Modified bitumen roofing systems usually require a minimum slope to ensure proper water drainage and avoid ponding, which can lead to leaks and premature material failure. The general recommendation for modified bitumen roofs is at least a 1/4 inch per foot (2%) slope. This slight angle helps water flow off the roof efficiently, preventing standing water that can cause the membrane to deteriorate over time. Low slope or flat roofs with modified bitumen often incorporate multiple layers or additional drainage solutions to compensate for limited angle. If you notice water pooling or slow drainage on your roof, it could be a sign that the slope is insufficient or compromised, warranting closer inspection.
Homeowners can check roof slope roughly by measuring the vertical rise over a horizontal 12-inch span using a carpenter’s level and a tape measure. If you see clear ponding or water flows slowly after rains, it’s time to call a professional to assess both slope and drainage. When consulting a roofing contractor about your modified bitumen system, ask specifically about their recommendations for slope and if your roof meets those minimums to maintain the warranty. Also, inquire how they plan to address any low areas or drainage issues to extend the roof’s life. Proper slope is key to the durability of these systems, so getting expert advice early helps avoid expensive repairs later.