Roof ponding is where water gathers on a roof. So how much ponding is acceptable on a roof? Specifically, water must remain on the roof for 24-48 hours to be considered ponding. If you have this leftover water, you may notice it collects in dips in the roof or around drains.
Why is this a problem? There are several reasons to consider.
If moisture has nowhere else to go, gravity will cause it to seek any tiny imperfection in the roofing system to move downward. This leads directly to leak development and resulting structural damage.
Stagnant water is a Petrie dish for the development of mould, mildew and other forms of vegetation. Many of these use a root system, which can penetrate the roofing system and cause the same defects that lead to the development of leaks. Additionally, some of this vegetation is highly unhealthy for the people the roofing system is designed to protect.
Standing water provides the opportunity for pests to gather. Not only is it a breeding ground for mosquitoes, but it also provides a gathering location for birds, rodents and other pests to find their necessary water.
Depending on the size of the ponding area, it could be adding unanticipated weight to the roofing structure. A single gallon of water weighs 8.34 pounds. You can quickly see how large areas of ponding can add substantial weight to specific structural locations. Those locations may not be sufficiently reinforced to support that weight.