount of pollution is produced during the manufacture of those man-made building materials. Slate, clay and thatch are all good examples of roofing materials which are naturally sourced, and wool is also a very effective insulator too.