Pre-fabricated waterproofing membrane made of distilled bitumen and elasto-plastic polymers (APP), reinforced with a woven non woven single strand composite polyester and rot proof fibre glass mat, which give the product very high mechanical characteristics and excellent dimensional stability. The fibre glass mat which is positioned in proximity of the surface guarantees a longer durability of the waterproofing sheet as well as enhancing the adhesion of coatings after application.