General Application Procedure It is generally agreed that the maximum strength between layers of felt is achieved by the thinnest continuous practical glue line. Too thin a layer starves the lap. The roofing bitumen should be applied such that in no place shall felt touch felt. However, excessive interply quantities can result in felt slippage on sloped roofs. Excessive top pour quantities can result in bitumen migration. Koppers will not be responsible for felt slippage or bitumen migration. NOTICE: Any deviations from these published specifications and requirements must be submitted in writing to your Koppers local representative. Final approval will come from Koppers Commercial Roofing Department headquarters in Pittsburgh.  Return to Application Considerations and Specifications