If you're a plant lover and want to build an entire home around your love for landscaping and gardening, you're likely considering the gathering and use of the water coming off of your roof. Yet, rooftop rainwater collection relies on a surface that doesn't shed material or chemicals into the water. Choosing the right roofing gives you a steady supply of water you can trust not to damage your plants or leave residue in the soil.
Some environmental groups warn against using asphalt shingle roofs for rainwater collection because they can shed granules into the water. This is easily resolved with a simple filter, and it doesn't have much of an effect on water used for garden purposes because the granules are inert. The shingles also release a lot of dissolved organic carbon (DOC) in the first few years after installation. This is a chemical that has little effect on plants or soil, but it can react if it mixes with the chlorine commonly used to disinfect water. Also, aging shingles can deposit iron and zinc, which can build up in the soil over time. Finding asphalt shingles tested for rainwater harvesting can help you avoid these problems.
In general, metal roofing produces the lowest amount of contaminants in the form of chemicals and metals dissolving into the rainwater as it passes over the roof. Many reports also show fewer bacteria flushing into the water, which can become a problem in the garden if you apply the harvested water to plants and fruits shortly before harvesting. Fecal bacteria is surprisingly prevalent on roofing of all types, including metal roofing. It can cause mild to serious illness, so you should either withhold using rainwater on produce leading up to harvest or treat the water in some way. UV systems sound complicated, but they're surprisingly easy to set up in a simple tank or barrel, so your water stays safe to use on all garden plants.
Finally, green roofs are a good option for harvesting rainwater for gardening. However, it's not without its problems either. The same DOCs that come off of new asphalt shingles are also released in relatively large amounts by these roofs. This means that you can't disinfect the water with chlorine safely due to chemical reactions that form other compounds. While it's fine to choose a green roof if you'd like to enjoy it's other benefits as well, it's not necessary to install and maintain one just to enjoy safe water harvesting for the garden.
Contact a local roofing company for more information and assistance.