The most common size for roofing felt is 36-inches wide and are known as the 15 pound paper. There are also some rolls of roofing felt that are pre-marked in order to help you with the felt roofing installation. The papers have markings in them so you can position the strips straight to the roof. Afterwards, you must drip the edge on the eaves and the flashing as well as the valley liner should all be installed right before you place the roofing felt. The process of installation is not that easy, so you better inquire with some rubber roofing contractors that might be able to do the installation for you.