4mm Insutech App Membrane.

APP (Atactic Polypropylene) membrane is a waterproofing product made from modified bitumen, used to create a durable and waterproof barrier on roofs, basements, and retaining walls. It is applied using a heat-welding (blow torch) method to bond it to the surface, and while it offers high temperature flexibility and is very effective, it is generally not trafficable unless covered with tiles or screed.