Most commercial buildings have flat roofs that use a system for draining water. They are usually built with a slight pitch, typically a minimum slope of ¼ of an inch every foot. Water drains from the roof into a scupper system or an internal drainage system. If this system is damaged or becomes clogged, then water will not drain properly and may begin to pond on the roof. Ponding water is water that remains on a roof for more than 48 hours. This can damage the roof in multiple ways.
The first and most apparent way ponding water will damage a roof is by its added weight. The ponding water can pool into certain areas of the roof which, when combined with the added weight of the water, will warp the structure and create a depression. This depression will allow even more water to pond and which will only add more weight and make the depression bigger. You can see how this can be a serious problem especially if a couple days of heavy rainfall occur. While this may be the most apparent damage ponding water can cause there are others.
Ice can form in water that ponds on roofs in cold temperatures. Damage to the roof can be done by the expansion and contraction as the ice forms and melts based on temperature fluctuations. This ice can also damage the roof by its constant movement within the ponding water. This can wear away the top level of the roof.
Algae and vegetation can also grow in ponding water which can also wear away the roof. Vegetation along with debris and other contaminants may also clog drainage systems that were previously working leading to further ponding water.
If you have a flat roof it is important to make sure that the drainage systems in place are working correctly. If they are not the problem needs to be corrected quickly to allow water to drain and prevent ponding. If ponding water has already occurred then your roof may already be damaged.