Nonwoven fabric, slip layer, or protective layer are different types of separation layers in a roof construction, intended to protect the roofing material, structure, or other waterproofing layers.

A migration barrier is used between two layers to prevent chemical interaction between them. A slip layer is placed between two layers to reduce excessive friction. A leveling layer is applied between two layers to conceal irregularities in the substrate. A protective layer is typically placed over the membrane to shield it from mechanical stress and similar impacts.