Should an underlay be installed?
A bitumen roof is a 2-ply system in which an underlay is always laid first, followed by a top layer. This ensures waterproofing. Make sure the top layer is staggered by half a metre with the underlay. Always install the underlay and top layer in the same direction. If you renovate an old roof, the existing roof covering can serve as an underlay. First repair blisters, cracks and tears and fix the remaining dust with a primer.