Black Sheathing Felt - 25 meter roll. Used as a separating membrane in mastic asphalt roofing systems.
The purpose of the separating layer is to isolate the asphalt from joint movement in the substrate but still provide a significant friction to help restrain the asphalt against contraction in cold weather. It must also allow a 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.
Separation is provided by Black Sheathing Felt which is laid entirely loose with lap joints of 50mm. However because Black Sheathing Felt is liable to compress slightly under heavy loadings, a separating membrane of Glass-Fibre Tissue should be specified in buried specifications such as Roof Gardens.