I'm going to have a go at replacing a tiled single pitch clay tiled roof extention. It has a cut-out round a window in the main house wall which has a flat roof, about 2 feet wide, 3 feet long, a sort of inverted dormer. I plan to use a cold fixed DIY felt system to cover this flat bit and the cheeks up to the pitched tiles. How should I seal the vertical joint between the felt and the main house wall please?