Ponding water sometimes occurs on membrane roof systems as a result of snow, rainfall or runoff from equipment. Although you may be held responsible for ponding on new or re-covered roof systems, many elements that determine how well a roof assembly will drain are outside your responsibility or control. Coordination with the building owner, roof system designer or general contractor often is necessary to address potential drainage deficiencies. Ponding water for short durations is unavoidable and considered acceptable by NRCA and membrane roof system manufacturers, but ponding water in excess of 48 hours can be detrimental to the roof assembly.
Slope and drainage requirements
NRCA recommends built-up, polymer-modified bitumen, single-ply and liquid-applied roof membrane assemblies be designed to provide positive drainage. The NRCA Roofing Manual: Membrane Roof Systems2011 states: "The criterion for judging proper slope for drainage is that there be no ponding water on the roof 48 hours after a rain during conditions conducive to drying."
Many building codes have a slope requirement for membrane roof systems. International Building Code,® 2012 Edition (IBC 2012) Section 1507Requirements for Roof Coverings states all membrane roof covering systems except coal-tar built-up roofs have a design slope minimum of one-fourth unit vertical in 12 units horizontal (2 percent slope) for drainage. Previous IBC editions have the same requirement.