The purpose of a separating layer is to isolate the asphalt from the movement in the substrate but still to provide significant friction to help restrain the asphalt against contraction in cold weather. It must also allow for the free lateral passage for hot air and moisture vapour during the application of the hot asphalt and act as a long term vapour pressure release layer.
Black Sheathing Felt
Black Sheathing Felt is normally used as a separating membrane in mastic asphalt roofing systems. It comprises a batt of jute fibre, which is impregnated with bitumen. Supplied in rolls, the product is loose laid onto the roof deck or thermal insulation, with minimum 50 mm side and end laps, prior to the application of the hot mastic asphalt.
Glass Fibre Tissue
Glass Fibre Tissue is generally used as the separating layer in asphalt flooring and paving applications. Unlike Black Sheathing Felt it does not compress under load, and is not affected by moisture from the ground.