EPDM (Ethylene Propylene Diene Terpolymer), HDPE (High-Density Polyethylene), TPO (Thermoplastic Polyolefin), and APP (Atactic Polypropylene) are all types of synthetic roofing membranes used for waterproofing and roofing applications. EPDM is known for its resilience and flexibility, HDPE is a durable plastic often used in industrial applications, TPO is appreciated for its energy efficiency, and APP is a bitumen-based membrane often reinforced for increased strength.