The following DIY green roof video shows the steps needed to ensure your green roof will not leak.
The most common building material used for roofing is asphalt shingles, and because of their poor durability they may spend only 15 or 20 years on your roof before heading to a landfill. A properly installed green roof has a longer lifespan than asphalt shingles and will absorb and retain a large portion of storm water, which otherwise would end up in sewers needing to be treated.
Storm water some regions is sent directly to waterways, taking with it pesticides as well as pollutants and debris from roads. Even in cities that treat their storm water, systems will occasionally be overloaded by heavy rains to a point where they just can't handle the volume and it ends up being dumped in rivers untreated, despite the efforts and finances invested to keep waterways clean.
Durability and cost:
A green roof will end up being more expensive to install than most other roofing products, but their durability makes them more affordable in the long run. By the time you have done a second round of asphalt shingles, the cost starts to even out. Expect at least a 50 year lifespan (and likely more) from a properly installed green roof, and if you are installing asphalt shingles, expect to replace them an easy 4 or 5 times over the lifespan of a building.