Generally, the paving fabric product is a non-woven polypropylene geotextile that retains sufficient liquid asphalt adhesion to form a waterproofing layer between the existing asphalt base material and the new addition. Nonwovens can be made of polyester or polypropylene, but most nonwovens used in the United States are made of polypropylene staple fiber and are needle punched. Nonwovens filter fabrics are used as separation fabrics when high water flows are required in applications. This can include acting as a separator between drainage gravel backfills after retaining walls, wrapping stones around pipes in French gutters, or as a bottom layer underneath the throw. Nonwovens can also be treated to resist water flow and create a waterproof barrier by combining with asphalt to serve as a pavement substrate