Such protection is often lacking when saturated felts are used exclusively as the space between insulation boards allows the hot asphalt to run down, exposing the fibres to moisture absorption if any moisture is already in the system or if it infiltrates after completion of the roofing. Saturated felts have an open texture and the hot bitumen can penetrate into the spaces between the fibres to ensure good adhesion between the two. Adhesion between coated sheets and hot asphalt occurs at the interface. To achieve satisfactory adhesion the asphalt must be applied at the correct temperature and the coated surface must be free of moisture, etc. Felts are used in built-up membranes to provide a uniform substrate for the asphalt pour coat and to give the membrane the necessary strength to withstand thermal stresses, small structural movements and light traffic.
Organic felts can lose much strength when they become saturated with moisture.
The installation should be carried out according to recognized standards, and the roofing should not be abused by other trades once completed. A section taken in the longitudinal direction usually shows 3 or 4 superimposed and continuous felts separated by a thin layer of bitumen. Thus the type, quality of adhesion, and quantity of asphalt are critical factors in the transverse direction and help to explain why failures such as ridging and splitting generally occur in the longitudinal direction. Sections through 4-ply membrane. If not enough asphalt is used the glue may be discontinuous.