A green roof starts with a waterproofing layer. For existing roofs, the existing waterproofing (asphalt shingles, tar and gravel, etc.) can be used. For new construction, a single-ply membrane such as EPDM (ethylene propylene diene monomer – a rubber typically used for pond liners) or TPO (thermoplastic polyolefin – an environmentally friendly and recyclable roofing product mostly used in large-scale commercial buildings, like the Rogers Centre) is usually used. EPDM and TPO are quick to install, and also act as a root repellent, preventing plant roots from compromising the waterproofing.