This method of drilling, arranging and nailing disks works great for any roofing that has wooden board underneath. Structures like sheds, dog houses, lean-tos and porches are easy to cover and it makes a great hard-wearing, long lasting (I hope), cheap and most importantly.... green, method of roofing.
CDs and DVDs are readily available if you scrounge around. Ask friends, post on local sites like freecycle or craigslist, ads in the free local paper; these ways soon add up to thousands if you're happy to do a little walking/cycling around your town to pick them up.
Using this method, you need as few as 120 disks per square meter, and the method of drilling and arranging used means no water runs through the holes in the CD!
Step 1: Pros and Cons
If you've got old disks you want to get rid of, first think whether there might be someone able to reuse them before drilling holes in them and tacking them to your shed roof! Software, music and films might be of interest to your friends or families and are often accepted by charity shops as a good product to sell on.
For junk mail CDs and badly damaged or scratched CDs/DVDs, it is better to reuse them than throw them away or recycle them. If they can be of use for another 5 years before needing to be replaced then that's better than transporting them to a recycling plant to be melted down and MUCH better than them just taking a trip to landfill. Landfill is a dark, scary place!